| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (SV) Class of 2023, Baylor College of Medicine, Houston, Texas |
| Transcript |
So we're going be talking about central scotoma. Scotoma is obviously a visual field defect and in this case it's in the center of our vision so the first thing we need to know is: it is it unilateral or is it bilateral? When it's bilateral central scotoma, we need to make sure it's not the macula or the papillomacular bundle. If it's the macula, that's red, and normally OCT can answer the ques-tion of macular disorders producing a unilateral or bilateral central scotoma. A macular hole, an epiretinal membrane, anything that's in that macula we'll normally be able to see if it is produc-ing a central scotoma on macular OCT. If it's not the macula and its optic nerve, the differential diagnosis for central scotoma that's bilateral is quite limited, and what it looks like on visual field is it's literally a central scotoma, a black spot right in the center of your vision. Now some-times the black spot extends to and involves the blind spot and that we call a cecocentral scoto-ma, but a central scotoma and a cecocentral scotoma have the same localizing significance. It's either macula or it's papillomacular bundle if it's an optic neuropathy. And so even though the chance of this being a compressive lesion is low, I usually recommend that we do an MRI of the head and orbit with gadolinium on every patient who has a central scotoma, regardless of wheth-er it's unilateral or bilateral, assuming the OCT doesn't show it's macula. If it's bilateral, then we're thinking about systemic things; toxic and nutritional are the most common and the most common toxin that we see is alcohol, so we can have toxic nutritional am-blyopia, a toxic nutritional optic neuropathy from alcohol consumption, and that usually leads to B12 and folate deficiency. So toxic and nutritional tend to run together in terms of ethanol be-cause it is B12 and folate are the vitamins that the papillomacular bundle needs the most to main-tain its metabolic activity. So when we have a central or cecocentral scotoma it's the highly ac-tive papillomacular bundle that feels the metabolic derangement first. And so when we do B12 and folate we also have to consider the addition of long-term tests for B12, which is methylma-lonic acid and plasma homocysteine. So in methylmalonic acid, that is involved in the conver-sion of the methylmalonic acid to succiny-coA, the entry molecule into the Krebs cycle, and so this is like a hemoglobin A1C of our B12 level. You cannot rely on the spot B12 or if the B12 level is borderline like 200 or 300 or 400, we will still do MMA and homocysteine. If the homo-cysteine is elevated but the MMA is normal, that usually suggests that it's folate and not B12 de-ficiency. Homocysteine is converted to methionine and that is a folate and B12-dependent path-way. The conversion requires methylene tetrahydrofolate reductase. The enzyme that assists in that conversion of dihydrofolate to tetrahydrofolate is a folate-dependent pathway. And so if we have elevations of both, we're really thinking about B12. If we just have homocysteine, folate. You need these to differentiate these because treatment of either condition with the other will mask the hematologic abnormality without improving the neurologic abnormality. So we really need to know: is it B12, folate, either, neither, or both? The hereditary thing that we're worried about with the central scotoma is Leber's hereditary optic neuropathy, and that's usually in a young male. That's a mitochondrial disorder; you can check out the youtube on the Leber, which is separate. And in the autosomal dominant form, which is also mitochondrial even though it's inherited in autosomal dominant fashion, optic atrophy genes, including the OPA one series. And the toxins in the acute setting are methanol, so that's alcohol but it's mixed with wood alcohol, methanol that's moonshining, or your drink has been poisoned for some reason or you're trying to kill yourself or someone's trying to kill you. And the other toxin that we'd be worried about are the medications that we are giving patients. So normally, the prototype is ethambutol, which is used for Mycobacterium including tuberculosis and non-tuberculous Mycobacteria family, so ethambutol is the central scotoma prototype for toxic optic neuropathy. If it's a unilateral central scotoma that could be any optic neuropathy, and so we still need to do the MRI scan on that person, and we're going to be doing the usual sus-pects. You can check out on the other video about optic atrophy what tests we would do: syphilis serologies, NMO, MOG, the other tests for an optic neuropathy that is unilateral or bilateral, which we call the usual suspects. So, when you're dealing with a central scotoma, you'd like to know if it's unilateral or bilateral. If it's bilateral, the hereditary conditions (Leber's, OPA one) and toxic nutritional. In the acute setting: methanol, and the chronic setting: alcohol. B12, folate, MMA and homocysteine, the hemoglobin A1C of B12, do an MRI scan and I usually treat the patients empirically with B12 and folate while I'm waiting for the tests if there's any suspicion that they have a nutritional deficiency. |
