Introduction to Visual Electrophysiology

Hello, my name is Quentin Davis and I’m the Vice President of Operations at LKC Technologies and today I’m going to talk about an introduction to visual electrophysiology.

Your brain works on electricity. All your senses (sight, sound, touch, smell, sense, balance), all of them have to be converted from those physical items into an electrical response that can then be interpreted by the brain. Visual electrophysiology uses some kind of light stimulus and then measures that naturally generated electrical response to the light stimulus.

If you record a visual electric electrophysiology response in under or on the surface of the eye, then you’re measuring an electroretinogram (ERG), the response of the retina to that stimulus of light. If instead you put your recording electrode on the back of the head, then you’re measuring a visual electrical potential (VEP) from the visual cortex. And not only by changing the location of where you record from, but also by changing the light stimulus, the test can be made sensitive or insensitive to issues involving refraction, cataracts, moving further into the eye, the rods and the cones, the photoreceptors, the bipolar cells, which receive the information from the photoreceptors, the retinal ganglion cells, which are also in the retina, which receive the information from the bipolar cells, and those retinal ganglion cells also form part of the optic nerve, going to the optic chiasm, and finally to the visual cortex. All these different areas of the visual system can be either made sensitive to or insensitive to in visual electrophysiology testing.

Some common ERG and VEP light stimuli include a full field stimulus, here’s a miniature handheld ganzfeld called RETeval^®, here’s a bigger ganzfeld for two eyes called a SunBurst, part of our UTAS system, and those full field stimuli illuminates the eye uniformly for a luminance stimulus.
You can also have a pattern-based stimulus like this checkerboard pattern that alternates which squares are white with squares are dark. For a contrast stimulus you can also go from a checkerboard to a uniform gray.
And you can also have a multifocal stimuli where you can generate either a luminance or a contrast stimulus that are presented in such a way as to be able to localize the response of the visual system, similar to how visual fields can localize the response.
We also have an EOG, an electrooculogram, which directed eye motion can be measured along with a visual stimulus in order to measure the retinal pigment epithelium (RPE) function.

A little bit about the ERG waveform and its origins. You can have light coming in, it hits the retina, the light goes through the retina to the photoreceptors. And then what’s shown here is time versus voltage; so this is similar to an electrocardiogram where you’re measuring this voltage with respect to time. And this first response here, this negative deflection, is driven primarily by the photoreceptors and also the off bipolar cells. The photoreceptors pass the information to the bipolar cells, which is represented functionally by this B wave, the first positive deflection, and then the bipolar cells pass information to the retinal ganglion cells, for this photopic negative response here, which is this which is this negative deflection here. You can get kind of all that information from an ERG waveform: from the photoreceptors to the retinal ganglion cells. And just as an example of another waveform here you can see this attenuated response and that attenuated response could reflect some disease state.

There are different flash stimuli that can be used for different purposes. If you first dark adapt the person, you can measure rod function with a dim white flash, where this is a response solely from the rod system. You can get a combined rod-cone response, where you can get a response that’s from the cone system and the rod system, where the early parts of the response are from the photoreceptors and the later responses are from the bipolar cells. If you’re not interested in rod function because you either have some cone dystrophies or some progressive diseases, then the cone functions you don’t have to dilate, it even provides you with a background light to help saturate out the rods. And you can get single flash responses that look like this, or responses to flickering light that look like this. Here is the response I showed you earlier with this photopic negative response, where this part here is sensitive to the retinal ganglion cells. And there are other more specialized tests that you can run. Tests that are sensitive to the functioning of on bipolar cells and off bipolar cells separately. You can test amacrine cells with an oscillatory potentials. You can look for things like enhanced S-conde syndrome with an S-cone test. And the RETeval device has a dedicated diabetic retinopathy assessment test looking for that disease in particular.

What are some of the general uses for visual electrophysiology testing? First, there’s just basic research: What are the mechanisms of vision? What are the origins of some of the psychophysical phenomena that we see? Are some of them originating in the retina or are they all in the brain?

You can also use visual electrophysiology testing for therapeutic research, like drug trials. They’re used in human clinical trials both for eye treatments, where it could be a performance indicator, or for other ones where it could just be looking for safety profiles. It can also be used earlier in the drug discovery pipeline testing, like mice or other non-human animals, to see if you can measure some benefit from the drug.

