Visual Evoked Potential (VEP) has been used for over 50 years in the clinical management of patients with a variety of vision problems. It was not until the last 10 years, however, that visual electrophysiology devices were made consistently available to the office-based eye care professional.
VEP test results give ophthalmologists and optometrists objective, functional information about the entire vision system. These test results often have a few components. Here is a quick review of the most important section: the pattern-reversal VEP waveform.
Visual Evoked Potential (VEP):
VEPs are electrical signals that are a measure of the electrophysiological activity at the visual cortex. Test results are a representation of the functional integrity of the visual pathway including the anterior segment, retina, optic nerve, lateral geniculate nucleus and visual cortex.1
A typical pattern-reversal VEP graph response will primarily consist of the N75-P100-N135 Complex. In a healthy vision system, the first major negative peak occurs around 75ms (N75), or 75ms after the pattern onset. The first major positive peak occurs around 100ms (P100), and the second major negative peak around 135ms (N135). Based on pathologies, the waveform shape, amplitudes and latencies of these components may change.
Amplitude, measured in microvolts (µv), indicates the amount of electrical energy reaching the visual cortex. Amplitude is calculated as the absolute difference between the N75 and P100 microvolt measurements.
Generally, amplitudes are a measure of “how much” information is reaching the visual cortex. For example, the number of retinal cells being stimulated will affect the amplitude. Amplitude will also vary with the vision system’s ability to process a particular size stimulus. An increase in amplitude typically correlates to a better ability to discriminate between different sized objects. The amplitude can be larger (better) or smaller (worse) depending on the disease state, treatment or refraction. In general, as the patient has more difficulty seeing the stimulus, a smaller electrical response will be evoked, thus creating smaller amplitudes.
P100 Latency (ms):
P100 Latency (Peak Time), measured in milliseconds (ms), indicates the time the electrical signal takes to travel from the retina to the visual cortex. Latency measurements can reveal issues affecting vision if it takes more time for the electrical signals to reach the visual cortex.
Anything that impedes conduction of the electrical signals can cause increased latencies, such as issues affecting the myelin sheath of the optic nerve like optic neuritis in multiple sclerosis. Latency measurements tend to have less variation than amplitude both within-subject and between subjects.2
The combination of amplitude and latency is helpful in determining the health of the visual pathway. VEP results can be used as part of the constellation of patient history, clinical signs and symptoms that lead to a diagnosis.
Visual Evoked Potential results are powerful because they are a representation of the functional integrity of the entire visual pathway, from the anterior segment and retina, all the way to the visual cortex. Reading the N75-P100-N135 Complex of a healthy vision system is often pretty straightforward. Building on the basic understanding of amplitude and latency will help with reading waveform results from patients with pathology. For support, Diopsys offers several educational resources for customers to advance their knowledge of visual electrophysiology.
1 Odom JV, et al. (2016) ISCEV standard for clinical visual evoked potentials – (2016 update). Doc Ophthalmol 133(1):1–9
Are you curious about how a Diopsys® VEP test is run? Check out this video: