Introduction to electrodiagnostic testing (EMG/NCS)

Calin I. Prodan, MD

Department of Neurology

University of Oklahoma Health Sciences Center

Electrodiagnostic testing in Neurology

Components:

• Nerve conduction studies (NCS)

• Needle electromyography (EMG)

• Other components:– Repetitive stimulation– Single-fiber EMG– Intraoperative monitoring

NCS

• Sensory nerves:– Amplitude– Velocity (distance divided by time)

• Motor nerves– Amplitude– Velocity– F-wave response

• Uses a gradually increased amount of electrical stimulation in order to obtain an motor/sensory response

Nerve Conduction Studies

• Amplitude of the response

Amplitude (mV)

Time (ms)

Sensory Pathways

• Receptor organs (sensory transducers)

• Peripheral nervous system pathway

• Spinal (ascending) pathway

• Thalamic “relay”

• Cortical representation

Pain & Temperature Pathway“Spinothalamic system”

• Receptors are naked nerve endings• Peripheral unmyelinated (C) and small-myelinated (A)

fibers• Ascending spinal cord pathway• Carried to the thalamus• The speed of conduction for pain and temperature

sensation is very slow

Proprioception Pathway

• Receptors are specialized organelles including muscle spindles, Golgi tendon organs, and Pacinian corpuscles

• Peripheral large-myelinated (A and A) fibers• Ascending spinal cord pathway• Carried to the thalamus• The speed of conduction is fast

Spinothalamic vs Proprioception Pathways

SPINOTHALAMIC

• Pain & temperature

• Naked nerve endings

• A and C fibers (slow)

PROPRIOCEPTION

• Proprioception

• Specialized receptor organelles

• A and A fibers (fast)

Electrodiagnostic studies:Nerve Conduction Studies

NCS will only study large-myelinated nerve fibers, distal to the DRG.Hence, they are normal in myelopathy, radiculopathy and small-fiberneuropathy, despite clinically evident sensory loss.

EMG

• Insertion of a needle electrode in various muscles

• Recording muscle activity at rest and during activity (volitional activity)

• Requires a lot of cooperation and cannot be performed accurately in patients with severe weakness

EMG

Normal voluntary activity

Fibrillation

Fasciculations

EMG/NCS

• Indications:– Polyneuropathy

– Entrapment neuropathy (carpal tunnel, etc.)

– Myasthenia gravis

– ALS

– Radiculopathy

– Myopathy/Myositis

– Plexopathy

– Any combination of the above

EMG/NCS

• Elective procedure

• Extension of clinical diagnosis – NOT to make a dx

• Grade severity (mild, moderate, severe) i.e. define the need for surgical intervention

• Prognosis (myasthenia, ALS, GBS)

When to refer for EMG/NCS

• Radiculopathy:– Part of surgical evaluation for cervical and LS spine

surgeries– Very low yield in pure sensory c/o or pain – Higher yield with weakness and reflex changes– Prior to repeating a surgery– Post-surgical complications (from the surgical service)

Radiculopathies

• “Numbness” and pain in the appropriate dermatomal distribution.

• Often associated with focal myotomal weakness, focal hyporeflexia and back pain.

Radiculopathy– Electrodiagnostic studies

Nerve conduction studies:• normal

EMG:• denervation changes (fibrillation potentials and fasciculations) within muscles innervated by the root involved and paraspinal muscles adjacent to the root• other muscles in the same extremity and any other muscles are normal

When not to refer for EMG/NCS

• Radiculopathy:– Low back/neck pain with or w/o radicular pattern –

patients should be treated medically and imaged (X-ray) – Low back/neck pain with sensory c/o or sensory findings -

patients should be 1st treated medically and imaged (X-ray)

– anticoagulation (cannot perform complete EMG)– Patient states that he does not want surgery or not a

surgical candidate

When to refer for EMG/NCS

• Polyneuropathy:– Consider risk factors (including DM and ETOH)– Not an early test– Serology and symptomatic treatment should come first – When considering biopsy

Peripheral Nerve Disease (Neuropathy)

• Small-Fiber Neuropathy• Large-Fiber Neuropathy

Spinothalamic vs Proprioception Pathways

SPINOTHALAMIC

• Pain & temperature

• Naked nerve endings

• A and C fibers (slow)

PROPRIOCEPTION

• Proprioception

• Specialized receptor organelles

• A and A fibers (fast)

Length-dependent (dying back) axonal neuropathy

Small-Fiber Neuropathy

Small-Fiber Neuropathy– Electrodiagnostic studies

COMPLETELY NORMAL

Large-Fiber Neuropathy

• History:– Constant unsteadiness

– Frequent falls

– slow (months/years) progression

– No numbness, tingling, pain

– first involves feet/legs then hands/arms

Large-Fiber Neuropathy• Examination:

– Decreased proprioception (joint position) and vibratory sense (tuning fork) in a “stocking and glove” distribution

– Normal OR decreased temperature/pain sensation in a “stocking and glove” distribution

– Reflexes are decreased (legs > arms)– Gait is very unsteady (wide-based) – Weakness in feet > hands is a late sign

Large-Fiber Neuropathy– Electrodiagnostic studies

Nerve conduction studies:• decreased amplitude or absent responses• changes are greater in the lower extremities than upper extremities

EMG:• normal• in longstanding disease it may show denervation in distal muscles

Electrodiagnostic studies

• Nerve conduction studies (NCS)– the most important non-serologic test for the diagnosis of

neuropathy

• Electromyography (EMG)– helps evaluating the effect on muscles

– rules out muscle disease

Segmental Demyelination

• Schaumburg, Berger & Thomas, 1992

Demyelinating

Polyneuropathies • Segmental demyelination is a random

process:– Statistically more likely to affect longer

nerves (i.e. legs) first and most severely. – But any nerve segment can be affected,

so proximal and facial weakness can occur early.

