UPDATED08.24.2021

RELEASED11.15.1999

EXPIRES FOR CME08.24.2024

Intraoperative neurophysiologicalmonitoring

Introduction

Overview

Multimodal intraoperative monitoring techniques are, at the current level of care and

technology, considered mainstay. They allow ongoing evaluation of the functional nature of

various neural pathways as well as clear identication of vital neural structures. This

approach enhances the likelihood of a more favorable postoperative outcome for various

neurosurgical, orthopedic, vascular, and other respective or ablative procedures.

Intraoperative monitoring involves a multidisciplinary eort with coordinated input from

anesthesiology, neurophysiology, and the operating surgical sta. Dierent modalities are

available to monitor, continuously, important anatomic pathways and to assure the proper

identication of eloquent neural tissue, which will be discussed in detail within this article.

Historical note and terminology

Intraoperative neurophysiological assessment has become an integral part of certain surgical

procedures. It can be divided into two basic activities: (1) monitoring is the continuous “on-

line” assessment of the functional integrity of neural pathways, and (2) mapping is the

AUTHOR

Richard P Knudsen MD FAASM CNP FAAP

EDITOR

Bernard L Maria MD

Subm

it U

AT

Feed

back

functional identication and preservation of neural structures (110). Among other

modalities, somatosensory-evoked potentials (SSEPs) and transcranial motor-evoked

potentials (TcMEPs; either magnetic or electrical) are of distinct clinical benet and

additively advantageous.

Somatosensory-evoked potentials are obtained by averaging the electrical signals generated

by multiple electrical stimulations of peripheral nerves. These measurements are made over

the scalp and skin on the neck and the back (over the spinous processes of the vertebrae) to

monitor these signals in the cortex and spinal cord. Median nerve stimulation at the wrist or

posterior tibial nerve stimulation at the popliteal fossa or medial malleolus (ankle) is

commonly used. This convention is in accord with upper extremity somatosensory-evoked

potentials (UESSEPs) and lower extremity somatosensory-evoked potentials (LESSEPs),

respectively. Intraoperative somatosensory-evoked potentials are obtained, by repetitive

measurement, via the elicitation of time-locked waveforms in order to monitor functionality

of the posterior column-medial lemniscus (PCML) pathway, resulting in detection of

possible impending spinal cord damage during surgery and in prevention of peri- and post-

operative neurologic decits.

Classically, the most common procedure for which intraoperative somatosensory-evoked

potential monitoring is utilized is during the spinal deformity corrective surgery, primarily for

scoliosis or kyphosis (44). Historically, before the use of intraoperative somatosensory-

evoked potentials, the Stagnara “wake up” test was employed in patients undergoing spinal

surgery (139). This test, still employed, involves liing the patient's anesthesia level during

the procedure and verifying intact lower and/or upper extremity motor function (ie,

digit/phalangeal movements) before deepening the anesthesia level repeatedly, once again.

Limitations include the lack of sensory information, the inability to obtain continuous

information, inadvertent extubation, loss of patient positioning with the risk for injury,

destabilization of vital signs, and the potential for psychologically unfavorable postoperative

memory recall (101). The wake-up test can at most be administered a few times throughout

the length of the surgical intervention. Uncooperative patients are also unable to participate

in the wake-up test. Further, the critical time window for reversal of a decit could be lost

when waiting for the patient to awaken. Before intraoperative somatosensory-evoked

potential monitoring was widely available, the incidence of postoperative paraplegia

consequent to Harrington rod placement or other spinal instrumentation or distraction was

0.5% to 1.6% (77).

Intraoperative somatosensory-evoked potential monitoring has been performed in adults for

many years, whereas in children, it became more widely used in the 1980s and early 1990s.

One of the limitations of intraoperative somatosensory-evoked potential monitoring is that

mainly the posterior-column somatosensory system is monitored; the dorsal spinocerebellar

tract is monitored to a lesser extent. Experts in the eld have looked for a means of

evaluating the motor system, especially the corticospinal tracts, cytoarchitecturally situated

more anterolaterally within the spinal cord parenchyma, during surgery. In the 1990s, two

techniques had eectively been studied that attempted to monitor the motor system during

central nervous system surgery: (1) magnetic stimulation and (2) electrical stimulation of the

motor cortex or spinal cord (motor-evoked potentials, or MEPs) (79). Further, the combined

approach (motor and sensory versus single modality, ie, motor or sensory) provides rapid

detection of cord ischemia and other risk factors for postoperative neurologic sequelae during

orthopedic spinal surgery or during thoracoabdominal aorta surgery. The neurosurgical team

at University of Aachen, aer reviewing their intra- and postoperative data, deduced that

motor-evoked potentials monitoring was superior to somatosensory-evoked potentials

monitoring in detecting impending impairment of the functional integrity of cerebral and

spinal cord motor pathways during surgery. In the clinical operative setting whereby the

SSEPs and the MEPs demonstrate stable signal activity over time, one can be reasonably

reassured that the pyramidal tract function has remained, equally, intact. Another advantage

of combined monitoring is that in the event one modality is rendered non-recordable,

another remains functional and available (141). To recap, combined SSEP/TcMEP “multi-

modality” monitoring provides a higher positive/negative predictive value than single-

modality monitoring techniques (51).

The level of care for patients is enhanced, and indications toward improved outcomes have

broadened. Intraoperative neuromonitoring was benecial in guiding resection of “even”

thalamic neoplasms, an otherwise presumed surgically inaccessible, deep-seated, crucial

structure (16). Regarding descending or thoracoabdominal aortic repair, intraoperative

neuromonitoring was found, surgically, to be possibly instrumental in the prevention of

postoperative paraplegia (70).

Clinical manifestations

Presentation and course

Postoperative paraparesis, or other serious neurologic decits, had been a feared

complication stemming from spinal surgery, especially consequent to corrective intervention

for scoliosis (142). The advent of intraoperative monitoring has reduced the risk of serious

neurologic decits (114). Somatosensory-evoked potential monitoring is now a standard of

care for monitoring the dorsal column sensory pathways. This approach can detect, early on,

potential impending damage to neural structures; thus, it is possible that injury may be

averted. Ischemia and mechanical injury are the most likely mechanisms. Damage may arise

from direct blunt trauma, excessive compression, distraction, stretching, or vascular

insuciency via embolus or thrombus formation. It should be noted that normal

somatosensory potentials or motor-evoked potentials at the end of surgery do not guarantee

the absence of delayed paraplegia. Thus, postoperative monitoring, especially aer vascular

procedures, is occasionally indicated.

Sensory-evoked potentials involve the electrical stimulation of peripheral nerves creating

action potentials that propagate cephalad from the periphery, subsequently over time, to the

then central nervous system. The amplitude of the sensory-evoked potentials is small when

compared to relative EEG activity (93). Computer averaging allows the dierentiation of the

somatosensory-evoked potential signals from the background electroencephalographic

activity (which is random in nature, rather than event-related; ie, “noise” rather than true

signal).

Monitoring provides services beyond simply the warning of the possibility of ensuing

complications. It oers advance insight toward prompt intervention (94). A surgeon can feel

reassured about the integrity of the spinal cord and can, therefore, extend the procedure to a

greater degree. Patients and families can be relieved knowing that certain feared

complications are screened for during surgery. Further, some patients may receive

technically challenging procedures that would have been avoided in the absence of such

feedback about the status of the nervous system. As Muthukumar states: “Considering the

enormous costs of health care and the human suering related to the development of

postoperative paraplegia/quadriplegia, there is enough evidence to prove that the cost of

performing IONM does not exceed that of providing health care to the injured patients”

(89). Ibrahim and colleagues at Neurosurgery, Penn State, take a dierent position in

expressing: “The aim of neuromonitoring during an operation is to provide the surgeon with

a real-time analysis of spinal cord function at a time when there is still a possibility to correct

any possibility of morbidity. Changes in intraoperative neuromonitoring measurements can

be due to changes in arterial pressure, cardiopulmonary function, and spinal cord function.

Potentials can also be inuenced by anesthetic regimen, perfusion pressure, hypothermia and

hyperthermia. Intraoperative neuromonitoring has been utilized in many contexts,

including spine surgery, arterio-venous malformations, thyroid and parathyroid surgery,

pediatric deformity correction surgery, epilepsy surgery, subarachnoid hemorrhage repair

and others. Although it has been used in the numerous contexts shown above, an obvious

benet of IONM providing optimal functional outcomes in patients has NOT been

demonstrated. Both low sensitivity and low specicity can have detrimental eects on the

surgery and adversely aect patient outcomes” (52). The debate burns on with reference to

thyroid/parathyroid surgery as well. Lombardi and associates state: “IONM should NOT be

considered the standard care in preventing recurrent laryngeal nerve palsy” (72) whereas

Sun defends its use: “IONM has become an eective adjunct for the golden standard of

naked-eye protection. It is simple, eective and practicable” (129). Jahangiri and colleagues

promote multimodality intraoperative neurophysiological monitoring during shoulder

surgeries in that it grants the identication – in real time – of signal changes consistent with

likely impending nerve injury (54).

Clinical vignee

G.W. was a neurodevelopmentally challenged teenager who suered from global

developmental delay, a mixed seizure disorder, and progressive thoracolumbar scoliosis

with associated issues of pain and positioning. Her treating orthopedic physician elected to

surgically intervene with planned anterior and posterior correction of the otherwise

progressive, clinically signicant spinal curvature or kyphoscoliosis. The extensive

distraction procedure, with instrumentation, was undertaken with assistance from the

neurophysiology monitoring team. Intraoperative monitoring included both upper extremity

and lower extremity somatosensory-evoked potentials. Transcranial motor-evoked potentials

were also performed as well as two channels of EEG monitoring. The patient had been

medicated with valproate acid for her outstanding epilepsy. Her antiepileptic drug levels

were therapeutic, and she had no history of bruisability or thrombocytopenia.

Intraoperatively, the surgical team was notied of a sudden loss in the TcMEPs followed by a

signicant increase in latency of the cortical waves of the lower extremity SSEP’s as well as a

greater than 50% to 60% reduction in amplitude. This was noted at a time when the

deformity was being corrected and there was some signicant bleeding. Coagulation studies

were sent to the lab and found to be unremarkable. The surgical team decided to be less

aggressive with the corrective procedure and reduced the amount by which the deformity

was corrected because of the change in the evoked potentials. Postoperatively, G.W. had a

spinal straightening of 15 degrees (preoperative curve was 60 degrees). She was able to be

more aptly positioned in her wheelchair but, unfortunately, had developed new-onset bowel

and bladder incontinence, presumably from an intraoperative myelopathic insult. Some

practitioners speculated that the disodium valproate, known to potentially induce a

thrombocytopathy without demonstrable thrombocytopenia, played a role--combined with

the mechanical stress of surgical distraction--in the initiation of an ischemic event, as

detected by the sensitive intraoperative monitoring of the dorsal column pathways.

Biological basis

Etiology and pathogenesis

Intraoperative neuromonitoring (IONM) aims to reduce the possibility of spinal cord injury

during procedures indicated toward overcoming symptoms related to spinal cord deformity.

The deformity may be congenital, acquired, traumatic, or neoplastic; the procedure could

include decompression, correction, instrumentation, or fusion, all of which are hazardous but

less so in the clinical or operative setting of intraoperative neuromonitoring. Intraoperative

neuromonitoring can consist of SSEP, TcMEP, and/or EMG--a multimodality combination

is preferred (28). To broaden, intraoperative neuromonitoring is also utilized in thyroid,

carotid, and aortic surgical cases, as well as central operations.

Somatosensory-evoked potentials (SSEPs). Somatosensory-evoked potentials are

obtained by repetitive electrical stimulations of peripheral nerves. The electrical signals

generated are detected at the level of the peripheral nerves (over the brachial plexus or

popliteal fossa), spinal cord (lumbar or high cervical), and cortex (scalp). The signals

generated by hundreds of stimulations are averaged to make the time-locked components

more evident and to produce cancellation of the random electrical activity that may

otherwise obscure the low amplitude signal of the potentials. This method relies chiey on

the stimulation of the large myelinated somatosensory bers, which transmit the impulses

orthodromically to the spinal cord via the posterior column medial lemniscus system,

although some have ascribed a role of the dorsal spinocerebellar pathways as well (112).

Methodology of intraoperative somatosensory-evoked potentials.

Intraoperative somatosensory-evoked potential monitoring should follow the American

Clinical Neurophysiology Society guidelines for intraoperative monitoring (02) and those of

the American Society of Neurophysiologic Monitoring (136). The American Scoliosis

Society’s position statement of 2009 in favor of intraoperative neuromonitoring should be

heeded as well. The electrode sites are demarcated using the International 10-20 system of

electrode placement. The preferred recording sites are C3, C4, CZ, FPZ, FZ, A1, and A2.

In most instances, utilization of gold cup electrodes in gauze soaked with collodion is favored

although subdermal needle electrodes may be used. Additional gold cup electrodes are

placed over the cervical spine at the level of C2 and C3 and over the shoulder to serve as

grounds. The use of properly sized electrodes appropriately proportioned to the child's head

size is essential toward obtaining optimally recorded data. Electrode impedances should be

maintained between 2000 and 5000 ohms.

The recommended stimulus is a monophasic pulse of 10 to 25 mA and 100 µs of duration

although this may be increased if clinically indicated (44). The stimulus pulse is applied to

the median nerve at the volar wrist or at the posterior medial malleolar region of the

respective ankle. Spinal cord intraoperative somatosensory-evoked potential monitoring

Intraoperative somatosensory-evoked potentials: stimulation(Contributed by Dr. Sotero de Menezes.)

oen involves the posterior tibial nerve; however, when surgery is caudad to the eighth

cervical spinal cord level, median nerve intraoperative somatosensory-evoked potential

monitoring is used above that level (17; 69). Rates of stimulation are typically near 5 Hz for

median nerve and 2 Hz for the posterior tibial nerve; however, exact divisors of 60 Hz are

not used so that 60 Hz noise in the signal will cancel out aer averaging (44; 125). Each limb

should have separate evoked potential testing performed. Generally, 350 to 500 repetitive

stimulations are averaged; however, up to 2000 repetitions may be necessary in order to elicit

optimal waveform morphology and, thus, reliable and reproducible data (17). An analysis

time of 100 msec is sucient for the lower extremity SSEPs and for the upper extremity

SSEPs 50 msec is appropriate (17; 44). The lter settings are oen 30 Hz (low pass) and

3000 Hz (high pass), but many groups use high frequency lters as low as 250 Hz in order to

reduce noise levels. Most institutions employ the convention of negative potentials producing

upgoing deections, but the opposite convention is used in certain parts of the United States

and Europe. In regard to the source generators of key signals, “N” denotes the negative

deection and “P” denotes, conversely, the positive deection.

