Intraoperative Monitoring

Behrouz Zamanifekri, MD

Neurophysiology Fellow

KUMC

March 2013

Intraoperative monitoring The most primitive method of monitoring the patient

50 years ago were continuous palpation of the radial

pulsations throughout the operation or wake up test!!

History 1921, Dr Penfield, intraoperative neurophysiology research

1950s, Dr Penfield, electrical stimulation to find epileptic foci

1970s, Dr Brown used SSEP for scoliosis operation

1974 , among 7,800 operations conducted with Harrington instrumentation, 87 patients had subsequently developed significant spinal cord problems

Early 1980, IOM in operations for large skull base tumors

1980, the American Society for Neurophysiological

Monitoring [ASNM])

Introduction

What is IOM ? use of neurophysiological recordings for detecting

changes caused by surgically induced insults

assess the function of specific parts of the nervous system continuously during an operation

It is becoming part of standard medical practice

What is the purpose of IOM?

1. Reduce the risk of postoperative neurological deficits

2. Identify specific neuronal structures and landmarks that cannot be easily recognized

3. Research purposes in basic science, pathophysiology and therapeutic management

What are the most common types of recording? Spontaneous activity EEG

EMG

Evoked responses (through external stimulation of a neural pathway)

Sensory : 1- visual

2-auditory

3-somatosensory

Motor

The type of test to be used and the sites of recording and stimulation are chosen on a case by case

Complications during surgery

ischemia mechanical insult

PRACTICAL ASPECTS OF

MONITORING SPINAL CORD

Spinal Monitoring Spinal cord, nerve roots, and blood vessels

are frequently placed at risk for injury

Electrophysiological modalities for monitoring:

SSEPs

MEPs

free run or spontaneous EMG (sEMG)

triggered EMG (tEMG)

Spine Surgery:

surgical insults to the ventral parts of the cord, using motor evoked potentials (MEPs)

dorsal columns of the spinal cord , SEP the purpose of IOM is to detect

response changes due to surgery, not to make a clinical diagnosis

Monitoring of Somatosensory Evoked Potential

Earliest used method in IOM 1970s in operations for scoliosis Stimulation of peripheral nerve and

recorded from scalp Only monitor dorsal(sensory) spinal cord patient sensory examination for position

and vibration is recommended prior to surgery

SSEP

By electrical stimulation of peripheral nerves Median nerve at wrist for injury above C8 Posterior tibial nerve at ankle for injury below C8

spinal cord through the dorsal roots,

ascending pathways, thalamus

and, finally, to the

primary sensory cortex

Recording

P9 from brachial plexus P11 Dorsal horn P14-16 Dorsal column nuclei P20 Primary sensory cortex(contralat.)

upper limb SSEP

N37 Primary sensory cortex(contralat.)

lower limb SSEP

Location of the stimulating and recording posterior tibial nerve SEPs.

It is important to note

- Earlier peaks tend to be less sensitive to anesthesia

- used to differentiate SSEP monitoring changes resulting from anesthetic effects from surgical manipulation.

Alarm criteria -50% reduction in amplitude -10% increase in latency

Factors that affect the SSEP amplitude include halogenated agents, nitrous oxide, hypothermia, hypotension, and electrical interference

Normal SSEPs from median nerves and posterior tibial nerves

SSEP in Peripheral nerves?

sciatic nerve injury during pelvic fracture

Injuries to brachial plexus in positioning of pt is common

Prolong latency of all peaks and decrease amplitude

Nerve root

SSEP: insensitive to changes

in nerve root function

Why?

SSEP in nerve root injury?

SSEP used during placement of pedicle screws

Risk of spinal nerve root injury

If one root damaged, no change in

SSEP

Dermatomal stimulation is better

Inhalational anesthetics, cortical responses

Intravenous Agents

- Propofol increases the latency by approximately10%

- Benzodiazepines reduce the amplitude of cortical SEP

- Etomidate : cortical SEP amplitude augmented 200–600%,

increases SEP latencies

- Opiates, cause a slight increase in SEP latency

- Muscle relaxants, not affect SEPs

SEP changes due to surgical maneuvers (e.g., spinal distraction) or ischemia (e.g., after placement of an artery clamp) are abrupt and localized and only one side of the body may be affected

whereas changes due to anesthesia or body temperature changes are relatively slower

Detection of cord injury due to misplaced instrumentation

just after placement of instrumentation,

both the cortical (peak N45) and cervical (peak N30) responses

disappear

SEPs obtained after cross-clamping of the internal carotid ,whichresulted in ischemia (time 9:45) that later deteriorated (9:55). After placement of a shunt,response amplitude is restored to within normal limits (time 10:01).

