| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (JS) Class of 2021, Baylor College of Medicine, Houston, Texas |
| Transcript |
So, just like the VEP talk, we can also use electrophysiology to measure the retinal function and an electroretinogram. But unlike the VEP, the ERG actually doesn't require that the patient participate, so we're just measuring the electrical activity. We don't need the patient to look at anything or say anything; we can, we can just measure the electrical activity of the retina against the cornea. So, the electrodes in ERG are on the front of the eye, versus VEP where we're measuring the potential occipital cortex. So we got different types of ERGs. We got a full-field ERG, and a full-field ERG is a total mass response, so it's when we have complete retinal whiteout. It's really a reasonable test, because it's measuring the whole function of the retina. And then we've got multifocal ERG, and multifocal ERG is multiple stimuli at different locations. And so we can measure focally and multifocally the function of the retina in different parts of the retina. In a full-field ERG, that's a mass response. So the diseases like retinitis pigmentosa, where we have a diffused wipeout of the photoreceptors, that's going to have a flat ERG. Or if we have it acquired rather than a hereditary process, carcinoma-associated retinopathy (CAR), that's going to have a flat ERG. However, if we have an ischemic event, like an ophthalmic artery occlusion, that will also produce a flat ERG; but if we have a retinal artery occlusion, like a central retina reclusion, that will only affect some parts of the waveform. So the a- and the b-wave are the important components of the ERG, similar to the negative and positive deflection waves and the VEP; these are representing actual function of photoreceptor elements and the bipolar cells. And so when we have a CRAO (a central retinal artery occlusion), but we don't get to see the cherry red spot, because it's months or weeks later, they just have a nerve that's pale, we can use the ERG to detect whether the b-wave is intact, which would tell us, oh look, we have an a-wave, but we have no b-wave. And that would be the finding that we had ischemia that preserved the choroid perfused part, but the non-perfused part (the inner retina) was starved of oxygen in central retinal artery occlusion, and that's going to cause the whole thing to be flat or the b-wave to be down. Likewise, we can use it for melanoma-associated retinopathy, which is like cancer-associated retinopathy, except the target again is bipolar cell, and so that would also cause a decrease b-wave. So multifocal ERG is really great when the problem is focal; for example, it's in the center part of your vision, a central scotoma, and we're trying to figure out if that's a cone dystrophy or not. If we're looking for a prior arterial occlusion; if we're looking for cancer or melanoma-associated retinopathy, or if we think the retina is the actual problem and the photoreceptors are damaged (vitamin A deficiency, retinitis pigmentosa). So for neuro-ophth, we're really using it in unexplained vision loss to establish whether or not it is the retina, and not the optic nerve, that is the causing the vision loss. We can use it in a full- field fashion, when we have diffuse retinal disease; or a multifocal fashion, when we're looking for focal disease. And you can tell from the waveform whether we're dealing with inner or outer retina or the bipolar cells. So ERG is still useful for non-retinal conditions, but in neuro-ophth, we're looking for neuro-ophth causes that are actually retinal. |
