Andrew G. Lee, MD, Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, TX; Professor of Ophthalmology, Weill Cornell Medicine; Dhanatcha Sadetaporn, Baylor College of Medicine Class of 2022
Dr. Lee lectures medical students on visual evoked potential (VEP).
We'll be talking about visual evoked potential, and it's exactly what it says it is. It's the electrical potential that is evoked by a visual stimulus. So a visual evoked potential measures the potential from the signal coming into the eye to the retina, optic nerve, chiasm, the tract, radiations, and all the way back to the occipital cortex. So we cannot tell if someone's VEP is decreased where the problem is. We only know that the signal is not being sent properly across the entire visual afferent pathway. And so the way that we use the VEP in the clinical setting is to try and establish what level of vision does the person have. And as you know most of the visual evoked potential is actually your macula so when we're measuring the VEP, it's really measuring a lot of the central vision. We're not really gonna use it for testing visual fields for example. So if someone has homonymous hemianopsia their VEP might be totally normal because you're only measuring the center part of the vision.So in our hospital we use different sized checkerboard stimuli and those stimuli correspond to different levels of visual acuity. So the smaller the stimulus size the closer we can approximate what acuity would be necessary for them to see that check size. So if we make the check very big, medium, or small we can see what they can see because we can measure the evoked potential in the occipital cortex produced by the visual stimulus and that's why it's called the visual evoked potential because the electrical potential that we are measuring in the occipital cortex was actually evoked by a visual stimulus and in this case a specific check size. We can make sure that the patient is fixating on the target to prevent them from not looking by having them put a little laser pointer right in the center there so we can make sure that they're fixating because if you look off to the side obviously you can degrade the VEP. So extracting the evoked potential is like multiple trials have to be done. So when we do enough trials we can make the background electrical activity go to zero because it'll eventually cancel out. So we'll have a negative deflection and a positive deflection. That positive deflection occurs at 100 milliseconds. And so the sum of multiple trials is what we're seeing when we see the visual evoked potential. And over time all the background noise just cancels itself out. So it's like an EEG in terms of measuring the electrical activity but in this particular example we are measuring only the evoked electrical activity in the occipital cortex and we do that by summating over multiple trials. So a visual evoked potential can be just a flash of light. So a flash VEP and that only tells you that lights are on or lights are not on. Or it can be checkerboard with a specific small pattern and I like the pattern. And so if someone has a normal 20/25 VEP but they're claiming that they can only see hand motions so we know they have nonorganic overlay. If someone has no light perception but we can record a 20/100 VEP that also suggests nonorganic overlay. But if someone is claiming that they're 20/50 and their VEP is in this range you cannot tell that they're telling the truth. You can only tell when people are seeing better than they actually claim. You cannot tell if they're seeing worse because they can just defocus or not look at the target. There's 10 million ways why you could defeat this. So if it's abnormal it suggests it's organic but does not prove it's organic. But if it's normal and disproportionate to their acuity loss that actually does prove they're nonorganic. And so that is how we use the VEP. If they have an optic neuropathy I would not do a VEP on that person because you already know that they're organic and they have an optic neuropathy. So we really don't use it for many many things other than just determining what the patient's visual potential is. Some people use it to measure latency so the… Positive wave at a hundred if it's delayed to 110 or 120 milliseconds that might suggest demyelination so you could use it for looking for another lesion separated in space and time in the frontal eye for example. But in general this is the way I use the VEP.
Spencer S. Eccles Health Sciences Library, University of Utah