Procedures involves the ICA, MCA, PCA, P.Com, or BA?

Median nerve SEPs

procedure involves the

ACA or the A.Com artery?

Posterior tibial nerve

MONITORING SPINAL

MOTOR SYSTEM

Introduction

SSEP for sensory pathway MEP for motor

SSEP + MEP: Small reversible changes in SSEP that occur when motor pathway are injured

MEP

1990s, TC-MEP as a method to monitor the corticospinal tracts

Prior to MEP monitoring,

the only way to assess corticospinal

tract during surgery was wake-up test

TC-MEPs stimulation through the skull with signal

recording at the level of

muscle (CMAP)

nerve (neurogenic MEP)

spinal cord ( D-wave )

-the newer technologies is continuous free-running EMG throughout the surgery

Recording of Muscle Evoked Potentials

Stimulation of cortex, activation of coticospinal, EMG of distal( Hand m., abd hallucis, tibialis anterior)

Muscle relaxant can not be used

MEP

Interpretation of MEP Recording

4 methods :

1) all-or-nothing criterion: the most used method,

complete loss of the MEP signal from a baseline recording is indicative of

a significant event

2) amplitude criterion: 80% amplitude decrement in at least 1

out of 6 recording sites

3) threshold criterion: increases in the threshold of 100 V or more

required for eliciting CMAP responses that are persistent for 1 h or more

4) morphology criterion: changes in the pattern and duration of MEP

waveform morphology

TcMEP monitoring

contraindicated in

-deep brain stimulators or cochlear implants

Tongue biting is the most common complication

Normal MEPs

Recording of the response from spinal cord(D, I wave)

Recording from epidural

electrodes D (direct activation of

corticospinal) I ( indirect, through transsynaptic)

Not affected by muscle relaxants, but latencies increase with cooling Subdural electrodes can be substituted for epidural electrodes Needle electrodes can be place in interspinous ligaments both sides of

surgery area major benefits reported during intramedullary spinal cord tumor

resection a complete loss of MEPs with at least 50% preservation of the D-wave

amplitude generally results in a transient paraplegia

Spontaneous EMG

monitor nerve roots recording electrodes placed in the

muscles no stimulation is performed monitoring of 2 muscles is

recommended C5 nerve root injury, The deltoid and

biceps brachii MEPs be obtained intermittently

sEMG

no paralytic agents

train-of-4 testing should indicate that at least 3 out of 4

Myasthenia gravis, Botox treatments, and muscular dystrophy are classic conditions that interfere with EMG

Abnormal sEMG spikes Bursts trains

Trains are continuous, repetitive EMG firing

caused by continuous force applied to the nerve

root.

Example of EMG activityindicating irritation of the nerve

Baseline recordings. Note the low

amplitude background activity

High amplitude spikes are present

Artifacts may be mistaken for spikes or trains

a neurostimulator

the surgical table

the surgeon’s head light

bipolar electrocautery device

Triggered EMG (Pedicle Screw Stimulation)

used to determine whether screws have breached the medial or inferior pedicle wall and thus pose a risk to the exiting nerve root at that level

When a pedicle screw is accurately placed, the surrounding bone acts as an insulator to electrical conduction, and a higher amount of electrical current is thus required to stimulate the surrounding nerve root.

When a medial pedicle wall breach occurs, the stimulation threshold is significantly reduced

False negative response

muscles relaxants

fluid, blood, or soft tissue around the head of the

screw , shunt current away from the screw

it is important that the stimulation probe be placed directly on the top of the screw and not the tulip, as these 2 structures are not structurally fused

Presence of preexisting nerve root injury. Injured nerve roots will have higher triggering thresholds,

Due to the variation in thickness and shape between thoracic and lumbar pedicles, different stimulation thresholds exist for these regions

A threshold < 10 mA for screw stimulation, suggest a medial wall breach in the lumbar pedicles

A thresholds > 15 mA indicate a 98% likelihood of accurate screw positioning

For thoracic pedicle screw placement, stimulation threshold < 6 mA suggest a medial pedicle breach

In cervical and thoracic procedures, the spinal cord are of greater importance

Conversely, in lumbar or sacral procedures the nerve roots are at greater risk of injury

Overview of IOM classified by spinal region

Conclusions

Multimodality neurophysiological monitoring is extremely valuable in the prevention of neurological injury

Knowledge of the benefits and limitations of each modality helps maximize the diagnostic value of IOM during spinal procedures

Neurosurg Focus / Volume 27 / October 2009

A concise guide to intraoperative monitoring / George Zouridakis, Andrew C. Papanicolaou.2001

Intraoperative neurophysiological monitoring / Aage R. Moller. -- 2nd ed.