When using the posterior tibial nerve as the stimulation site, a channel derived from a

popliteal fossa recording electrode is referenced to one that is placed 4 cm above this topical

recording landmark. That particular pair of electrodes is used to verify the integrity of the

signal generated distally and to rule out problems that could interfere with the nerve

conduction, such as clinically signicant changes in temperature. Other important recording

montages include the upper cervical cord (CS lead) referenced to CZ and the ipsilateral ear

as well as a scalp lead at CZ referenced to FZ. The upper cervical cord recording may

require an inserted needle electrode due to the low amplitude of the signal stemming from

posterior tibial nerve stimulation.

Advocated recommendations include the use of at least three channels for median nerve

intraoperative somatosensory-evoked potential monitoring (44). The rst channel records

over the ipsilateral Erb's point at the level of the brachial plexus. The more proximal channels

include mid-cervical cord referenced to CZ and, subsequently, in a cephalad array,

contralateral central cortex to contralateral Erb's point or to FZ.

Median nerve somatosensory-evoked potential waveforms and generators monitored

during surgery. Erb’s point potential (P9), is the name given to the waveform generated by

the passage of the stimulus volley through the distal brachial plexus. The montage used to

record this potential is ipsilateral to contralateral Erb’s point. The Erb’s point potential may

manifest itself as a double negativity, especially in children.

Cervical potential (N13). This potential is maximal at the low- to mid-cervical region. It is

stationary because its latency does not change when one moves the recording electrode

proximally or distally. It shows negativity posteriorly and shows positivity anteriorly (ie, an

anterior-to-posterior phase reversal in the cervical area). The N13 represents the

postsynaptic activity in the dorsal gray matter of the cervical cord or dorsal column as the

sensory volleys reach the cervicomedullary junction. N13 is recognized as the most

prominent of several negative potentials, but it may be confused with the N11, which is

generated in the root entry zone. Unlike N13, the N11 does not show anterior-to-posterior

phase reversal in the cervical area.

N20 is a contralateral parietal near eld potential. It is the initial corticothalamic response to

the sensory volley. The use of the contralateral central cortex to reference and the

contralateral central cortex allows the separation of N18 (a subcortical brainstem potential)

from N20. It is generated in the post-central gyrus with an activation of the 3b area

(posterior bank of the central sulcus) with subsequent spread of the depolarization to area 1

and area 2 (post-central sulcus), area 3a (bottom of the central sulcus), and area 4 (anterior

bank of the central sulcus) (75). There is also thought to be a thalamic component to the

N20. Around 1.5 to 2 ms aer N20, a positive deection is seen over the centroparietal

region (P22) that is also thought to have a cortical source generator, but the exact location

remains controversial.

Posterior tibial nerve somatosensory-evoked potential waveforms and generators

monitored during surgery. This includes the popliteal fossa potential that has a triphasic

morphology. This peak is also known as the knee potential, and its negative peak is alluded

to as N7 or N8 (81). This waveform is generated locally as the stimulus traverses the nerve at

the region of the popliteal fossa.

N22 is a negative potential maximally recorded between T10 and L1, using the contralateral

iliac crest as reference. It reects the postsynaptic activity at the level of anatomically normal

lumbar enlargement in the spinal cord. This potential is important when monitoring surgical

procedures near the pelvic area.

P31 is a component of the posterior tibial somatosensory-evoked potentials that reects the

activity on the medial lemniscal system. It is characterized by a small positive deection, and

it is a far eld potential.

P37 (also known as P38, P39, or P40) is a potential recorded over CZ-PZ or the ipsilateral

central region due to the positivity generated as a dipole in layer 4 of the contralateral leg,

cytoarchitecturally, area of the primary sensory cortex. P37 is a near-eld potential

restricted to the central and parasagittal region. FZ is inactive aer 34 ms; therefore, it can

be utilized selectively as a reference for this particular component.

Aer P37, two other main waveforms are recordable in a caudo-cephalad direction: (1) N45

and (2) P60. P60 is also referred to as P2 (148); it is maximal at the vertex and may persist

when P37 is absent (81). The P37, N45, and P60 combination produces the typical

W-shaped morphology of the evoke potential.

Cervical potentials. One of the main dierences between the potentials generated by the

median nerve and by the homologous posterior tibial nerve is the presence of decided

cervical potentials. Median nerve somatosensory-evoked potentials generate clearly

identiable cervical potentials (N13). On the other hand, by the time the stimulus generated

at the posterior tibial nerve reaches the cervical spinal cord, the temporal dispersion will

cause a signicant amplitude reduction of the cervical signal. The use of needle electrodes

implanted at the interspinal ligament can aid in the accurate detection of these otherwise

small waveforms. When needle electrodes are used, the cervical potentials are seen as a

major negativity followed by a positive wave (74; 75; 17). Throughout the surgery (from

wound opening to wound closure), the waveforms are recorded at approximately 3- to

5-minute intervals, but more frequent (and nearly continuous) recordings should be routinely

performed during the more intensied surgical periods, such as the critical times of either

instrumentation or of distraction, both being mechanically intrusive and, thus, potentially

necrogenic, given the heightened risk of ischemic insult.

Criteria for abnormality (aka, what constitutes warning criteria or an ALERT). It is

important to obtain baseline data, including pre-incision tracings and early (surgical site

preparation) intraoperative data. All subsequent electrographic ndings, as collected, are

then compared to the baseline. In essence, the subject serves as their own ‘internal’ control.

That is, the usual subject variables, such as age, limb length, and body height, play a minor

role in the operative setting and are eclipsed by the patient’s own individual baseline values

and deviation from that index criterion. Stimulus intensity as well as rate and duration of the

square wave need to be regarded (101). A multifactorial analysis of the eects of physiologic

parameters on intraoperative somatosensory-evoked potentials yielded a correlation between

variations in latency and amplitude in the order of inuence as temperature, then paCO2,

then heart rate, followed by diastolic blood pressure, and then systolic blood pressure (23).

The patient's underlying condition may prevent the recording of intraoperative

somatosensory-evoked potentials in about 5% of the cases (46). The most frequent

conditions leading to the problem of poor data acquisition are neural tube defects and

severe spastic quadriparesis with atrophy of the lower extremities. Other disorders associated

with unrecordable intraoperative somatosensory-evoked potentials include sequelae of spinal

cord trauma, advanced spondylosis with myelopathy, scoliosis with myelopathy, peripheral

neuropathy, and spinal cord tumor (46). In one study, spinal cord monitoring using

somatosensory-evoked potentials was reliably achieved in 31 out of 34 patients with

cerebral palsy undergoing corrective surgery for progressive scoliosis (30). Proposed

pathogenetic mechanisms of signal change during monitoring include temporal dispersion of

the aerent volley and conduction block in damaged axons stemming from ischemia and

anoxemia with resultant abnormalities in the electrophysiological parameters of amplitude of

the response potential or prolongation of onset time of the evoked signal. Further,

hypoperfusion can injure the white matter of the central nervous system with loss of

waveforms arising from calcium inux into the intracellular space. Hypothermia aer

exposure of the spine but before instrumentation may render an increase in false negative

outcomes (117).

Intraoperative neuromonitoring, multimodality, provides data that prompts reevaluation in

approximately 10% of patients with pediatric spinal deformity (106).

Some variability in latencies and amplitudes between dierent stages of scoliosis surgery is

normal. Principally, the SSEP (when the spine is exposed) may be used as the reference

baseline to determine whether future potentials are subnormal at the subsequent stages of

surgery. Of note, the amplitude of the SSEP decreased in most patients when the spine was

exposed, although there was no injury per se to the spinal cord. This must be heeded in order

to avoid declaring undue false positives.

The criteria commonly used in signicant wave form change (in relation to the baseline

tracing) are either an increase in latency equal to or greater than 10% of the preoperative

baseline (95; 44), or a decrease in amplitude of more than 50% (95; 92; 148; 44). Of late,

there is an advocacy movement toward revising the threshold for alarm to, more adequately,

a 60% amplitude attenuation compared to baseline (aer skin incision or spine exposure)

rather than 50%, based on data (49). The literature suggests that amplitude may be the more

sensitive indicator of neurologic decit, but the criteria for change vary with the type of

procedure and the type of expected injury, so careful consideration must be made regarding

what criteria will be used in each case.

Nuwer's experience was that patients with persistent amplitude reduction of greater than

50% maintain a 25% chance toward a new neurologic decit in the postoperative period

(92). The same author mentions that amplitude reductions of less than 50% are not as

concerning but do warrant careful neurologic follow up; a similar conclusion was reached by

another study including 81 patients (148).

Regarding indications of intraoperative neurophysiological monitoring, the neurosurgical

team at the University of Wisconsin examined the diagnostic and therapeutic utility of

intraoperative neurophysiological monitoring in the surgical treatment of cervical

degenerative disease (109). They deemed evoked potential monitoring as a sensitive tool

during anterior spinal surgery for cervical spondylotic myelopathy. Further, they

concluded that intraoperative signal worsening does not tightly correlate with clinical

worsening and that its recognition does not necessarily prevent neurologic insult. That is,

intraoperative neurophysiological monitoring does not seem to forecast outcome with

reliability. This must be considered in the appropriate context. Anterior cervical discectomy

and fusion is associated with a low risk of neurologic injury, so it will be dicult for any tool

to improve outcomes (133; 123). When there is a higher risk of neurologic injury, any tool

Intraoperative posterior tibial evoked potential-baselineBaseline measurement immediately aer anesthetic induction. Le side cortical

potentials: P37 31.7 msec; N45 36.7 msec; 1.15 microvolts; Le sided cervical potentials 23.8msec; 0.45 microvolts; Right sided cortical potentials:...

becomes much more useful.

The value or power of intraoperative neurophysiological monitoring has been evaluated in

the setting of surgical remediation of tethered cord syndrome in which the risk of injury to

nerves embedded in the tether is signicant. Of note, neurologists at the Aga Khan

University investigatively utilized – beyond the routine tibial SSEPs – clitoral and dorsal

penile SSEPs during monitoring (59). They deduced that few data exist to support the

merits of intraoperative neurophysiological monitoring in tethered cord syndrome. This has

been challenged by data from the University of Virginia (107). That center contends that

electrophysiological monitoring provides, for untethering, an ecient, eective, and reliable

method for intraoperative guidance with the goal of reducing iatrogenic injury. They also

purport that monitoring, in the form of both a threshold-based interpretation system and

continuous EMG, can localize the “autonomous placode” in secondary tethered spinal cord

syndrome. For example, clinically, if a newborn has an operation in the perinatal period for

myelomeningocele, there is the possibility of subsequent onset secondary tethered spinal

cord syndrome. The symptoms, of remote or latent onset, can typically include progressive

bowel or bladder dysfunction with associated lumbago and lower extremity paresthesia and

spasticity. The monitoring can detect the tethering placode, a combination of scar and

neural structures. The surgeon can, more aggressively, section out the placode and, thus,

grant the patient more postoperative relief. In Pouratian and colleagues’ series, the patients

benetted greatly in terms of both intraoperative utility and postoperative outcome (107).

Again, as a guidepost for the valid interpretation of EP data, when alluding to cervical and

cortical signals, suspect potential impending pathologic alteration if: (1) there is an increase

in latency equal to greater than 10% of the baseline value or (2) there is evidence of a

decrease in amplitude greater than 50%. These are certainly, at the least, warning criteria

(91; 94).

Transient and relatively less signicant changes in the amplitude (30% to 50% of baseline)

or latency (less than 2 ms) of intraoperative somatosensory-evoked potentials may be seen

during surgery due to hypotension/rendering of ischemia, hemodilution,

hypothermia/anemia, hypo- or hyper-thermia, hemodilution, hypothermia, or irrigation with

cold uid or due to certain anesthetics; return to the baseline values is seen when these

alterations are corrected (46; 44). These alterations oen occur gradually over a period of 30

to 60 minutes. On the other hand, clinically signicant changes, such as intraoperative

somatosensory-evoked potential amplitude reductions of greater than 50% of baseline or

increases in latency of 10% (2 ms or greater), tend to be acute and not associated with any

change in temperature, blood pressure, or the amount or nature of anesthetic administered

(44). The Scottish National Spine Deformity Centre espouses an algorithm of action in

response to intraoperative monitoring events; in the patient consent forms, they state the

possible need to abandon surgery depending on the intraoperative monitoring ndings and

complete it in a staged manner at a later date (137). One should realize that increasing

concentrations of halothane can quickly produce a decrease in the amplitude of the cortical

potentials, which is directly proportional to the end tidal concentration of that gas (148).

Patients younger than 10 years of age are particularly susceptible to the eects of high

concentration--or boluses--of general anesthetics, producing attenuation of the cortical

potentials (46). Cortical potentials in children are also more likely to be attenuated by a

combination of anesthetics, such as isourane and nitrous oxide, especially in high doses

(46). Improved and more assured scalp recordings are obtained through the avoidance of

combination anesthetics or by keeping the concentration of nitrous oxide at less than 50%

and isourane at less than 0.6% when these agents are used together (46). When one uses

the latter range of anesthetic doses and adds narcotics (ie, fentanyl) during the induction,

intraoperative somatosensory-evoked potentials remain unaltered (46; 44). Other anesthetics

that have been shown not to aect the intraoperative somatosensory-evoked potential

recordings are propofol, etomidate (44), and ketamine (04). A bolus of lidocaine may

produce a transient, but signicant change in the cortical response (19). The greater

instability of the cortical responses in children is thought to be due to the lack of symmetry

and synchrony in the myelination process (36). Due to the unreliability of the cortical

response in children, recording of the cervical potential has been recommended (46).

Cervical potentials are more resistant to the eects of general anesthetics and can be used to

monitor the integrity of the spinal cord above the surgical level when the cortical potentials

are absent (57; 39). If the posterior cervical recording electrode is within the actual operative

eld, an anterior neck recording electrode can be placed, which is usually referenced to CZ

or FZ (45).

When interpreting intraoperative somatosensory-evoked potential abnormalities, in real-

time, one has to be attentive to certain factors that may generate recording artifacts. These

include electrical stimulator failure, electrode problems (high impedance, detachment from

the skin), and electrical or magnetic interference from other equipment in the typically

electrically “hostile” operating room setting. Additionally, certain physiological factors, such

as obesity, diabetes with comorbid neuropathy, peripheral vascular disease, seizure disorders,

and closed head injuries have been associated with less than optimal responses (154).

