What are visual evoked potential (VEP) tests?

Updated: Oct 25, 2019
  • Author: Andrew B Evans, MD; Chief Editor: Selim R Benbadis, MD  more...
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Answer

Answer

The visual evoked potential (VEP) tests the function of the visual pathway from the retina to the occipital cortex. It measures the conduction of the visual pathways from the optic nerve, optic chiasm, and optic radiations to the occipital cortex. It is important to keep in mind that although the axons from the nasal half of the retina decussate at the optic chiasm, the temporal axons do not. Therefore, retrochiasmatic lesions may not be detected by full-field checkerboard stimulation.

The usual waveform is an initial negative peak (N1 or N75), followed by a large positive peak (P1 or P100), followed by another negative peak (N2 or N145). Maximum value for P100 is 115 msec in patients younger than 60 years; after this age, it rises to 120 msec in women and 125 msec in men. Even though published norms are available in the medical literature, each individual laboratory should have its own norms to control for laboratory-to-laboratory variations in technique.

The W morphology, in the author’s experience, is most often an individual variation, though decreasing the stimulation frequency from the ubiquitous 2 Hz to 1 Hz usually converts the W shape into a conventional P100 peak. Check size and alternation rate are factors in this; the responses can be manipulated to a W or a conventional P100 response by changing these parameters. Large checks tend to produce VEPs similar to those produced by flash stimulation.

VEPs are most useful for testing optic nerve function and less useful for assessing postchiasmatic disorders. In patients with retrochiasmatic lesions, MRI is a more useful test. Partial-field studies may be useful for retrochiasmatic lesions; however, they are not performed routinely in clinical settings. It is also important to note that the macula projects to the occipital pole, whereas the rest of the retina projects to the mesial calcarine cortex.

Although the VEP is very useful for detecting an anterior visual conduction disturbance, it is not specific with regard to etiology. A tumor compressing the optic nerve, an ischemic disturbance, or a demyelinating disease may cause delay in the P100; only additional clinical history and, often, MRI are needed to uncover the etiology.


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