| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (SP) Class of 2023, Baylor College of Medicine, Houston, Texas |
| Transcript |
So, I'm going to talk to you today about non-organic vision loss. And so that's on the afferent side, and one of the overarching things we need to know is that they can't have anything structurally wrong with them. So, that means they really can't have an RAPD or an organic cause for whatever they're complaining about. So, the first thing that we have to establish is that they have a normal exam or that the exam does not equal their complaint. Once we've established that they don't have an RAPD, they don't have optic atrophy, they have a normal exam, then you must prove that the person is non-organic before labeling them as non-organic. So, it's a two part thing: they have to have a normal exam, and then you have to prove that they see better than they claim. So, the ones that are easy, the easy ones, are if I have no light perception in only one eye but I'm 20/20 in the other eye. Well, you better have an RAPD here. So, the absence of RAPD in this scenario where it's markedly asymmetric really means that it's non-organic if you don't have the RAPD. However once you start getting to levels of vision that are like 20/100, there might not be an RAPD, or if it's bilateral (both eyes are 20/100, then there's no "R"), then we might not have an RAPD, or if the acuity loss is from the macula and not in the afferent pupillary pathway (like in a macular hole or macular edema or if they have a cone dystrophy), they might not have an RAPD. If the lesion is cortical or retrogeniculate (that's behind the pupil pathway), and so they might not have an RAPD. So, these people might have juxtaposed homonymous hemianopsia where they have a bilaterally occipital defect, and they're cortically blind. So, the absence of the RAPD only is helpful when you have markedly asymmetric or unilateral vision loss. If it's bilateral or not that bad or it's cortical, the absence of the RAPD does not prove that the person is non-organic. So, when we're dealing with patients who have asymmetric acuity like 20/100 and 20/15 vision in the fellow eye, that's a lot easier to get that person to show you that they actually see better, and one of the ways we do that is with a prism. So, we'd have a four base down prism, and we put that prism in front of their good eye, and we tell the patient that "this is a prism. It's going to bend the light" (which it does), and if this eye is 20/100 and this eye is 20/15 and they're looking at a 20/20 letter, when we put the four base down prism over the good eye, we're gonna ask this patient, "what do they see?", and if they see two "E's" then not only have you measured the acuity in the eye that they think you're testing, that is actually measuring this other eye as well. So, we can tell that someone actually has 20/20 vision as soon as they say they see two "E's." The organic response when looking at a 20/20 letter (if this eyes is 20/100) is to only see one "E." So, when we ask the patient what do they see, if they see only one "E" that also suggest it's organic. If they see two "E's", that suggests it is non-organic. In addition, the most common visual field defect we see for non-organic vision loss is a constricted field, so normally it's like a tunnel. So, it's like a five degree field (a tunnel field), and the reason you need to know this field is it's also relatively easy to prove that you're non-organic. Because as you know, if you're looking at a target, your visual field expands the further away you are form the target. So really, the field is a funnel because that angle subtended is the same, and so, if someone has a five degree field at one meter, and we retest that person at two meters, and it's still a five degree field, then that is a non-organic tunnel field because it really should expand and be a funnel field. In addition, we can put targets out into the supposed blind eye field by checking their eye movement, and we would tell the patient, "just say when you the finger" and if you put it in the five degree they'll say "yes", and then you say, "okay look at my finger," but if you put it out here and they look at the finger then that means they're not really blind out there. So we're using the saccade testing. If they can saccade to the target, that means they actually can see out there. So accurate and normal saccades into a blind field that you've established as a five degree island also is a very good, fast test to establish that someone with a tunnel visual field is non-organic. So, you need to prove the person is non organic before calling them that. You have to have a normal eye exam. Use the RAPD, but be aware that bilateral and retrogeniculate body lesions don't actually have RAPD's. You need a four base down prism (a vertical prism) put over their good eye. It's a very quick and easy test that tells you what the other eye is doing, and you should test tunnel versus a funnel field but one meter and two meter testing, and also doing saccades outside of their claimed visual field. |