When cervical or cortical potential abnormalities (with an amplitude drop greater than 50%

or latency increase greater than 2 ms from baseline) are seen during surgery, the patient and

the recording apparatus should be checked for sources of artifact; the anesthesiologist should

be questioned about the subject's vital signs (especially blood pressure and body

temperature) and relevant changes in the pharmaco-anesthetic regimen. Remediable causes,

such as hypotension should be dealt with appropriately. Intraoperative somatosensory-

evoked potentials should be monitored frequently to realize, optimistically, a return to

baseline. The entire review process should be accomplished within 5 to 10 minutes, and, if

no other probable reason is present to explain the intraoperative SSEP alterations, the

surgical team should be notied promptly. It is also sound practice to have someone from the

surgical team tell the neurophysiology monitoring team of the impending onset of high-risk

manipulations, such as spinal distraction, instrumentation, and sublaminar wire placement

(144; 46). When cortical or cervical intraoperative somatosensory-evoked potential

abnormalities return to baseline within 15 minutes of the acute changes, postoperative

neurologic sequelae are unlikely to be realized. The opposite is true if the waveform

abnormalities persist for periods of time greater than 15 minutes (44).

In circles of physicians performing cervical spine surgery, controversy and debate surround

the issue of whether intraoperative somatosensory-evoked potentials are clinically useful in

uncomplicated, non-upper cervical spine procedures. One group, launching a retrospective

review of a large number of cases of anterior cervical discectomy with fusion (ACDF),

concluded that the surgical procedure itself was safe and that intraoperative somatosensory-

evoked potentials had no utility and was, thus, withdrawn from use (133). Certain groups are

exploring S100B, a serum marker for glial injury, as an adjunct with evoked potentials, to

predict long-term neurologic alteration postoperatively (131).

Some surgeons favor somatosensory-evoked potential monitoring during lumbar root

decompression. Yue and Martinez found that during L4-5 root decompression, supercial

peroneal nerve SSEP (SPN-SSEP) is more reliable than post tibial nerve SSEPs (PTN-

SSEP) (150).

A survey was orchestrated from the neurosurgery department at the University of

Saskatchewan (104). Canadian neurosurgeons and orthopedic surgeons were asked revealing

questions, through the Canadian Spine Society. Respondents stated that monitoring was

performed to reduce the risk of an adverse operative event rather than because of liability

concerns, and they collectively favored monitoring in cases of reduction of major deformity

(scoliosis), symptomatic and asymptomatic spinal cord compression, spinal cord tumors, and

instrumentation. Availability was an issue, as was the lack of neurophysiology specialists

within neurology.

Motor-evoked potentials (MEPs). Over time, direct electrical stimulation and, to a lesser

extent, magnetic transcranial stimulation of the cortex or spinal cord have become a more

standard method of evaluation of the integrity of the motor pathways during spinal surgery,

and, thus, we have the term “transcranial motor-evoked potential” or “TcMEP” (68; 03).

Customarily, two main reasons have ushered in the need for intraoperative motor

monitoring. The rst stems from the fact that postoperative motor decits are oen

profoundly clinically disabling. The second reason centers on the selective vulnerability of

the anterior spinal cord to hypoperfusion when compared, in contradistinction, to the

posterior columns (87). The ventral spinal cord seems to have fewer anastomotic vascular

communications and, further, contains gray matter, predisposing it to hypotensive damage

during surgery (87). From a practical standpoint, the MEP changes tend to precede the

SSEP changes, making them an earlier warning sign for the neurophysiology team to deal

with; this allows greater time for the neuro-orthopedic surgeon and anesthesiologist to rectify

aspects of the operation and, thus, mitigate irreversible injury (87).

Transcranial electrical stimulation of the motor cortex via motor-evoked potentials

(TcMEPs) is a desirable method, but its interpretation has several caveats. This method

requires careful assessment of the degree of neuromuscular blockade during surgery and of

the type of anesthetics used (03). In general, inhalational anesthetics and neuromuscular

blockade have been shown to limit the ability of the TcMEP monitoring to detect signicant

changes. Hypothermia, as well, can negatively aect intraoperative neurophysiological

monitoring. Opioids have little inuence on TcMEPs. Further, a stable concentration of

inhalational or intravenous anesthetics optimizes TcMEP monitoring (140). TcMEPs have

been successfully used to monitor intramedullary spinal surgery in children (61). The

combination of electrical TcMEPs and SSEPs in spinal surgery appears to be safe and

accurate to predict neurologic decits in children (127).

A relatively high voltage is utilized to generate the transcranial motor response, oen a few

hundred volts (88). This level of stimulation can be painful in an awake patient. The charge

density utilized for TcMEPs is 10% to 15% of that required to induce seizures in humans

(78). Reportedly, the incidence of seizures during MEP monitoring for cranial procedures

was overall 1.8%. The incidence rose to 5.4% when direct cortical stimulation was

additionally applied (138).

Voltage-based stimulation is not commonly undertaken in other settings due to safety issues.

Current intensity-based parameters are used for direct cortical stimulation (see “Functional

brain mapping” section below). The stimulating electrodes are located at C3-C4 (of the

International 10-20 system of electrode placement) for elicitation of upper extremity

responses and C1-C2 when looking for activity over innervated lower extremity

musculature. The anode (positive electrode) is the one stimulating as cathodal (negative

electrode) stimulation requires relatively higher currents, so the anode should be placed at

C1 or C3 for le lower or upper extremity responses (88). When lower extremity response is

not seen with C1-C2 placement, an alternative electrode positioning using CZ as the anode

and with the cathode located on the midline 6 cm anterior to CZ (between CZ and FPZ)

can be eective. These techniques allow a more vertically oriented vector, which enables a

better excitation of the descending axons’ cortical spinal tract neurons (88). The closer the

stimulating electrodes are, the more horizontal the stimulus is and more robust the I waves

are.

TcMEPs use somewhat short pulse durations ranging from 50 to 100 microseconds for

recovery of the D waves between the also short interpulse intervals of 2 milliseconds (88).

Using trains of 4 to 9 stimuli with 2 to 4 milliseconds interpulse intervals tends to overcome

anesthetic inhibition of anterior horn cells (87).

Stimulation of the motor cortex produces two patterns of negative waveforms D and I waves

with direct recording from the dorsal spinal cord using epidural electrodes. The D wave can

be identied as a single negativity, and the I waves are characterized by up to 4 negative

peaks named N2 to N5 (88). D waves are caused by direct stimulation of the corticospinal

tract. I waves are caused by stimulation of neurons of deeper layers of the primary motor

cortex, which ultimately will cause indirect and “transsynaptic” (cortical-cortical

connections) activation of the corticospinal tract.

D waves are less inuenced by anesthetic or neuromuscular blocking agents, especially when

they are averaged (87).

For practical reasons, oen only peripheral recordings are used through needle electrodes

inserted on the muscles of interest (the muscle-MEP). Because the focus is on the

corticospinal tract, the preference is to monitor the distal muscles of the extremities with

EMG needles. The hand muscles used are usually the abductor pollicis brevis (APB) and

abductor digiti minimi (ADM). On the lower extremities, corticospinal tract converges over

the abductor hallucis brevis and tibialis anterior, which are oen used during TcMEPs (88).

The muscle responses vary more than the direct spinal cord waveforms (D and I waves)

because of the synapse at the anterior horn cell. Three criteria for interpreting changes in the

Tc-MEP are used. The rst is the threshold method (14) in which the typical criterion for

abnormality is the need to increase the stimulating voltage more than 100 V from baseline to

obtain responses. Another criterion is either absence or a signicant reduction in the

amplitude of the Tc-MEP (65; 62). The third criterion is a change in the complexity of the

Tc-MEP waveform (108). The criterion for the change in the D-wave is generally a 50%

decline in amplitude (61).

The interpretation of the TcMEPs should take into account all the factors that may alter the

signal, such as anesthesia and neuromuscular blocking agents. Barbiturates,

benzodiazepines, and propofol are the intravenous anesthetics that are more likely to

decrease the amplitude and increase the latency of the TcMEPs (55). Inhalatory anesthesia,

either halogenated or nitrous oxide, may also decrease the amplitude of the TcMEPs in a

concentration-dependent manner (55).

The neuroscience group at Singapore General Hospital discerned that cross-scalp

stimulation was oen an augmentative approach, especially in infratentorial and spinal cord

surgical cases, because the summation of the ipsilateral and contralateral stimulation signals

would be easier to realize (71).

A practical way of troubleshooting a poor signal from myogenic TcMEPs is to consider the

main causes (87):

Calancie and colleagues found that transcranial electrical stimulation of the motor cortex was

more ecient in detection of postoperative motor decits but missed some of the sensory

decits; the opposite was true for intraoperative somatosensory-evoked potentials (15).

Electrical stimulation of pedicle screws. The goal of neurophysiological testing during

screw pedicle placement is to attempt to nd the tip (135). The closer the screw tip is to the

nerve root, the higher the likelihood of neurologic sequelae. Cadaver studies have

demonstrated a 20% chance of screw misplacement, such as misdirection and piercing of the

bone wall (48). Radiological verication of the screw placement with plain x-rays is not

more precise that neurophysiological testing. The “gold standard” is the computed

tomography in real time that is not a practical alternative in the operating room.

• Signal acquisition method issues

• Anesthetic inuence, such as the high-dose inhalatory anesthesia, propofol

• System variables, such as hypotension

• Patient’s physiopathology, such as a preexisting CST dysfunction

The threshold to a response is recorded with progressively higher current intensity, which is

delivered at the outer end of the screw. The spontaneous muscle (EMG) activity is also

recorded during the screw instrumentation.

The surgeon and the neurophysiology team should be aware of the factors that alter the bone

and so tissue impedance during the surgery. A “wet” surgical eld tends to lower the

response threshold (88). Some authorities have used a voltage-constant system to circumvent

this problem.

Aer drilling the hole in the pedicle, a needle is inserted just half-way (48). This method

allows for greater proximity to the nearby nerve root. Inserting the tip of the needle all the

way into the vertebral body would make it more removed from the site; this is where many

of the bone wall perforations occur. When using the needle electrode, an EMG response

threshold of 4 mA or lower has been associated with bone perforation, provided that the

adjacent nerve is normal (48).

Subsequently the screw itself is electrically stimulated. When stimulating the screw, an

EMG response threshold of 6 mA or lower has been associated with bone perforation,

provided that the adjacent nerve is normal (48).

The eect of pedicle screw instrumentation on the functional outcome has been studied

primarily in lumbar surgery (135). The lack of electrical stimulation of the pedicle screws is

associated with higher risk of neurologic sequelae aer spinal surgery (135; 88).

The data related to cervical spine surgery are more limited, but even the most optimistic

evaluations show pedicle perforation with 4% to 10% of the screws placed and a smaller

incidence (approximately 1%) of radiculopathy as a consequence of the procedure (01).

More screws inducing bone wall perforation are seen with surgery at the C4 and C7 levels

(01). Thus, there is some room for improvement, and further studies are necessary to

establish the role of pedicle testing in cervical surgery.

Transcranial magnetic motor-evoked potentials (TcMMEPs). Transcranial magnetic

stimulation produces a motor response by inducing an electrical current on the nerve.

Magnetic stimulation causes less discomfort than its electrical counterpart, so it can be done

while the patient is awake.

TcMMEPs recorded from the spinal cord will elicit the D and I waves similar to but dierent

from the responses to electrical stimulation (88). As with intraoperative somatosensory-

evoked potentials, transient changes in the magnetically-evoked response were not

associated with postoperative neurologic decits. The preliminary experience using cortical-

spinal magnetic stimulation in children during spinal surgery is encouraging. In a study with

a mixed adult-pediatric population, the use of motor-evoked potentials seemed to improve

the sensitivity and sensibility of the intraoperative somatosensory-evoked potentials to detect

postoperative neurologic decits (102). Among the 500 cases in the latter study, no case of

false negative results (normal monitoring recording with postoperative neurologic decit)

was seen when intraoperative somatosensory-evoked potentials were used in combination

with motor-evoked potentials, and the specicity of normal data predicting normal ndings

in a neurologic examination was 100%. The Department of Neurosurgery at Baylor College

of Medicine set an index whereby a MEP signal decrease of 50% or greater was predictive of

a postoperative neurologic decit involving the motor- or corticospinal pathways (32). Krieg

and colleagues found that continuous MEP monitoring provides reliable data during

resection of metastases in motor-eloquent brain regions. However, they established the

warning criteria of an amplitude decline greater than 80% of the baseline rather than the

conventional 50% as quoted by most others (63).

EMG. Electromyographic monitoring in the form of free-running EMG is also commonly

used in a number of surgeries, including spinal surgery for scoliosis and anterior discectomy

with fusion. Cranial nerve distribution muscles are monitored during some intracranial

operations, such as surgery at the base of the skull (acoustic neuroma, meningioma).

Whether the cranial nerves are monitored depends on the surgery. The muscles innervated

by cranial nerves III to XII are the ones monitored most oen during surgery. Additionally,

the optic nerve (cranial nerve II) and the visual system can be monitored by the use of

visual-evoked potentials (VEPs, see below).

During motor mapping, the response can be further substantiated by using EMG, which also

allows for lower stimulus intensity with a visible response (see Motor and cognitive mapping

through direct cortical electrical stimulation below).

Electromyographic monitoring also has been useful during selective dorsal rhizotomy for the

treatment of spasticity. Stimulation of the dorsal rootlets with a 50-Hz frequency is used

when looking for exaggerated responses, such as lower stimulus threshold, sustained activity

aer discharges, or the spread of the response to other muscles (44).

Several texts provide data regarding the innervation of muscles used (24; 82), some citing

their practical application for intraoperative monitoring (87).

Table 1. EMG Monitoring of Muscles According to Spinal Levels

Level Muscle(s)

C2 Sternocleidomastoid (also CN XI)

C3 Trapezius and sternocleidomastoid (also CN XI)

C4 Trapezius (also CN XI) and elevator scapulae

C5 Deltoid, brachioradialis, and biceps

C6 Biceps, triceps, exor carpi radialis, pronator teres, brachioradialis

C7 Triceps, forearm extensors, exor carpi radialis, pronator teres

C8** Triceps, abductor digiti minimi (ADM), abductor pollicis brevis (APB),

interossei

T1 Flexor carpi ulnaris, abductor digiti minimi (ADM), abductor pollicis

brevis (APB), interossei, intercostal muscles

T2-3-4-5-

6

Intercostal and paraspinal muscles

T6-7-8 Upper rectus abdominis, intercostal and paraspinal muscles

T8-9-10 Middle rectus abdominis, intercostal and paraspinal muscles

T10-11-1

2

Lower rectus abdominis, intercostal and paraspinal muscles

L1 uadratus lumborum, iliopsoas, cremaster, internal oblique and paraspinal

muscles

L2 Iliopsoas, quadriceps, adductor longus, adductor magnus, and quadratus

lumborum

Visual-evoked potentials (VEPs). The optic nerve (cranial nerve II) and the visual system

can be monitored intraoperatively by the use of visual-evoked potentials. Nonetheless,

intraoperative visual-evoked potentials are somewhat challenging due to poor reproducibility

of ash visual-evoked potentials in this setting (88). Visual-evoked potentials performed with

high-intensity light ashes can improve the reliability of this test during surgery. Visual-

evoked potentials with direct recording from the optic nerve also may be easier to obtain

during surgery than with scalp leads.

When used during surgery, visual-evoked potentials are done through light-emitting diodes

that are attached to contact lenses (88). Using green or high-intensity lights rather than a

red-colored source will decrease the intraoperative dark adaptation.

Alternative monitoring methods for spine surgery. Direct electrical stimulation of the

spinal cord during surgery has also been performed. The stimulation may be either epidural

or transosseous (115). The transosseous method employs a needle near the spinous process of

a vertebra above the level of surgical intervention when concurrently monitoring below that

level (115). False positives may be seen with this technique due to shunting of electrical

current from the stimulating needles through the metal rod to the ground electrode, thus,

preventing adequate stimulation to the spinal cord. A modication of the transosseous

technique with epidural stimulation has been suggested to improve reliability of the

L3 uadriceps, adductor longus, adductor magnus, iliopsoas, and quadratus

lumborum

L4 uadriceps, tibialis anterioris, adductor longus, adductor magnus and

iliopsoas

L5 Tibialis anterioris, peroneus longus, and adductor magnus

S1 Gastrocnemius, abductor hallucis and peroneus longus

S2 Gastrocnemius and abductor hallucis

S2-5 Anal sphincter

From: (24; 82; 87).

procedure (115). Pereon and colleagues used direct electrical stimulation of the spinal cord

via needle electrodes in the rostral part of the surgical eld and recorded from both sciatic

nerves (09; 105; 73). The latter study found no false-negative cases of intraoperative spinal

cord damage among the 112 consecutive patients being monitored during surgery for spinal

deformity. Neurogenic motor-evoked potentials appear to be faster than intraoperative

somatosensory-evoked potentials in the detection of spinal insults during surgery (105).

From a frontier biomedical engineering standpoint, the neuroengineering laboratory at the

Georgia Institute of Technology devised a superior stimulator that not only is exacting in the

specic tract and white matter funiculi stimulated but also is stretchable and conformational

to the spinal cord, so it is less apt to induce mechanical damage to the so tissue. The

SMEA, or stretchable microelectrode array, derived the same range of evoked cap

conduction velocities and stimulus resolution as rigid tungsten microelectrodes but aorded

the circumferential contact with less mechanical incompatibility to the cord itself (86).

Dermatomal somatosensory-evoked potentials could be considered ideal for monitoring the

avoidance of radicular injury that is subadequately detected by intraoperative

somatosensory-evoked potentials. However, dermatomal responses do not have good

reproducibility. This technique has not been explored suciently in children (44).

Anesthesia can readily compromise dermatomal responses.

During carotid endarterectomy (CAE) some surgeons advocate triple monitoring; namely,

combined somatosensory-evoked potential, motor-evoked potential, and

electroencephalography monitoring, citing evidence of predicable synergy (05). In the

setting of CAE, the diagnostic accuracy of MULTImodality intraoperative neuromonitoring

was higher than an approach using solely single modality intraoperative neuromonitoring

such that neuroprotective therapies to prevent periprocedural strokes could be based on the

changes in SSEP and EEG, according to the University of Pittsburgh study (134).

Additionally, during carotid endarterectomy, some vascular surgeons rely on vagus nerve

neuromonitoring as well in order to avoid potential vocal fold paralysis postoperatively.

Intraoperative brainstem auditory-evoked potential monitoring. Intraoperative

brainstem auditory-evoked potential monitoring has been shown to be useful for preservation

of hearing and vestibular nerve function during the resection of acoustic

neuroma/intracanalicular schwannoma and other posterior fossa surgeries (42). The

experience with this procedure in children is limited. The types of operations in which the

brainstem auditory-evoked potentials are used include not only acoustic neuroma resections,

but also: extirpation or revision of posterior fossa and petroclival skull-base tumors,

arteriovenous malformations, and aneurysms. Also, microvascular decompression, and

decompressive procedures in patients with Chiari malformations (suboccipital craniectomy).

As in diagnostic recordings done in the laboratory, the patients receive auditory stimulation

delivered by a series of clicks at intensities of 60 to 70 dB hearing level (17). Earphones,

transducers, or even direct middle ear inserts deliver the sounds (80). The signals from many

stimuli are averaged due to the low amplitude of each individual auditory-evoked response,

which is oen less than 0.1 µv. The recording can be done on the scalp in the nonoperative

brainstem auditory-evoked potentials or directly from the acoustic nerve with special cotton

wick electrodes (69). The scalp electrodes are placed on both ears or mastoids, and vertex,

and ground. A contralateral (to the side of stimulus) ear-mastoid to vertex montage can help

dierentiate the wave form IV and V peaks that may be fused in the ipsilateral channel. The

signal phase (rarefaction, condensation, or mixed) should be chosen to maximize brainstem

auditory-evoked potential wave forms. The American Clinical Neurophysiology Society

Committee on Guidelines for Intraoperative Monitoring of Sensory-Evoked Potentials (02)

suggests using click frequencies between 5 and 50 per second. Higher frequencies allow for

faster results, which are important in the operating room setting. Nonetheless, signal

frequency equal to or higher than 30 clicks per second may produce degradation of the wave

forms (20). Using click frequencies that are not multiples of 60 (ie, 11.3 clicks per second)

may prevent time-locked summation eects from 60-Hz artifacts generated by multiple

electrical devices used in the operating room (69). Overall, the goal should be the

identication of the most important components of the brainstem auditory-evoked

potentials, namely wave I and wave V, in the shortest time possible (17).

The eects of surgery on the brainstem auditory-evoked potentials are interpreted noting the

generators of the wave forms to help localize where the problem is taking place.

Table 2. Brainstem Auditory-Evoked Potential Wave Generators

Wave I:

Wave II:

Wave III:

Wave IV:

Wave V:

Distal acoustic nerve

Proximal acoustic nerve/cochlear nucleus

Superior olivary complex at the level of the lower pons

Lateral lemniscus

Inferior colliculus or upper pons

Similar to intraoperative somatosensory-evoked potentials, intraoperative brainstem

auditory-evoked potentials can be inuenced by temperature. When the temperature drops

below 35°C, the latency of all the wave forms will increase wave V (the most sensitive wave

to the thermal eect will disappear with temperatures below 28°C) (44).

Age also has a strong inuence in the brainstem auditory-evoked potentials, with latencies

getting shorter and the wave forms becoming better formed with maturation. Wave form I,

wave form III, and wave form V achieve mature (adult) latencies sequentially.

The brainstem auditory-evoked potentials are resistant to the eects of medications

including benzodiazepines, barbiturates, narcotics, and nitrous oxide. Inhalation general

anesthetics (ie, isourane, halothane, and enurane) produce only a mild latency delay and

mild decrease in amplitude. Wave V is the most sensitive to the eect from these agents.

Intraoperative brainstem auditory-evoked potential abnormalities. Absolute criteria for

intraoperative brainstem auditory-evoked potential abnormalities are not available (17).

Reversible disappearance of all brainstem auditory-evoked potential wave forms is

compatible with complete recovery, and persistent loss of the brainstem auditory-evoked

potential pattern is usually associated with lingering neurologic decit (17; 56). Typically,

changes in wave V are most easily monitorable, and prolongation in latency of more than

10% are considered early changes. Changes up to 20% or 1 msec are considered more

signicant. Brainstem auditory-evoked potential changes seen during surgery can be divided

into 3 types (44):

Table 3. Brainstem Auditory-Evoked Potential Waveform Maturation

Wave Age (to reach adult latency range)

I

III

V

Neonatal period (full-term)

12 to 18 months

3 months to 5 years

• Type 1. Gradual and persistent prolongation of the wave forms of 1 ms or more.

This type of abnormality may or may not be followed by a return to the baseline

Other strategies used to monitor posterior fossa surgery. Somatosensory-evoked

potentials and EMG of the muscles within the cranial nerve distribution usually have been

monitored in co-association with brainstem auditory-evoked potentials. Somatosensory-

evoked potential parameters used during posterior fossa, regarding infratentorial lesions,

surgery are similar to the ones used for spinal cord monitoring (44). The EMG parameters

used for posterior fossa surgery monitoring include a vertical display range of 200 to 500 µv,

sweeps of 10 msec per screen, low frequency lter of 30 Hz, and high frequency at 3 KHz to

10 kHz.

The cranial nerve distribution muscles commonly sampled for intraoperative monitoring

EMG are: inferior rectus (cranial nerve, or CN, III), superior oblique (cranial nerve IV),

masseter (cranial nerve V), lateral rectus (cranial nerve VI), orbicularis oris (cranial nerve

VII), stylopharyngeal (cranial nerve IX), cricothyroid or vocalis (cranial nerve X), trapezius

(cranial nerve XI), and tongue (cranial nerve XII) (44; 87).

values. Postoperative type 1 brainstem auditory-evoked potential abnormalities are

not accompanied by clinically signicant hearing decits, but careful audiological

testing may reveal some minor hearing loss.

• Type 2. Sudden loss of wave I through wave V ipsilaterally to the side of the

surgery without return to the baseline. Hearing impairment is oen observed

postoperatively on the same side of the surgery when type 2 changes are seen

during surgery. When this happens, there is a good chance that blood supply of the

ear, especially the cochlea, will be compromised.

• Type 3. When the contralateral brainstem auditory-evoked potential wave forms

become abnormal during posterior fossa surgery, the prognosis is poor. Type 3

changes are oen associated with other signs of brainstem dysfunction. When this

pattern is not accompanied by return to the baseline, it has been correlated with

poor outcome, such as death or postoperative survival with severe neurologic

decit, including hearing impairment.

Table 4. Recordings for Cranial Nerve Monitoring, Including EMG, Visual-Evoked Potentials, and Direct Nerve/Nucleus Compound Action Potential

Intraoperative-EMG monitoring evaluation includes survey for the presence of spontaneous

electrical activity from muscles that may indicate injury or depolarization of the innervating

nerve. Intraoperative monitoring of BAEP is of established benet in the prevention of

hearing loss or cochlear nerve impairment during microvascular decompression of the eighth

cranial nerve for primary hemifacial spasm (119).

Direct recording from the exposed VIIIth cranial nerve or nearby cochlear nucleus through

monitoring of compound action potentials can be also helpful (88). Compound action

potentials (CAPs) monitoring had shown that the cochlear component of cranial nerve VIII

is sensitive to stretching and heat injuries, which can be a problem due to the use of bipolar

Cranial

Nerve

Muscle(s) or Type of Monitoring

I Usually not monitored

II Usually not monitored. VEP used but oen not reliable during surgery.

Use high intensity ashes, green LED-contact lenses

III-IV-VI EMG - extraocular muscles

V EMG - masseter and temporalis

VII EMG - orbicularis oris; orbicularis oculi, mentalis, frontalis

VIII BAERs, direct VIIIth nerve or cochlear nucleus compound action

potential (CAP)

IX EMG – stylopharyngeus

X EMG - pharyngeal and laryngeal muscles

XI EMG - trapezius and sternocleidomastoid (also C2-3)

XII EMG - glossopharyngeus (tongue muscle)

From: (24; 82; 87)

electrocoagulation near the auditory portion of cochlear nucleus VIII.

During surgery for the resection of acoustic neuromas, in addition to monitoring the VIIIth

nerve function with intraoperative BAEP, free running EMG of cranial nerve VII innervated

muscles such as orbicularis oris, orbicularis oculus, mentalis and frontalis should be used.

Because oen the surgeon needs to inict damage on the acoustic nerve to resect the tumor,

monitoring the facial nerve distribution muscles may be the most important task to perform

during this type of procedure.

Laryngeal nerve monitoring. Intraoperative neuromonitoring (IONM) has gained

widespread acceptance among head and neck surgeons as an adjunct, during thyroid

surgery, to direct visual (recurrent laryngeal) nerve identication (“gold standard”).

Laryngeal nerve monitoring is becoming essentially routine practice for both select and

standard thyroidectomy and parathyroidectomy (120). Dralle and colleagues summarize the

“Recommendations of the Surgical Working Group for Endocrinology” regarding

intraoperative neuromonitoring in thyroid and parathyroid surgery (29), and Barczynski and

colleagues review the standards guideline statement from the “International Neural

Monitoring Study Group” in reference to monitoring of the external branch of the superior

laryngeal nerve during thyroid and parathyroid surgery (10). Some authors stress that

neuromonitoring does not indeed reduce the risk of postoperative laryngeal nerve palsy, at

least in complicated cases such as redo thyroid surgery or thyroid cancer surgery (06). The

general consensus, however, is that intraoperative neuromonitoring has a favorable eect in

terms of decreasing the prevalence and severity of upper aerodigestive symptoms typically

involving altered swallowing and change of phonation or voice pattern (118). Dionigi and

colleagues espouse that continuous, as compared to intermittent, monitoring of the recurrent

laryngeal nerve in thyroid surgery is cutting-edge technology and needs further assessment

in an evidence-based fashion (26).

Intraoperative neuromonitoring or laryngeal nerve monitoring is then a risk minimization

tool. It can be used eectively to verify the functional integrity of the recurrent laryngeal

nerve (27). Recurrent laryngeal nerve palsy, a potentially catastrophic complication of

thyroid surgery, can be averted with laryngeal nerve monitoring; the incidence without

monitoring is up to 3% for permanent palsy and 5% to 8% for transient palsy. For persons

who professionally depend on their voice this would be especially taxing. Bilateral recurrent

laryngeal nerve trauma can lend toward the eventual need for tracheostomy, stemming from

vocal cord paralysis. Intraoperative neuromonitoring can also be benecial in identication

of the anatomy and physiology of the nonrecurrent laryngeal nerve. The movement is toward

C-IONM (continuous), over I-IONM (intermittent), to potentially enable the surgeon to

react before irreversible damage to the recurrent laryngeal nerve arises.

Regarding animal studies, Wu and colleagues investigated with a porcine model. Recurrent

laryngeal nerve traction resulted in eventual loss of signal, similar to the change rendered

from clamping or transection or electrothermal injury. But if the traction stress was relieved

before the loss of signal, there was typically recovery of the EMG tracings recorded.

Repeated traction resulted in lost amplitudes, lending support to the notion that undue

retraction of the tissue is deleterious (145).

