Evoked Potentials

Authors: Flaherty, Alice W.; Rost, Natalia S.

Title: Massachusetts General Hospital Handbook of Neurology, The, 2nd Edition

Copyright 2007 Lippincott Williams & Wilkins

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Evoked Potentials

A. General

AKA evoked responses. They record latency, amplitude, R/L discrepancies from scalp during stimulation of sensory modalities. They can detect clinically silent lesions and give objective proof of sensory deficits, but a positive MRI usually makes EPs unnecessary.

B. Visual evoked potentials (VEP)

Stimulate with flashing light or checkerboard pattern. Detects vision when pt. can not communicate; delayed in optic neuritis even after recovery. P100 is prolonged in retinal or optic nerve lesion need ERGs (electroretinograms) to differentiate between them.

C. Somatosensory evoked potentials (SSEP)

Stimulate peripheral nerves (usually median, common peroneal, posterior tibial), record on scalp, sometimes spine. Prolonged if lesion anywhere along nerve, plexus, nerve root, spinal cord, brainstem, thalamus, cortex.

D. Brainstem auditory evoked responses (BAER)

Measure function of auditory nerves and brainstem auditory pathways. Peripheral vestibular dz does not affect BAER. There are seven waves, but only I, III, and V are important. I is from CN VIII; III = bilateral superior olive, V = inferior colliculus. IPL = interpeak latency.

• 1. All waves absent: Suggests peripheral deafness.

• 2. I-III prolonged (or III absent): Suggests pontomedullary junction lesion, e.g., MS, CPA tumor, pontine glioma, brainstem infarct.

• 3. III-V prolonged (or V absent): Pons/midbrain lesion, e.g., MS, extrinsic mass compressing brainstem (includes contralateral CPA tumor).

• 4. All waves prolonged: Suggests diffuse dz, e.g., MS, big glioma. Not usually metabolic.