Norm of Visual Evoked Potential
P-100 is of normal latency. Latency is equivalent bilaterally (test results require expert interpretation).
Usage of Visual Evoked Potential
Diagnosis or monitoring of demyelinating diseases, glaucoma, maculopathy, migraine headaches, nitrous oxide toxicity (chronic), papilledema, Parkinson's disease, pressure on the optic pathway as a result of tumors or granulomas, pseudotumor cerebri, retinal diseases of the optic nerve, toxic optic neuropathies, and vitamin B deficiency. Also used intraoperatively during eye surgery to provide early warning of potential optic nerve damage.
Description of Visual Evoked Potential
Visual evoked potential (VEP) is a low-amplitude, electrical waveform representation of the brain's response to a visual stimulus. Because the amplitude is too low to be noted on a traditional electroencephalogram (EEG), sophisticated computer signal-averaging techniques are used to average out the effect of other brain activity during testing. The test involves placing repetitive, patterned stimuli such as a striped, checkerboard, or dotted pattern in the visual field while VEP waveforms are recorded and the amount of time taken for the VEP to occur is measured. Variations of the technique include varying the pattern size, intensity, and visual field size as well as alternating the pattern itself in an effort to selectively stimulate portions of the visual field. Results are analyzed according to an algorithm and are related to the “P-100” wave. The P-100 wave occurs at about 100 milliseconds after each stimulus in normal clients. Of significance is the amount of time required for the VEP to occur after stimulation (latency) and a comparison of latency measurements of both eyes. Factors that affect latency include head size, electrode location, visual field position of the stimulus, and integrity of the visual nerve pathways.
Professional Considerations of Visual Evoked Potential
Consent form NOT required.
- See Client and Family Teaching.
- Obtain EEG electrodes, a machine and cap, and electroconductive gel.
- Remove jewelry and metal objects from the client's head.
- The client is positioned sitting with his or her eyes located about 1 meter away from the screen. One eye is patched.
- Scalp electrodes are placed in occipital, parietal, and midline locations.
- The client is instructed to focus the eyes on the screen.
- The chosen pattern(s) is (are) displayed in a rapid, flashing sequence as the client gazes at the screen, and a recording of VEPs is made. A computer signal average of the brain's electrical activity at a specifically chosen time after each stimulus is displayed.
- The other eye is patched, and the test is repeated on the opposite eye.
- Remove the electrodes and cleanse the scalp of electroconductive gel.
Client and Family Teaching
- The client must gaze continuously at a lighted screen of flashing patterns.
- Hair should be shampooed the night before the examination and should be free of hair spray or other hair fixatives.
- The test may take more than 1 hour.
Factors That Affect Results
- Results must be compared with the norms of the laboratory performing the test, as different patterns and variations of the test will be performed, depending on the client's history and the purpose of the test.
- Cataracts or a miotic pupil may increase the latency of the response.
- Female P-100 latency has been shown to be shorter than that of male latency.
- After 50 years of age, latency increases by about 2 milliseconds every 10 years.
- The client must be able to concentrate on the test. Breaking the gaze on the screen hinders the usefulness of the results.
- An acute migraine attack produces prolonged peak latency.
- Multifocal pattern electroretinography can help differentiate VEP delays caused by macular degeneration from delays resulting from optic nerve disease.