Electrocorticography (ECoG). Electrocorticography is commonly used during epilepsy

surgery. The goal of the electrocorticography is to help determine the areas that need to be

resected during epilepsy surgery (99; 97; 98). Electrocorticography can give prognostic

information during surgery by indicating the areas of residual discharges aer resection of a

brain tumor or epileptogenic focus. Nonetheless, this is controversial; some believe that when

no interictal discharges are seen aer resection, the patients are more likely to be seizure-free

than those with persisting discharges (99; 97). In one study of electrocorticography in

intractable frontal lobe epilepsy, a higher percentage of Engel’s classication Class I

outcome was associated with pre-excision spikes recorded from two gyri or less (p < 0.05)

and post-excision spikes absent or limited to the resection border with a p < 0.01 (143).

Complete lesion excision correlated with Class I outcome with a p < 0.001 (143). The same

study found that only 2% of the cases had Class I outcome when spikes were seen distant

from the lesion border and no good outcome if more than 24 spikes/minute were seen;

combining completeness of lesion excision with electrocorticography risk factors was highly

correlated with class I/II outcome.

In a series of patients with temporal lobe epilepsy, electrocorticography was found to help by

predicting poor outcome (83). Baseline electrocorticography with less than one spike per 4

minutes was associated with a poorer prognosis. Conversely, pre-resection

electrocorticography showing more than 18 spikes per minute was typically associated with

a good outcome.

Spikes with a major positive component are commonly derived from both depth electrodes

(78%) and from subdural monitoring strips (72%) (83). Berger and colleagues also found

that using electrocorticography to dene areas of epileptogenic cortex in and around brain

tumors increases the likelihood of satisfactory postoperative seizure control (12).

Using electrocorticography to dene the seizure focus during low-grade glioma removal may

be more eective in children (11).

Intraoperative ECoG is now being utilized during MRI-guided Laser-Interstitial Thermal

Therapy (LITT) for intractable epilepsy, which is an exciting frontier (76).

Asano and colleagues found that intraoperative electrocorticography in children with

intractable neocortical epilepsy is reliable only when spike frequency is greater than 10

spikes/minute (07). A spike frequency of less than one spike/minute is largely unreliable for

localization of seizure foci.

Salanova and colleagues found that in patients undergoing surgery for the treatment of

medically refractory occipital lobe epilepsy, electrocorticography helped improve the

outcome (111). Residual spikes on the post-resection electrocorticography were associated

with worse outcome.

The adequacy of resection of temporal-lobe mass lesions such as gangliogliomas, cavernous

angiomas, and dysembryoplastic neuroepithelial tumors can be aided by intraoperative

electrocorticography. Resection of spike foci aer lesionectomy improved the 3-year seizure-

free outcome (128).

One study used intraoperative hyperventilation or overbreathing to increase the amount of

epileptiform discharges in children with relative success aer removal of those foci, which

were not located in eloquent cortex (132). Occasionally, infusions of the proconvulsant

barbiturate methohexital are also used.

In electrocorticography of cortical dysplasia, certain patterns appear to predict the outcome

more precisely aer resections for intractable seizures. In 1995, Palmini and colleagues found

that when areas in which the ECoG showed ictal-like or continuous or quasi-continuous

patterns were le unresected, the outcome for seizure-freedom was worse.

Table 5. ECoG Patterns Associated with Cortical Dysplasia

1. Repetitive electrographic seizures

• Recruiting/derecruiting frequency around 12 to 16 Hz.

2. Repetitive bursting patterns

• High frequency (10 to 20 Hz) lasting for 5 to 10 seconds

Although intraoperative electrocorticography is not oen used by many epilepsy centers and

has been found non-useful in cases of surgical management of symptomatic mesial temporal

sclerosis (116). Hippocampal electrocorticography (HECoG) may permit a temporal

lobectomy to be performed in a tailored fashion. Guided by hippocampal

electrocorticography in a temporal lobectomy, a surgeon can minimize the amount of

hippocampus removed to minimize postoperative memory decline while maximizing seizure-

free outcome (84).

Several combinations of anesthetics have been administered to facilitate intraoperative

electrocorticography with and without functional brain mapping. The alpha 2-adrenergic

receptor agonist dexmedetomidine has been useful (124), and dexmedetomidine tends to

produce a natural sleep pattern on the EEG and tends to reduce the need for propofol as well

as inhalational and opiated anesthetics (124).

Functional brain mapping. Intraoperative somatosensory-evoked potentials are a valuable

tool for functional brain mapping during various types of resective surgery. The SSEPs are

oen the rst step in cortical localization of the eloquent cortex. SSEPs are recorded during

intracranial surgery with a 6- or 8-contact strip of subdural electrodes. Median nerve SSEPs

show a phase reversal over the central sulcus near the area of cortical representation of the

hand.

The negativity located over the sensory cortex is oen alluded to as N1 (75) and

corresponds to the N20 on scalp recordings. N1 tends to be relatively small but has a more

gradual spatial fall o when compared to P2. Thus, N1 is seen several centimeters around its

point of maximal negativity. A positivity located over the hand somatomotor cortex is noted

1 to 2 milliseconds later and corresponds to the P22 of the scalp recordings. The component

has been named P2, with a major positivity peaking around 2 to 3 msec aer N1 and which

was maximal range over the post-central gyrus but may extend to the precentral gyrus. The

absolute latency of P2 was calculated to be 22.3+/-1.6 msec (25). P2 has a fast fall-o,

disappearing 1 to 2 cm posterior to the central sulcus (75). The post-rolandic potential (N1)

3. Continuous or quasi-continuous rhythmic spiking

• Prolonged trains of rhythmic 2-8 Hz spikes

(103)

can be dierentiated because it is negative and peaks earlier and has amplitude that is twice

that of the pre-rolandic primary cortical potential (25). Waveform morphology is best when

subdural strip electrodes are perpendicular to the central ssure.

The position of the phase reversal of the cortical potentials of the median SSEP (recorded in

a reference montage) tends to be across the rolandic ssure as veried by motor response to

direct electrical stimulation (25). Nonetheless, the phase reversal is sometimes anterior to the

rolandic ssure (25). In these recordings, it is important to make sure that the recording strip

is oriented perpendicular to the sulcus being studied.

Motor and cognitive mapping through direct cortical electrical stimulation. During

neurosurgical procedures, brain mapping can be also done using direct electrical cortical

stimulation to provide an improvement in otherwise intractable epilepsy, facilitating more

aggressive and complete removal of the epileptogenic tissue, a brain tumor, or both.

When performing electrical stimulation, a couple of electrical principles are helpful to

remember: (1) the stimulus intensity decreases (attenuates) with the square of the distance

from the stimulating electrode; and (2) the strength of a stimulus is thought to be dependent

on the charge density. Most cortical stimulation is done with systems that vary the current

intensity.

The charge (in Coulombs) during cortical stimulation also can be calculated by measuring

the “area under the curve” of one of the phases of the pulse. So, for the usual rectangular

pulse, the charge equals the current intensity multiplied by the pulse duration.

Charge C= I x D (I = current intensity; D = pulse duration).

Ohms law relates the current intensity (I) to the resistance (R) and voltage (V).

Ohms law: I = V/R

The current intensity (I) is measured in Amperes (Amp) OR milliamperes (mAmp). The

resistance is measured in Ohms (Ω), and the voltage is measured in Volts (V) or millivolts

(mV).

The energy (E) required to move an electrical current through the tissue is expressed (in

Joules) and is related to the square current intensity, pulse duration, and resistance.

E = I2 x D x R.

The charge density (CD) is calculated dividing the charge (C) by the area of the stimulating

electrode (ASE) assuming the transmission media is uniform.

Charge density CD = C/ASE

The surface of a bipolar wand electrode (4 cm2 surface) is higher when compared with a

subdural grid lead (12 to 13 cm2 surface). The charge density of a hemispheric ball electrode

is 159 to 796 microcoulombs/cm2 per phase for peak currents of 13.6 to 15 mA (66).

Subdural grid leads will deliver charge densities of 54 to 57 microcoulombs/cm2 per phase

for peak currents of 13.6 to 15 mA.

While in the operating room, the surgeon oen uses a bipolar stimulator with a range of

current intensity from 1 to 15 mA and with a pulse duration of 200 to 1000 microseconds,

using a biphasic pulse set at a 50 to 60 Hz frequency. Young children and toddlers may

require longer pulse durations of 500 to 1000 microseconds and somewhat higher currents of

10 to 15 mA. The stimulus delivery is generally three seconds long. One should be careful

with longer durations of four to ve seconds and amperage higher than 15 mA as they may

cause activation of neighboring tissues. Some stimulators such as the Ojemann Cortical

Stimulator are set to deliver an amperage that is half of the total current because the dial

takes into consideration the current from the baseline to the rst peak and not from peak to

trough.

This type of mapping minimizes the risk of neurologic morbidity, whether it be speech and

language decit or motor or sensory loss. Adults and older children (usually 12 years and

older) can undergo “awake craniotomies” in which they are under anesthesia for opening and

closing of the skull and dura but awake during the resection of the tumor or epileptic focus.

Our approach entails the use of two modalities of motor-mapping. During awake

craniotomy, if our patients are completely cooperative, we perform motor and cognitive

testing (such as picture naming) during most of the surgery aer cortical exposure. In some

Cortical Stimulation Parameters:

Current Intensity Pulse Duration Pulse Frequency Pulse Type

1-15 mA 200-1000 msec 50-60 Hz Biphasic

cases, we also perform ongoing motor stimulation as witnessed by EMG recordings, which

allows for the use of a weaker cortical stimulus and fewer patient movements, thus,

enhancing the safety of the procedure. Furthermore, we can also stimulate the white matter

before resections looking for motor and cognitive dysfunction. This is especially important

during temporal and paracentral resections in which the bers connecting eloquent cortex

are vulnerable to operative insult.

In cases of tumor surgery, brain mapping allows more aggressive resections. The decrease of

the tumor burden or metastatic disease aords a survival advantage and lesser likelihood of

subsequent risky operative intervention. Because, in the presence of tumor and cerebral

edema, the CNS anatomy can be distorted, it becomes critical to rely on intraoperative-

evoked potentials for more accurate identication of sensorimotor cortex. One particular

surgical procedure seems to warrant MEP over SSEP. With aneurysm repair is the risk of

perforating arteries at the corticospinal tract region, within the corona radiate of the internal

capsule. This vulnerable area is best monitored with motor- rather than sensory-evoked

potentials, given the neuroanatomy and neurocerebrovasculature (41).

The multipulse technique used to obtain the Tc-MEP has many advantages over the

Peneld technique discussed above. In particular, it can map motor pathways during general

anesthesia. It also has a lower likelihood of generating seizures. However, the longer

stimulation times used with the Peneld technique are useful in studying cognitive functions.

Prevention

The goal of intraoperative somatosensory-evoked potential monitoring is prevention of

neurologic decits during and aer surgery.

Management

Outcomes

The limitations of intraoperative evoked potential monitoring must be considered when used

during neurosurgical orthopedic, vascular, or other resective or ablative procedures. One

factor that must be taken into account when monitoring intraoperative somatosensory-

evoked potentials is that only the posterior column somatosensory pathways are assessed.

Intraoperative somatosensory-evoked potentials may be misleading at times; there may be

false positives (18), and postoperative neurologic decits may be seen despite unchanged

ndings during monitoring (67). Overall, intraoperative somatosensory-evoked potentials

tend to be more sensitive for the spinal insults that involve multiple regions. More focal

intraoperative spinal lesions, either compressive or vascular in nature, are less likely to be

detected by intraoperative somatosensory-evoked potentials (44). To optimize care, certain

neurosurgery groups have designed, developed, and implemented a checklist for responding

to intraoperative neuromonitoring alerts in spine surgery (153).

The most common pediatric procedure in which intraoperative somatosensory-evoked

potential monitoring is used is the surgical correction of scoliosis (44). In one series,

intraoperative somatosensory-evoked potential monitoring was used to monitor orthopedic

procedures in 326 children (46). Among the cases undergoing the latter procedure, 63.7%

had idiopathic scoliosis, and 31.2% had neuromuscular scoliosis (46; 44). The presence of

cerebral palsy was common among these patients; thus, a complete preoperative baseline

neurologic examination is essential for the comparison with the patient’s status aer the

surgery. As follows, the same is also true for most of the other conditions for which this type

of surgery is done, such as myelodysplasia, neuromuscular disorders, and brain and spinal

cord malformations.

In the Boston Children's Hospital study, nine of 326 cases had acute changes in the

intraoperative somatosensory-evoked potentials during surgery; in eight cases the surgical

manipulation (traction-distraction) was reversed suciently to prevent new postoperative

neurologic decit (46; 44). In one case the patient had postoperative paraplegia, most likely

due to the need for resection of a spinal cord tumor. The same study noticed three cases

(0.9%) of new neurologic decits that were not detected by the intraoperative

somatosensory-evoked potential monitoring. Two of these cases were focal L4 to L5

radiculopathies (one resolved spontaneously and the other required surgical correction of a

loose lamina). One patient had a new onset of urinary retention that resolved in one week.

Harper also found that radiculopathies, which were dicult to detect by intraoperative

somatosensory-evoked potential monitoring, occurred in four of 184 cases aer surgery for

scoliosis repair (43). Earlier work by Wilber and colleagues showed that transient

postoperative neurologic decits were more dicult to predict with intraoperative

somatosensory-evoked potential monitoring (144). Dermatomal somatosensory-evoked

potentials would theoretically be ideal to monitor radicular injury. However, dermatomal

responses do not have good reproducibility, and this technique has not been utilized in

children (44).

The likelihood of neurologic deterioration aer surgical repair of scoliosis is greater in

patients with neuromuscular spinal curvature or severe scoliosis (77). Postoperative

deterioration is more likely to be seen in patients with pre-existing neurologic decits, such

as patients with meningomyelocele (44). The risk of neurologic injury is also higher in

patients who need skeletal traction, Harrington rod instrumentation, or sublaminar wire

placement, and aer a second spinal surgery (144; 44). These high-risk patients are more

likely to benet from intraoperative somatosensory-evoked potential monitoring during these

neuro-orthopedic procedures (44). Intraoperative neuromonitoring can be challenging –

indeterminate or unreliable – in young patients with immature neural pathways or

underlying preexistent malacia (37). The group at Seoul National University Hospital

arms that EMG, MEP, SEP, and BCR (bulbocavernosus) reex are essential modalities in

intraoperative neurophysiological monitoring for untethering of tethered cord in spinal

dysraphism (60). They contend that early intervention is better than waiting for onset of

neurologic decits. Findings include that pathophysiology of the sacral nervous system is

both rst in onset and principally foremost. McKinney and Islam stress that when evaluating

a patient with pediatric scoliosis – as a potential surgical candidate with intraoperative

monitoring – be attentive to possible concurrent polyneuropathy; the presence of otherwise

unrecognized polyneuropathy could prompt a change in recording parameters (85).

