Visual Evoked Cortical Potentials
Visual evoked potential (VEP; also, VECP or VER, for visual evoked cortical potential or response) testing measures electrical signals produced in the brain in response to stimulation of the retina by either light flashes or patterned stimuli (usually a black-and-white checkerboard that reverses its pattern on a TV monitor). The signals are recorded via electrodes placed on the occipital scalp. The VEP is extracted from the larger background electroencephalogram by averaging the responses to multiple reversals or flashes. Pattern-reversal VEPs (the black-and-white squares interchange without change in luminance) have a similar waveform across a population and a remarkably consistent timing; amplitudes show greater variability. Flash VEPs are far more variable across a population but can be useful when comparing eyes or hemispheric responses in the same patient; hemispheric comparison requires multiple recording channels. A normal pattern-reversal VEP (Fig 3-7) contains a major positive component at approximately 100 ms, P100. Measurement is usually taken of P100 amplitude and peak time (sometimes called latency).
In adults, VEPs are often used to demonstrate optic nerve conduction delay, particularly in patients with suspected multiple sclerosis; patients with demyelinating optic neuritis almost invariably show VEP delay even when visual acuity recovers (see Fig 3-7). However, there can be subclinical delay in patients without any history or signs of optic neuropathy. Most optic nerve diseases show VEP delay, but an abnormality can be confined to amplitude (interocular asymmetry), for example, in patients with nonarteritic anterior ischemic optic neuropathy. VEPs are crucial in patients with medically unexplained vision loss when the vision loss is suspected to be nonorganic. VEPs can objectively demonstrate normal function in the presence of symptoms that suggest otherwise. It is important to note that a delayed VEP is not diagnostic of optic nerve disease. Macular dysfunction can cause similarly abnormal findings, and assessment of macular function with mfERG or PERG enables improved VEP interpretation.
Figure 3-7 Pattern-reversal visual evoked potential from a patient with a 4-month history of right eye (RE) optic neuritis and recovery of right visual acuity to 20/20. Left eye (LE) response is normal; RE P100 component shows profound delay with preservation of amplitude and waveform.
(Courtesy of Graham E. Holder, PhD.)
VEPs are indispensable in examining children, particularly those who are preverbal, who have apparent vision loss, or who present in infancy with roving eye movements or unexplained nystagmus. VEPs are used in conjunction with ERGs to discover any retinal or post-retinal dysfunction. However, caution must be exercised in any patient with nystagmus because that eye movement disorder can itself induce abnormal responses.
Pattern-appearance stimulation, in which the stimulus appears from a uniformly gray background and then disappears, maintaining isoluminance throughout, can also be used to elicit VEPs. VEPs recorded in this manner are particularly useful in demonstrating the intracranial misrouting associated with ocular or oculocutaneous albinism (requiring multiple recording channels) because they are less affected by nystagmus than reversal VEPs. Flash stimulation is effective for babies and infants who cannot maintain adequate fixation on a pattern stimulus. If the check size and contrast levels are varied, pattern-appearance VEPs can also be used to objectively assess visual system resolution, providing a surrogate measure of visual acuity.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.