In veterinary uses, people like with pet dogs or other animals, that might get a cataract. You might want to test to make sure the retina still is functioning before you proceed with a cataract surgery. Or it could be used to look for inherited diseases or sudden vision losses.

And of course for human clinical use. It can be used in three different ways: it can be used as an aid in diagnosis of some visual pathway disorder, it can be used to monitor disease or its treatment, if I’m treating someone for glaucoma it could be used to see whether this treatment is actually helpful or not, and it can also be used to aid in predicting disease progression. There’s been papers published for using that for progression of inherited diseases, like retinitis pigmentosa, as well as acquired diseases, like CRVO.

Why would you use an ERG at all when you have fundus imaging, OCTs? Why would you bother to use an ERG? There are many times when anatomy doesn’t tell the whole story. For example if you look at this OCT or this fundus image, you’re thinking this person is totally blind, this is a terrible looking retina. When in fact this person had 20/60 vision, the ERG was normal, this is almost good enough vision for them to drive without corrective lenses. It’s really not that bad. This is an example of North Carolina macular dystrophy. Digging in a little closer, you could imagine that this is a cartoon of the membrane in the rod and the membrane in the bipolar cell and you have these ion channels that are passing messages from the rods to the bipolar cells. And if you have genetic mutation and any of these proteins here, you could imagine that you could greatly affect the ion channel’s efficiency. But that’s a thousand times too small, maybe a million times too small, for it to be visible in an OCT or a fundus photograph. Everything can look great macroscopically, but when you get down to the ion channel, it just doesn’t work and that’s why anatomy doesn’t tell the whole story.

Another issue with imaging techniques is cataracts. And if you have a cataract, you can’t see through it. The OCT doesn’t work, the fundus photography doesn’t work, whereas the ERG is is largely independent of media opacities, if you pick the right ERG technique.

You can have functional changes that occur early, much earlier than seen in structural tests. There are publications on like five years out from seeing retinal nerve fiber layer thickness changes. You can have changes in your ERG and in glaucoma subjects.

And another reason to use it is the functional parameters and high predictive value. And that’s kind of together or alone from fundus imaging. In diabetic retinopathy there’s papers published showing that used by itself, it is the most predictive parameter for who’s going to need an ocular surgery.

What are some of the indications where an ERG is helpful? In the medical retina area, it’s useful for diabetic retinopathy, central retinal vein occlusion, retinal detachments, cancer-associated retinopathies, for the optic nerve, for glaucoma, optic neuropathies, optic atrophies. For small children, if you have inherited diseases. Maybe the child has some nystagmus. You want to figure out “Do they have a retina disorder or not?” There are other inherited diseases, such as retinitis pigmentosa, X-linked retinochisis sciences, CSNB, Lebers, or other kinds of unexplained vision loss that the ERG can help provide some insight to what’s going on. And there are diverse other applications as well, for example, assessing the retina before cataract surgery, vitamin A deficiency in people with getting bariatric surgery when you lose a lot of your stomach, that’s where the vitamin A is absorbed, so maybe a decade after the bariatric surgery, they could start losing their night vision due to vitamin A deficiency. And then there’s toxicity monitoring, for example hydroxychloroquine.

How do you interpret the ERG? It’s kind of like an electrocardiogram, where there is time on the x-axis and voltage on the y-axis. This can be the response from a flicker ERG and LKC provides a nice rectangular box showing you where this peak is expected to occur kind of 95 percent of the time for someone with normal vision. A diseased result could look like this, where it’s delayed and smaller amplitude and this could indicate that something is going on with the retina.

Here’s another example of how you can interpret the data. This is being plotted as time and amplitude of that ERG shown on the last page; the black circles are people with normal vision, the red circles are ones with inherited diseases. And you can see that the inherited diseases have lower amplitudes and longer times

We have this inbuilt reference data that you can use. It’s nice and color coded for are these peaks inside the expected range for both in time and an amplitude. Where they’re only colored red, if you have an extended time or a reduced amplitude. If you have a remarkably fast time than most normal people, that could just be an indication that you’re a professional ping-pong player or some other kind of Olympic athlete, where reaction time is important. And this color coding is based on reference data collected in 2 countries, from 6 trial sites, using over 400 healthy subjects.