Table 6. Risk Factors for Postoperative Neurologic Decit Aer SpinalSurgery

Related to the diagnosis Related to the procedure

Neuromuscular scoliosis Skeletal traction

Congenital scoliosis

Tethered cord

Harrington rod placement

Intraoperative somatosensory-evoked potentials, as a procedure, is rather benign as long as

the technologist performing it complies with the usual safety rules of bioelectrical and EEG

equipment. However, burns do occur occasionally (126). The most critical causes of these

injuries include improperly grounded electrocautery, defective equipment, and the

placement of recording electrodes too near sources of high voltage.

Allergic and contactant-related skin changes, such as contact dermatitis, are uncommon

problems with EEG lead placement. Skin abrasion (to reduce impedance) and the various

chemical components of the leads (silver, gold, and copper) as well as the paste or collodion

used may contribute to these dermatological complications.

One report called for an electrocardiogram artifact that can be produced by intraoperative

somatosensory-evoked potential monitoring (21). The report describes a 3-year-old girl with

Goldenhar syndrome in whom the somatosensory-evoked potentials produced an artifact in

the electrocardiogram resembling supraventricular tachycardia leading to inappropriate

treatment. Another rather odd occurrence was a case report revealing the onset of an acute

postprocedural compartment syndrome as a complication of the use of intraoperative

neuromonitoring needle electrodes in the arm, prompting multiple emergent surgical

fasciotomies (31).

Communication has been an issue. There needs to be familiarity and trust amongst the

neurophysiologist, the surgeon, and the anesthesiologist. The “interventional cascade”

should follow: test, then interpretation, then communication, then intervention, then

outcome (121).

Special considerations

Pre-existing neurologic decit Sublaminar wire placement

Spinal cord tumors Previous spinal surgery

Anesthesia

Because the most disabling postoperative decits are motor decits, one should be aware of

the drugs that can alter TcMEPs. GABAergic drugs, such as barbiturates, benzodiazepines,

and propofol, are the intravenous anesthetics that are more likely to decrease the amplitude

and increase latency of the TcMEPs (55). As a general rule, barbiturate anesthesia precludes

TcMEP monitoring and should not be used unless the possible benets of neuroprotection

outweigh the risks of a lack of motor monitoring.

Inhalational anesthesia with either halogenated or nitrous oxide may also decrease the

amplitude in a concentration dependent manner TcMEPs (55). In general, even

concentrations 0.5 MAC may aect TcMEPs by halogenated agents with preferential

suppression of the motor tracts at the level proximal to the anterior horn cells.

A few combinations have been used including a “nitrous-narcotic” combo remifentanil

infusion of 0.2 to 0.5 microgram/kg/minute with 60% nitrous oxide although nitrous oxide

also has signicant deleterious eects on the Tc-MEPs. Other combinations are variations of

total intravenous anesthesia (TIVA) using some propofol infusion associated with ketamine

or etomidate (55). Total intravenous anesthesia using ketamine or etomidate is especially

interesting due to low potential of these two drugs to depress either SSEPs or TcMEPs. In

fact, etomidate may even increase the SSEP and TcMEP amplitudes. Nonetheless,

etomidate may be a proconvulsant and may cause adrenal suppression (55).

High doses of boluses or inhalational anesthetics may produce a decrease in the amplitude of

the cortical potentials during intraoperative somatosensory-evoked potential monitoring

(40). Increasing concentrations of halothane can quickly produce a decrease in the

amplitude of the cortical potentials, which is directly proportional to the end tidal

concentration of that gas (148). Patients less than 10 years of age are particularly susceptible

to the eects of high concentration boluses of general anesthetics, which may produce

attenuation of the cortical potentials (46). Cortical intraoperative somatosensory-evoked

potentials in children are also more likely to be attenuated by a combination of anesthetics,

such as isourane and nitrous oxide (46). The most optimal scalp recordings are obtained by

avoiding the combination of anesthetics and by, as stated, keeping the concentration of

nitrous oxide less than 50% and the concentration of isourane less than 0.6% (46). The

greater instability of the cortical responses in children is thought to be due to the lack of

symmetry and synchrony in the myelination process (36). Due to the unreliability of the

cortical response in children, the use of recording sites over the cervical spine has been

recommended to monitor the cervical potentials (46). Cervical potentials are more resistant

to the eects of general anesthetics and can be used to monitor the spinal cord integrity

above the surgical level when the cortical potentials are absent (57; 39).

Overall, the most commonly used and validated anesthetic protocol with Tc-MEP and

SSEP recording is now total intravenous anesthesia with propofol. It was determined that

methadone has a statistically signicant eect on SSEPs (latency and amplitude) but not

TcMEPs; regardless, the dierence did not translate into clinical signicance (47). Biscevic

and colleagues state, emphatically, that the monitoring of motor pathways with transcranial

electric motor-evoked potentials requires the avoidance of halogenated anesthetics

(halothane, sevourane, isourane, etc.) and neuromuscular blockade (vecuronium,

rocuronium, etc). Further, they state that ketamine-based anesthesia allows for appropriate

MEP recordings but that total intravenous anesthesia with propofol is preferred (13).

As stated previously, the brainstem auditory-evoked potentials are fairly resistant to the

eects of medications including benzodiazepines, barbiturates, narcotics, and nitrous oxide.

Inhalational general anesthetics (eg, isourane, halothane, enurane) produce only a mild

latency delay and decrease in amplitude. Wave V is the most sensitive to the eects of these

drugs (64).

Further various applications. Intraoperative monitoring is broadening in its indications and

strengthening as an important element toward optimal patient care. It is accepted for,

inclusively now, the monitoring of pelvic autonomic nerves during laparoscopic low anterior

resection of rectal cancer, for example (152). Multimodality intraoperative

neurophysiological monitoring is utilized in anterior hip arthroscopic repair surgeries (100).

It is employed for brachial plexus neurolysis during delayed xation of a clavicular fracture

(08). Its use – and clinical benet with optimal resection and safe outcome – in thyroid

surgery is well accepted (146; 22; 90). The Japanese group designed an electromyography

endotracheal tube for successful intraoperative neurophysiological monitoring dentication

and preservation of an extralaryngeal bifurcation of the recurrent laryngeal nerve (147).

Schneider and colleagues state that, in experienced hands, continuous intraoperative neural

monitoring in thyroid surgery can diminish permanent vocal fold palsy, a devastating

complication in terms of compromise in quality of life, rates to 0% (113). As a powerful risk

minimization tool, it can oer some protection in a medicolegal litigious environment (151).

Pelvic intraoperative neural monitoring has been associated with a signicantly lower rate of

fecal incontinence aer total mesorectal excision for rectal cancer (58). The utility of

intraoperative neural monitoring has been evaluated, by cardiothoracic (aortic arch) surgery,

and was found to have both a high sensitivity and specicity, but also a high negative

predictive value is reassuring for low risk of stroke in the absence of alerts (35).

Intraoperative neural monitoring of the facial nerve has been applied, by EMG, in cases of

tympanomastoidectomy for chronic ear disease (96); it was found to be both feasible and

eective for facial nerve stimulation and identication. The facial nerve is also monitored

during surgery at the CP or cerebellopontine angle, along with the cochlear nerve. As stated,

intraoperative neurophysiological monitoring is essentially the mainstay (‘standard of care’)

in various spine surgeries, including adult and pediatric procedures (33; 50; 53; 130; 149).

Controversy remains within certain neurosurgical circles whether intraoperative

neurophysiological monitoring (SSEP/TcMEP) is key in elective microsurgical clipping of

unruptured intracranial aneurysms (38).

The Journal of Clinical Monitoring and Computing has released provocative dialogue

regarding the “new” American Society of Neuromonitoring supervision guideline and is

under scrutiny or attack, principally by SA Skinner from Northwestern and colleagues. He

contends that leniency exists in the new guidelines in favor of the telemedicine industry. He

favors, as proper science and proper care, a personal-in-room approach toward quality

intraoperative neurophysiological monitoring that enhances communication (122). The

response – by Gertsch and associates from UCSD – applauded Dr. Skinner’s pursuit of

important concepts such as teambuilding, collaboration, and eective communication (34).

The resources toward personal-in-room intraoperative neurophysiological monitoring are

sparse.

Media

01 Abumi K, Shono Y, Ito M, Taneichi H, Kotani Y, Kaneda K. Complications of pedicle screw xation inreconstructive surgery of the cervical spine. Spine 2000;25(8):962-9. PMID 10767809

02 ACNS. Guideline eleven: guidelines for intraoperative monitoring of sensory evoked potentials.ANCS 2004.

03 Adams DC, Emerson RG, Heyer EJ, et al. Monitoring of intraoperative motor-evoked potentialsunder conditions of controlled neuromuscular blockade. Anesth Analg 1993;77:913-8. PMID8214726

04 Agarwal R, Roitman KJ, Stokes M. Improvement of intraoperative somatosensory-evokedpotentials by ketamine. Paediatr Anaesth 1998;8:263-6. PMID 9608975

05 Alcantara SD, Wuame JC, Lantis JC 2nd, et al. Outcomes of combined somatosensory evoked

References

potential, motor evoked potential, and electroencephalography monitoring during carotidendarterectomy. Ann Vasc Surg 2014;28(3):665-72. PMID 24495327

06 Alesina PF, Rolfs T, Hommeltenberg S, et al. Intraoperative neuromonitoring does not reduce theincidence of recurrent laryngeal nerve palsy in thyroid reoperations: results of a retrospectivecomparative analysis. World J Surg 2012;36(6):1348-53. PMID 22411090

07 Asano E, Benedek K, Shah A, et al. Is intraoperative electrocorticography reliable in children withintractable neocortical epilepsy. Epilepsia 2004;45(9):1091-9. PMID 15329074

08 Ashman BD, Tewar A, Castle J, Hasan SS, Bhatia S. Intraoperative neuromonitoring for brachialplexus neurolysis during delayed xation of a clavicular fracture presenting as thoracic outletsyndrome: a case report. JBJS Case Connect 2018;8(4):e85. PMID 30601768

09 Balzer JR, Rose RD, Welch WC, Sclabassi RJ. Simultaneous somatosensory-evoked potential andelectromyographic recordings during lumbosacral decompression and instrumentation.Neurosurgery 1998;42(6):1318-25. PMID 9632191

10 Barczynski M, Randolph GW, Cernea CR, et al. External branch of the superior laryngeal nervemonitoring during thyroid and parathyroid surgery: International Neural Monitoring Study Groupstandards guideline statement. Laryngoscope 2013;123 Suppl 4:S1-14. PMID 23832799

11 Berger MS, Ghatan S, Haglund MM, Dobbins J, Ojemann GA. Low-grade gliomas associated withintractable epilepsy: seizure outcome utilizing electrocorticography during tumor resection. JNeurosurg 1993;79(1):62-9. PMID 8315470

12 Berger MS, Ojemann GA, Leich E. Neurophysiological monitoring during astrocytoma surgery.Neurosurg Clin North Am 1990;1:65-80. PMID 2135974

13 Biscevic M, Sehic A, Krupic F. Intraoperative neuromonitoring in spine deformity surgery:modalities, advantages, limitations, medicolegal issues – surgeons’ views. EFFORT Open Rev2020;5(1):9-16. PMID 32071769

14 Calancie B, Harris W, Brindle GF, Green BA, Landy HJ. Threshold-level repetitive transcranialelectrical stimulation for intraoperative monitoring of central motor conduction. J Neurosurg2001;95:161-8. PMID 11599831

15 Calancie B, Harris W, Broton JG, Alexeeva N, Green BA. "Threshold-level" multipulse transcranialelectrical stimulation of motor cortex for intraoperative monitoring of spinal motor tracts:description of method and comparison to somatosensory-evoked potential monitoring. JNeurosurg 1998;88:457-70. PMID 9488299

16 Carrabba G, Bertani G, Cogiamanian F, et al. Role of intra-operative neurophysiologicalmonitoring in the resection of thalamic astrocytomas. World Neurosurg 2016;94:50-6. PMID27338215

17 Celesia GG, Allison T, Bodis-Wollner I, et al. American electroencephalographic society commieeon guidelines for intraoperative monitoring of sensory evoked potentials. Guideline eleven:guidelines for intraoperative monitoring of sensory evoked potentials. J Clin Neurophysiol1994;11:77-87.

18 Chatrian GE, Berger MS, Wirch AL. Discrepancy between intraoperative SSEP's and postoperativefunction. Case report. J Neurosurg 1988;69:450-4. PMID 3404244

19 Chaves-Vischer V, Brustowicz R, Helmers SL. The eect of intravenous lidocaine on intraoperativesomatosensory-evoked potentials during scoliosis surgery. Anesth Analg 1996;83:1122-5. PMID8895300

20 Chiappa KH. Brainstem auditory evoked potentials: methodology. In: Chiappa KH, editor. Evokedpotentials in clinical medicine. 3rd ed. Philadelphia: Lippinco-Raven Publishers, 1997:157-97.

21 Choudhry DK, Stayer SA, Rehman MA, Schwartz RE. Electrocardiographic artefact with SSEPmonitoring unit during scoliosis surgery. Paediatr Anaesth 1998;8:341-3. PMID 9672934

22 Cirocchi R, Arewzzo A, D’Andrea V, et al. Intraoperative neuromonitoring versus visual nerveidentication for prevention of recurrent laryngeal nerve injury in adults undergoing thyroidsurgery. Cochrane Database Syst Rev 2019;1:CD012483. PMID 30659577

23 Cui H, Luk KD, Hu Y. Eects of physiological parameters on intraoperative somatosensory-evokedpotential monitoring: results of a multifactor analysis. Med Sci Monit 2009;15(5):CR226-30. PMID19396037

24 DeJong RN, Magee KR. Motor strength and power. In: DeJong RN, Magee KR. The NeurologicalExamination. 4th ed. Hargerstown: Harper and Row Publishing Inc., 1979:334-74.