• Disruption of large-fiber nerve function: early weakness, areflexia and sensory ataxia.

• Schaumburg, Berger & Thomas, 1992

Features of Acquired (Segmental) Demyelination:

(1) Partial motor conduction block (PMCB)

(2) Temporal dispersion (TD)

(3) Asymmetric conduction slowing between nerves and in proximal/distal segments of the same nerves

Acquired Demyelinating PolyneuropathiesSegmental Demyelination

• Usually immune mediated

• ACUTE (nadir within 4 weeks)– Guillian-Barré Syndrome– Treat with IvIgG or PE

• CHRONIC (progress slowly for > 6 weeks)– CIDP– Treat with corticosteroids, IvIgG or PE

When to refer for EMG/NCS

• Entrapment neuropathy:– Part of surgical evaluation (NES, Ortho, plastics)

– Part of initial evaluation if sensory/motor changes are present

– After at least 6-8 weeks of smx

• When not to refer for EMG/NCS:– if only sensory c/o (treat – splints, pads, NSAIds, pain management)

– patient refuses surgery

Focal Neuropathies in the Upper Limb

Carpal Tunnel Syndrome (CTS)

Carpal Tunnel Syndrome (CTS)

• Commonest focal neuropathy encountered

• Entrapment of the median nerve beneath the flexor retinaculum at the wrist

• Symptoms:

– numbness and pain the wrist and hand

– worse at night and with repetitive motions

– relieved by shaking

• Signs:

– numbness in median nerve distribution

– weakness/wasting of APB, sparing FDI/ADM

– Tinel’s and Phalen’s signs

Carpal tunnel syndrome

• Diagnosis:– clinical– NCS (80-90% even in mild cases)

• Labs:– always screen for diabetes and thyroid disease

• NCS:– Slow conduction velocity (sensory/motor) across the wrist– +/- decreased amplitude– +/- prolonged F-wave

• EMG– +/- denervation (acute and/or chronic)

Carpal tunnel syndrome

• Treatment:– wrist splints

– NSAIDs

– treat the predisposing condition

• Surgical release:– surgical release (90% success rate)

– Indicated mostly in moderate and severe cases

– Outpatient surgery (plastics, ortho, NES, GS)

When to refer for EMG/NCS

• Myopathy/myositis:– Not an early test– Serology (including myositis antibodies) and treatment should

come first – When considering biopsy

Myopathy

Werwerwr

Myopathy/Myositis– Electrodiagnostic studies

Nerve conduction studies are normal (motor and sensory nerves)

EMG - pathy: No denervation (spontaneous activity at rest)Small units when symptomatic muscles are activated even with maximal effort

EMG – itis: Denervation

When to refer for EMG/NCS

• ALS, MG, plexopathy:– Not an early test– Serology/treatment/imaging should come first

MND

MND

MND– Electrodiagnostic studies

Nerve conduction studies:• sensory nerves are normal• motor nerves +/- low amplitude response • conduction velocities are normal

EMG:• the crucial test for diagnosis• widespread denervation changes (spontaneous activities at rest) – fibrillation potentials and fasciculations in all extremities/thorax and even face/tongue

Muscular dystrophy

– Family history of similar issues– Most often EMG/NCS NOT indicated– DNA testing available

= Myopathy with positive FH

Neuromuscular Junction Disease

Post-synaptic NMJ

Myasthenia Gravis (MG)

Pre-synaptic NMJ

Eaton-Lambert syndrome (ELMS)

MG

• History:– double vision (diplopia) and droopy eyelids – very common (60-70)– difficulty swallowing, chewing and talking – common (15-20%– limb weakness – less common (10%)

– weakness fluctuates during the day– strength normal in the AM, weakness most pronounced in PM– weakness is triggered by repetitive activities – strength improves with rest– severe cases may lead to respiratory failure and death

• Hallmark is fluctuating fatigable weakness.

MG – Electrodiagnostic studies

Nerve conduction studies are normal (motor and sensory nerves)

EMG is normal

Repetitive stimulation (2-3 Hz) of a distal motor nerve may produce a decrease in the amplitude of the motor response ( > 10%)

MG – Repetitive stimulation

ELMS

• History:– Heaviness/fatigue in the upper portion of the limbs– Trouble going up/down stairs or getting in/out a chair – NO double vision/droopy eyelids/difficulty

swallowing, chewing or talking

Hallmark is improvement with exercise

ELMS – Electrodiagnostic studies

Nerve conduction studies are normal (motor and sensory nerves)

EMG is normal

Repetitive motor stimulation at 2-3 Hz may produce a decrease in the amplitude of the motor response (5- 20%)

Repetitive motor stimulation at 20-50 Hz will produce an increase in the amplitude of the motor response (200-500%)

ELMS – Repetitive stimulation

ELMS – Repetitive stimulation

EMG/NCS

• Involves significant discomfort

• Relatively expensive ( = MRI)

• Requires patient cooperation

• Takes approx. 1 hour

Carpal tunnel screen (CTS)

• Carpal tunnel screen clinic – EMG lab

• Screening test: both median nerves across the wrists and both ulnar nerves at the wrist (motor and sensory component)

• No needle testing

• Can be done on a same-day basis