25 Dinner DS, Lüders H, Lesser R, Morris HH. Cortical generators of somatosensory-evoked potentialsto median nerve stimulation. Neurology 1987;37:1141-5. PMID 3110649

26 Dionigi G, Donatini G, Boni L, et al. Continuous monitoring of the recurrent laryngeal nerve inthyroid surgery: a critical appraisal. Int J Surg 2013;11 Suppl 1:S44-6. PMID 24380551

27 Donatini G, Carnaille B, Dionigi G. Increased detection of non-recurrent inferior laryngeal nerve(NRLN) during thyroid surgery using systematic intraoperative neuromonitoring (IONM). World JSurg 2013;37(1):91-3. PMID 22955954

28 Drake J, Zeller R, Kulkarni AV, Strantzas S, Holmes L. Intraoperative neurophysiological monitoringduring complex spinal deformity cases in pediatric patients: methodology, utility, prognostication,and outcome. Childs Nerv Syst 2010;26(4):523-44. PMID 20213189

29 Dralle H, Lorenz K, Schabram P, et al. Intraoperative neuromonitoring in thyroid surgery.Recommendations of the Surgical Working Group for Endocrinology. [Article in German]. Chirug2013;84(12):1049-56.

30 Ecker ML, Dormans JP, Schwartz DM, Drummond DS, Bulman WA. Ecacy of spinal cordmonitoring in scoliosis surgery in patients with cerebral palsy. J Spinal Disord 1996;9:159-64. PMID8793785

31 Eli IM, Gamboa NT. Acute compartment syndrome as a complication of the use of intraoperativeneuromonitoring needle electrodes. World Neurosurg 2018;112:247-9. PMID 29408593

32 Fulkerson DH, Satyan KB, Wilder LM, et al. Intraoperative monitoring of motor evoked potentials invery young children. J Neurosurg Pediatr. 2011;7(4):331-7. PMID 21456902

33 George J, Das S, Egger AC, Chambers RC, Kuivila TE, Goodwin RC. Inuence of intraoperativeneuromonitoring on the outcomes of surgeries for pediatric scoliosis in the United States. SpineDeform 2019;7(1):27-32. PMID 30587317

34 Gertsch JH, Moreira JJ, Lee GR, et al. Response to: is the new ASNM intraoperativeneuromonitoring supervision “guideline” a trustworthy guideline? A commentary. J Clin MonitComput 2019;33(2):193-4. PMID 30806935

35 Ghincea CV, Anderson DA, Ikeno Y, et al. Utility of neuromonitoring in hypothermic circulatoryarrest cases for early detection of stroke: listening through the noise. J Thorac Cardiovasc Sur2020;S0022-5223(20):30455-4. PMID 32204911

36 Gilmore RL, Bass NH, Wright EA, et al. Developmental assessment of spinal cord and corticalevoked potentials aer tibial nerve stimulation: eects of age and stature on normative dataduring childhood. Electroencephalogr Clin Neurophysiol 1985;62:241-51. PMID 2408871

37 Glover CD, Carling NP. Neuromonitoring for scoliosis surgery. Anesthesiol Clin 2014;32(1):101-14.PMID 24491652

38 Greve T, Stoecklein VM, Dorn F, et al. Introduction of intraoperative neuromonitoring does notnecessarily improve overall long-term outcome in elective aneurysm clipping. J Neurosurg2019:1-9. PMID 30925469

39 Grundy BL. Intraoperative monitoring by evoked potential techniques. In: Amino ML, editor.Electrodiagnosis in clinical neurology. 3rd ed. New York: 1992:649-82.

40 Gugino V, Chabot RJ. Somatosensory-evoked potentials. Int Anesthesiol Clin 1990;28:154-64. PMID2197230

41 Guo L, Gelb AW. The use of motor evoked potential monitoring during cerebral aneurysm surgeryto predict pure motor decits due to subcortical ischemia. Clin Neurophysiol 2011;122(4)648-55.PMID 20869304

42 Harper CM, Harner SG, Slavit DH, et al. Eect of BAEP monitoring on hearing preservation duringacoustic neuroma resection. Neurology 1992;42:1551-3. PMID 1641152

43 Harper CM Jr, Daube JR, Litchy WJ, Klassen RA. Lumbar radiculopathy aer spinal fusion forscoliosis. Muscle Nerve 1988;11:386-91. PMID 3398884

44 Helmers SL. Intraoperative neurophysiological monitoring in pediatrics. In: Chiappa KH, editor.Evoked potentials in clinical medicine. 3rd ed. Philadelphia: Lippinco-Raven Publishers,1997:661-74.

45 Helmers SL, Carmant L, Flanigin D. Anterior neck recording of intraoperative somatosensory-evoked potentials in children. Spine 1995;20:782-6. PMID 7701390

46 Helmers SL, Hall JE. Intraoperative somatosensory-evoked potential monitoring in pediatrics. JPediatr Orthop 1994;14:592-8. PMID 7962499

47 Higgs M, Hackworth RJ, John K, et al. The intraoperative eect of methadone on aomatosensoryevoked potentials. J Neurosurg Anesthesiol 2017;29(2):168-74. PMID 26669838

48 Holland NR. Lumbosacral surgery. In: Husain AM, editor. A Practical Approach toNeurophysiological Intraoperative Monitoring. New York: Demos, 2008:139-54.

49 Huang SL, Qi HG, Liu JJ, Li JL, Huang YJ, Xiang L. Alarm value of somatosensory-evoked potentialin idiopathic scoliosis surgery. World Neurosurg 2016;92:397-401. PMID 27241092

50 Huang ZF, Chen L, Yang JF, Deng YL, Sui WY, Yang JL. Multimodality intraoperativeneuromonitoring in severe thoracic deformity posterior vertebral column resection correction.World Neurosurg 2019;127:e416-26. PMID 30981802

51 Hyun SJ, Rhim SC, Kang JK, Hong SH, Park BR. Combined motor- and somatosensory-evokedpotential monitoring for spine and spinal cord surgery: correlation of clinical andneurophysiological data in 85 consecutive procedures. Spinal Cord 2009;47(8):616-22. PMID19223859

52 Ibrahim T, Mrowczynski O, Zalatimo O, et al. The impact of neurophysiological intraoperativemonitoring during spinal cord and spine surgery: a critical analysis of 121 cases. Cureus2017;9(11):e1861. PMID 29375947

53 Ishida W, Casaos J, Chandra A, et al. Diagnostic and therapeutic values of intraoperativeelectrophysiological neuromonitoring during resection of intradural extramedullary spinal tumors:

a single-center retrospective cohort and meta-analysis. J Neurosurg Spine 2019:1-11. PMID30835707

54 Jahangiri FR, Blaylock J, Qadir N, Ramsey JA. Multimodality intraoperative neurophysiologicalmonitoring during shoulder surgeries. Neurodiagn J 2020;60(2):96-112. PMID 32298207

55 James ML. Anesthetic considerations. In: Husain AM, editor. A Practical Approach toNeurophysiological Intraoperative Monitoring. New York: Demos, 2008:55-66.

56 James ML, Husain AM. Brainstem auditory evoked potential monitoring: when is change in wave Vsignicant . Neurology 2005;65(10):1551-5. PMID 16301480

57 Johnson RM, McPherson RW, Szymanski J. The eects of stimulus intensity on somatosensory-evoked potentials during intraoperative monitoring. Anesthesiology 1983;59:A365.

58 Kau DW, Roth YDS, Bezieche RS, Kneist W. Fecal incontinence aer total mesorectal excision forrectal cancer- impact of potential risk factors and pelvic intraoperative neuromonitoring. World JSurg Oncol 2020;18(1):12. PMID 31941505

59 Khealani B, Husain AM. Neurophysiologic intraoperative monitoring during surgery for tetheredcord syndrome. J Clin Neurophysiol 2009;26(2):76-81. PMID 19279498

60 Kim K. Intraoperative neurophysiology monitoring for spinal dysraphism. J Korean Neurosurg Soc2021;64(2):143-50. PMID 32905697

61 Kothbauer K, Deletis V, Epstein FJ. Intraoperative spinal cord monitoring for intramedullarysurgery: an essential adjunct. Pediatr Neurosurg 1997;26:247-54. PMID 9440494

62 Krammer MJ, Wolf S, Schul DB, Gerstner W, Lumenta CB. Signicance of intraoperative motorfunction monitoring using transcranial electrical motor evoked potentials (MEP) in patients withspinal and cranial lesions near the motor pathways. Br J Neurosurg 2009;23:48-55. PMID 19234909

63 Krieg SM, Schaner M, Shiban E, et al. Reliability of intraoperative neurophysiological monitoringusing motor evoked potentials during resection of metastasis in motor-eloquent brain regions:

clinical article. J Neurosurg 2013;118(6):1269-78. PMID 23521547

64 Kumar A, Bhaacharya A, Makhija N. Evoked potential monitoring in anaesthesia and analgesia.Anaesthesia 2000;55(3):225-41. PMID 10671840

65 Langeloo DD, Journee HL, deKleuver M, Grotenhuis JA. Criteria for transcranial electrical motorevoked potential monitoring during spinal deformity surgery: a review and discussion of theliterature. Neurophysiology Clin 2007;37:431-9. PMID 18083499

66 Lesser RP, Gordon B. Methodologic considerations in cortical stimulation in adults. In: Luders HO,Noachtar S, editors. Epileptic Seizures. New York: Churchill Livingston, 2000:153-65.

67 Lesser RP, Raudzens P, Luders H, et al. Postoperative neurological decits may occur despiteunchanged intraoperative somatosensory-evoked potentials. Ann Neurol 1986;19:22-5. PMID3947036

68 Levy WJ Jr. Clinical experience with motor and cerebellar evoked potential monitoring.Neurosurgery 1987;20:169-82. PMID 3808259

69 Linden DR, Zappulla R, Shields CB. Intraoperative evoked potential monitoring. In: Chiappa KH,editor. Evoked potentials in clinical medicine. 3rd ed. Philadelphia: Lippinco-Raven Publishers,1997:601-38.

70 Liu LY, Callahan B, Peterss S, et al. Neuromonitoring using motor and somatosensory evokedpotentials in aortic surgery. J Card Surg 2016;31(6):383-9. PMID 27193893

71 Lo Y, Dan Y, Tan Y, Teo A, et al. Clinical and physiological eects of transcranial electricalstimulation position on motor evoked potentials in scoliosis surgery. Scoliosis. 2010;5:3. PMID20175933

72 Lombardi CP, Carnassale G, Damiani G, et al. “The nal countdown”: is intraoperative, intermientneuromonitoring really useful in preventing nerve palsy? Evidence from a meta-analysis. Surgery2016;160(6):1693-1706. PMID 27566947

73 Lubitz SE, Keith RW, Crawford AH. Intraoperative experience with neuromotor evoked potentials. Areview of 60 consecutive cases. Spine 1999;24(19):2030-4. PMID 10528380

74 Lueders H, Andrish J, Gurd A, Weiker G, Klem G. Origin of far-eld subcortical potentials evokedby stimulation of the posterior tibial nerve. Electroencephalogr Clin Neurophysiol 1981;52:336-44.PMID 6169510

75 Lueders H, Lesser R, Hahn J, Lile J, Klem G. Subcortical somatosensory-evoked potentials inresponse to hand stimulation. J Neurosurg 1983;58:885-94. PMID 6850273

76 Luedke MW, Pietak MR, Serani S, Haglund MM, Sinha SR. Intraoperative ECoG During MRI-Guided Laser-Interstitial Thermal Therapy for Intractable Epilepsy. J Clin Neurophysiol2016;33(4):e28-30. PMID 27261642

77 MacEwen GD, Bunnell WP, Sriram K. Acute neurological complications in the treatment of scoliosis.A report of the Scoliosis Research Society. J Bone Joint Surg Am 1975;57:404-8. PMID 1123394

78 MacDonald DB. Safety of intraoperative transcranial electrical stimulation motor evoked potentialmonitoring. J Clin Neurophysiol 2002;19(5):416-29. PMID 12477987

79 Macri S, De Monte A, Greggi T, et al. Intra-operative spinal cord monitoring in orthopaedics. SpinalCord 2000;38(3):133-9. PMID 10795932

80 Martin WH, Stecker MM. ASNM Position Statement: Intraoperative Monitoring of Auditory EvokedPotentials. J Clin Monit Comput 2008;22(1):75-85. PMID 18058246

81 Mauguiere F. Somatosensory-evoked potentials. In: Niedermeyer E, Lopes da Silva F, editors.Electroencephalography: basic principles, clinical applications and related elds. 4th ed. Baltimore:Williams and Wilkins, 1999.

82 Mayo Clinic Department of Neurology. Motor function II: specic study of muscle. In: Wiebers DO,Dale AJ, Kokmen E, Swanson JW, editors. Mayo Clinic Examinations in Neurology. 7th ed. Mosby,1998: 177-240.

83 McBride MC, Binnie CD, Janota I, Polkey CE. Predictive value of intraoperative electrocorticogramsin resective epilepsy surgery. Ann Neurol 1991;30:526-32. PMID 1789682

84 McKhann GM 2nd, Schoenfeld-McNeill J, Born DE, Haglund MM, Ojemann GA. Intraoperativehippocampal electrocorticography to predict the extent of hippocampal resection in temporallobe epilepsy surgery. J Neurosurg 2000;93(1):44-52. PMID 10883904

85 McKinney JL, Islam MP. Neurophysiologic intraoperative monitoring in pediatric patients withpolyneuropathy. Childs Nerv Syst 2020;36(11):2801-5. PMID 32215716

86 Meacham KW, Guo L, Deweerth SP, Hochman S. Selective stimulation of the spinal cord surfaceusing a stretchable microelectrode array. Front Neuroeng 2011;4:5. PMID 21541256

87 Minahan RE, Mandir AS. Basic neurophysiological intraoperative monitoring techniques. In:Husain AM, editor. A Practical Approach to Neurophysiological Intraoperative Monitoring. NewYork: Demos, 2008:21-44.

88 Moller AR. Intraoperative Neurophysiological Monitoring. 2nd ed. Totowa, NJ: Humana PressSecond Edition, 2006.

89 Muthukumar N. Multimodal intraoperative neuromonitoring during surgery for correction ofspinal deformity: standard of care or luxury? Neurol India 2017;65(1):80-2. PMID 28084244

90 Naytah M, Ibrahim I, da Silva S. Importance of incorporating intraoperative neuromonitoring ofthe external branch of the superior laryngeal nerve in thyroidectomy: a review and meta-analysisstudy. Head Neck 2019;41(6):2034-1. PMID 30706616

91 Norcross-Nechay K, Mathew T, Simmons JW, Hadjipavlou A. Intraoperative somatosensory-evokedpotential ndings in acute and chronic spinal canal compromise. Spine 1999;15;24(10):1029-33.PMID 10332797

92 Nuwer M. Spinal cord monitoring. In: Nuwer M, editor. Evoked potential monitoring in theoperating room. New York: Raven Press, 1986:126.

93 Nuwer MR. Spinal cord monitoring with somatosensory techniques. J Clin Neurophysiology1998;15(3):183-93. PMID 9681556

94 Nuwer MR. Spinal cord monitoring. Muscle Nerve 1999;22(12):1620-30. PMID 10567073

95 Nuwer MR, Dawson E. Intraoperative evoked potential monitoring of the spinal cord: enhancedstability of cortical recordings. Electroencephalogr Clin Neurophysiol 1984;59:318-27. PMID 6203721

96 Oh SJ, Choi SW, Lee S, et al. Application of aachable magnetic nerve stimulator in intraoperativefacial nerve monitoring during ear surgery. Otolaryngol Head Neck Surg 2020 162(5):773-5. PMID32151184

97 Ojemann GA. Intraoperative electrocorticography and functional mapping. In: Wyler AR, HermannBP, editors. The Surgical Management of Epilepsy. Boston: Buerworth-Heinemann, 1994:189-96.

98 Ojemann GA. Surgical treatment of epilepsy. In: Wilkins RH, Rengachary SS, editors. Neurosurgery.2nd ed. New York: McGraw-Hill, 1996:4173-83.**

99 Olivier A, Awad IA. Extratemporal resections. In: Engel J Jr, editor. Surgical Treatment of theEpilepsies. New York: Raven Press, 1993:489-500.

100 Overzet K, Kazewych M, Jahangiri FR. Multimodality intraoperative neurophysiological monitoring(IONM) in anterior hip arthroscopic repair surgeries. Cureus 2018;10(9):e3346. PMID 30473979

101 Padberg AM, Bridwell KH. Spinal cord monitoring: current state of the art. Orthop Clin North Am1999;30(3):407-33, viii. PMID 10393764

102 Padberg AM, Wilson-Holden TJ, Lenke LG, Bridwell KH. Somatosensory- and motor-evokedpotential monitoring without a wake-up test during idiopathic scoliosis surgery. An acceptedstandard of care. Spine 1998;23:1392-400. PMID 9654631

103 Palmini A, Gambardella A, Andermann F, et al. Intrinsic epileptogenicity of human dysplastic

cortex as suggested by corticography and surgical results. Ann Neurol 1995;37:476-87. PMID7717684

104 Peeling L, Hentschel S, Fox R, Hall H, Fourney DR. Intraoperative spinal cord and nerve rootmonitoring: a survey of Canadian spine surgeons. Can J Surg 2010;53(5):324-8. PMID 20858377

105 Pereon Y, Bernard JM, Fayet G, et al. Usefulness of neurogenic motor evoked potentials for spinalcord monitoring: ndings in 112 consecutive patients undergoing surgery for spinal deformity.Electroencephalogr Clin Neurophysiol 1998;108:17-23. PMID 9474058

106 Polly DW Jr, Rice K, Tamkus A. What Is the Frequency of Intraoperative Alerts During PediatricSpinal Deformity Surgery Using Current Neuromonitoring Methodology? A Retrospective Study of218 Surgical Procedures. Neurodiagn J 2016;56(1):17-31. PMID 27180504

107 Pouratian N, Elias WJ, Jane JA Jr, Phillips LH 2nd, Jane JA Sr. Electrophysiologically guideduntethering of secondary tethered spinal cord syndrome. Neurosurg Focus 2010;29(1):E3. PMID20594001

108 Quinones-Hinojosa A, Lyon R, Zada G, et al. Changes in transcranial motor evoked potentialsduring intramedullary spinal cord tumor resection correlate with postoperative motor function.Neurosurg 2005;56:982,93. PMID 15854246

109 Resnick DK, Anderson PA, Kaiser MG, et al; Joint Section on Disorders of the Spine and PeripheralNerves of the American Association of Neurological Surgeons and Congress of NeurologicalSurgeons. Electrophysiological monitoring during surgery for cervical degenerative myelopathyand radiculopathy. J Neurosurg Spine 2009;11(2):245-52. PMID 19769504

110 Sala F, Krzan MF, Deletis V. Intraoperative neurophysiological monitoring in pediatricneurosurgery: why, when, how. Childs Nerv Syst 2002;18(6-7):264-87. PMID 12172930

111 Salanova V, Andermann F, Olivier A, Rasmussen T, Quesney LF. Occipital lobe epilepsy:electroclinical manifestations, electrocorticography, cortical stimulation and outcome in 42patients treated between 1930 and 1991. Surgery of occipital lobe epilepsy. Brain 1992;115(Pt6):1655-80. PMID 1486456

112 Schieppati M, Ducati A. Eects of stimulus intensity, cervical cord tractotomies and cerebellectomy

on somatosensory evoked potentials from skin and muscle aerents of cat hind limb.Electroencephalogr Clin Neurophysiol 1981;51:363-72. PMID 6164535

113 Schneider R, Machens A, Lorenz K, Dralle H. Intraoperative nerve monitoring in thyroid surgery--shiing current paradigms. Gland Surg 2020;9(suppl 2):S120-8. PMID 32175252

114 Schwartz DM, Auerbach JD, Dormans JP, et al. Neurophysiological detection of impending spinalcord injury during scoliosis surgery. J Bone Joint Surg 2007;89-2440-9. PMID 17974887

115 Schwartz DM, Drummond DS, Ecker ML. Inuence of rigid spinal instrumentation on theneurogenic motor evoked potential. J Spinal Disord 1996;9:439-45. PMID 8938615

116 Schwartz TH, Bazil CW, Walczak TS, Chan S, Pedley TA, Goodman RR. The predictive value ofintraoperative electrocorticography in resections for limbic epilepsy associated with mesialtemporal sclerosis. Neurosurgery 1997;40(2):302-11. PMID 9007862

117 Seyal M, Mull B. Mechanisms of signal change during intraoperative somatosensory-evokedpotential monitoring of the spinal cord. J Clin Neurophysiol 2002;19(5):409-15. PMID 12477986

118 Silva IC, Neo Ide P, Vartanian JG, Kowalski LP, Carrara-de Angelis E. Prevalence of upperaerodigestive symptoms in patients who underwent thyroidectomy with and without the use ofintraoperative laryngeal nerve monitoring. Thyroid 2012;22(8):814-9. PMID 22780215

119 Sindou MP. Microvascular decompression for primary hemifacial spasm: Importance ofintraoperative neurophysiological monitoring. Acta Neurochir 2005;147(10 ):1019-26. PMID16094508

120 Singer MD, Rosenfeld RM, Sundaram K. Laryngeal nerve monitoring: current utilization amonghead and neck surgeons. Otolaryngol Head Neck Surg 2012;146(6):895-9. PMID 22399282

121 Skinner S, Holdefer R, McAulie JJ, Sala F. Medical error avoidance in intraoperativeneurophysiological monitoring: the communication imperative. J Clin Neurophysiol2017;34(6):477-83. PMID 29023306

122 Skinner SA, Aydinlar EI, Borges LF, et al. Is the new ASNM intraoperative neuromonitoringsupervision guideline’ a trustworthy guideline? A commentary. J Clin Monit Comput2019;33(2):185-90. PMID 30612285

123 Smith PN, Balzer JR, Khan MH, et al. Intraoperative somatosensory evoked potential monitoringduring anterior cervical discectomy and fusion in nonmyelopathic patients: a review of 1,039 cases.Spine J 2007;7:83-7. PMID 17197338

124 Souter MJ, Rozet I, Ojemann JG, et al. Dexmedetomidine sedation during awake craniotomy forseizure resection: eects on electrocorticography. J Neurosurg Anesthesiol 2007;19(1):38-44. PMID17198099

125 Stecker MM. Generalized averaging and noise levels in evoked responses. Comput Biol Med2000;30(5):247-65. PMID 10913772

126 Stecker MM, Paerson T, Netherton B. Mechanisms of electrode induced injury. Part 1: theory. Am JElectroneurodiagnostic Technol 2006;46(4):315-42. PMID 17285816

127 Stephen JP, Sullivan MR, Hicks RG, et al. Cotrel-Dubousset instrumentation in children usingsimultaneous motor and somatosensory-evoked potential monitoring. Spine 1996;21:2450-7. PMID8923630

128 Sugano H, Shimizu H, Sunaga S. Ecacy of intraoperative electrocorticography for assessingseizure outcomes in intractable epilepsy patients with temporal-lobe-mass lesions. Seizure2007;16(2):120-7. PMID 17158074

129 Sun H, Tian W, Jiang K, et al. Clinical guidelines on intraoperative neuromonitoring during thyroidand parathyroid surgery. Ann Transl Med 2015;3(15):213. PMID 26488009

130 Suer M, Eggspuehler A, Jeszenszky D, et al. The impact and value of uni- and multimodalintraoperative neurophysiological monitoring on neurological complications during spine surgery:a prospective study of 2728 patients. Eur Spine J 2019;28(3):599-610. PMID 30560453

131 Szelenyi A, Heukamp C, Seifert V, Marquardt G. S100B, intraoperative neuromonitoring ndings

and their relation to clinical outcome in surgically treated intradural spinal lesions. Acta Neurochir(Wien) 2014;156(4):733-9. PMID 24390083

132 Tanaka T, Hashizume K, Kunimoto M, Yonemasu Y, Chiba S, Oki J. Intraoperativeelectrocorticography in children with medically intractable epilepsy. Neurol Med Chir1996;36(7):440-6. PMID 8741373

133 Taunt CJ Jr, Sidhu KS, Andrew SA. Somatosensory-evoked potential monitoring during anteriorcervical discectomy and fusion. Spine 2005;30(17):1970-2. PMID 16135987

134 Thirumala PD, Natarajan P, Thiagarajan K, et al. Diagnostic accuracy of somatosensory evokedpotential and electroencephalography during carotid endarterectomy. Neurol Res2016;38(8):698-705. PMID 27342607

135 Thomsen K, Christensen FB, Eiskjaer SP, et al. The eect of pedicle screw instrumentation on thefunction outcome and fusion rates in posterolateral lumbar spine fusions: a prospectiverandomized clinical study. Spine 1997;22:2813-22. PMID 9431617

136 Toleikis JR, American Society of Neurophysiological Monitoring. Intraoperative monitoring usingsomatosensory evoked potentials. A position statement by the American Society ofNeurophysiological Monitoring. J Clin Monit Comput 2005;19:241-58. PMID 16244848

137 Tsirikos AI, Duckworth AD, Henderson LE, Michaelson C. Multimodal intraoperative spinal cordmonitoring during spinal deformity surgery: ecacy, diagnostic characteristics and algorithmdevelopment. Med Princ Pract 2020;29(1):6-17. PMID 31158841

138 Ulkatan S, Jaramillo AM, Téllez MJ, Kim J, Deletis V, Seidel K. Incidence of intraoperative seizuresduring motor evoked potential monitoring in a large cohort of patients undergoing dierentsurgical procedures. J Neurosurg 2016:1-7. PMID 27341047

139 Vauzelle C, Stagnara P, Jouvinroux P. Functional monitoring of spinal cord activity during spinalsurgery. Clin Orthop 1973;93:173-8. PMID 4146655

140 Wang AC, Than KD, Etame AB, La Marca F, Park P. Impact of anesthesia on transcranial electricmotor evoked potential monitoring during spine surgery: a review of the literature. NeurosurgFocus 2009;27(4):E7 PMID 19795956

141 Weinzierl MR, Reinacher P, Gilsbach JM, et al. Combined motor and somatosensory-evokedpotentials for intraoperative monitoring: intra- and post-operative data. Neurosurg Rev2007;30(2):109-16. PMID 17221265

142 Weiss DS. Spinal cord and nerve root monitoring during surgical treatment of lumbar stenosis.Clin Orthop 2001;(384):82-100. PMID 11249183

143 Wennberg R, Quesney LF, Lozano A, Olivier A, Rasmussen T. Role of electrocorticography atsurgery for lesion-related frontal lobe epilepsy. Can J Neurol Sci 1999;26(1):33-9. PMID 10068805

144 Wilber RG, Thompson GH, Shaer JW, Brown RH, Nash CL Jr. Postoperative neurological decits insegmental spinal instrumentation. A study using spinal cord monitoring. J Bone Joint Surg Am1984;66:1178-87. PMID 6490694

145 Wu CW, Dionigi G, Sun H, et al. Intraoperative neuromonitoring for the early detection andprevention of RLN traction injury in thyroid surgery: a porcine model. Surgery 2014;155(2):329-39.PMID 24084598

146 Wu SY, Shen HY, Duh QY, Hsieh CB, Yu JC, Shih ML. Routine intraoperative neuromonitoring of therecurrent laryngeal nerve to facilitate complete resection and ensure safety in thyroid cancersurgery. Am Surg 2018;84(12):1882-8. PMID 30606343

147 Yano, M, Saito Y, Yoshida M, et al. Usefulness of intraoperative neuromonitoring for preservation ofan extralaryngeal bifurcation of the recurrent laryngeal nerve: a case report. Int J Surg Case Rep2018;53:330-2. PMID 30471624

148 York DH, Chabot RJ, Gaines RW. Response variability of somatosensory-evoked potentials duringscoliosis surgery. Spine 1987;12:864-76. PMID 3441833

149 Yoshida G, Ushirozako H, Kobayashi S, et al. Intraoperative neuromonitoring during adult spinaldeformity surgery: alert-positive cases for various surgical procedures. Spine Deform2019;7(1):132-40. PMID 30587306

150 Yue Q, Martinez Z. Monitoring supercial peroneal nerve somatosensory evoked potential during

L4-5 lumbar root decompression. Spine J 2013;13(8):922-5. PMID 23623638

151 Zhang D, Pino Am Caruso E, Dionigi G, Sun H. Neural monitoring in thyroid surgery is here to stay.Gland Surg 2020;9(Suppl 1):S43-6. PMID 32055497

152 Zhou MW, Huang XY, Chen ZY, et al. Intraoperative monitoring of pelvic autonomic nerves duringlaparoscopic low anterior resection of rectal cancer. Cancer Manag Res 2018;11:411-7. PMID30643466

153 Ziewacz JE, Berven SH, Mummaneni VP, et al. The design, development, and implementation of achecklist for intraoperative neuromonitoring changes. Neurosurg Focus 2012;33(5):E11. PMID23116091

154 Zouridakis G, Papanicolaou AC, Simos PG. Intraoperative neurophysiological monitoring. Part2:Neurophysiological background. J Clin Eng 1997;22(5):321-7. PMID 10174604

Author

Richard P Knudsen MD FAASM CNP FAAPDr. Knudsen of the Pacic Sleep Tech in Aiea, Hawaii has no relevant nancial relationships todisclose.SEE PROFILE

Editor

Bernard L Maria MDDr. Maria of Thomas Jeerson University has no relevant nancial relationships to disclose.SEE PROFILE

Contributors

Former Authors

Sandra Helmers MD, Y Helen Han PhD, Mark M Stecker MD PhD, and Marcio ASotero de Menezes MD