| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (BM) Class of 2023, Baylor College of Medicine, Houston, Texas |
| Transcript |
Eyes here, optic nerve, the chiasm, the crossing point, the optic tract, and then we have the geniculate body. And so, today we are going to be focusing on the geniculate and behind. If you want more information on the optic tract you can watch the different video that's assigned to just that topic. So, when we're behind this line as you know we get a homonymous hemianopsia. Hemi (half), an (not), opsia (see): "don't see half"; and homo (same), nymous (name): "same name half no see"-a homonymous hemianopsia. And so, if this is the right and this is the left side, any lesion behind the chiasm on the left side will produce a right homonymous hemianopsia. So, from the retrogeniculate pathway we've got the optic radiations. And so, as you know the radiations travel both in the temporal lobe as a Myers loop and in the parietal lobe which doesn't have a name. So, in the temporal lobe the Myers loop (Myers loop), contains the inferior fibers. And that means in a temporal lobe lesion, our homonymous hemianopsia will be denser superiorly, sometimes colloquially referred to as "pie-in-the-sky". So, a temporal lobe lesion causes "pie-in-the-sky" homonymous hemianopia because of inferior fibers in the Myers loop. And similar, the parietal lobe lesion produces a field defect that is denser inferiorly so that the "pie-on-the-floor" in the homonymous hemianopsia. So, when we have a homonymous hemianopic field defect, we can be denser superiorly or we can be denser inferiorly. And optic radiations lesions tend to be incongruous. And the reason they're incongruous is it's hard to get the radiations to damage the pathway in the exact same way because the fibers are radiating out that's why they're called the "radiations". And so, damage in the radiations causes a homonymous hemianopsia that is not the same in each eye, and that we call "incongruous" (not the same in each eye). So, the congruity refers to the shape of the field. But as we're getting closer and closer to the occipital cortex, in the occipital cortex the fibers are getting very very very compact. And so, lesions in the occipital cortex tend to produce field defects that are homonymous hemianopsias but are very congruous so you could literally just cut one field out and paste it on the other. And that congruity is one of the features of a retrogeniculate body pathway lesion in the occipital lobe. There are two more features of the field that you need to know about with occipital lobe lesions: one is because the tip has a dual blood supply and also the occipital tip has a large representation in the visual field. Field defects that are produced by occipital lobe lesions often produce a specific finding called macular sparing. So that sparing of the tip is a tip-off that you have an occipital lobe lesion because the tip is represented by a large area and has a dual blood supply. And so, macular sparing is a sign in the homonymous hemianopic field that we might be dealing with a occipital lesion. And then finally, even though we often draw the fields as circles, they're really not circles they're ovals like this. And so, the nasal portion of the field is not as big as the temporal portion of the field. So, the temporal field is actually slightly larger than the nasal field. It's not really a circle, and the amount that it's larger is this temporal crescent right here. So, the field of the temporal portion of the field is bigger than the nasal, and there's no correlate of this temporal crescent in the contralateral nasal field because they're not circles, they're ovals. And that temporal crescent is represented in the most anterior portion of the occipital cortex on the contralateral side. So, if we have a field defect that is homonymous but spares this temporal crescent, or we could have a field defect that is not homonymous and just involves the temporal crescent (so, sparing or involvement of that temporal crescent, the piece that's not got any correlate in the nasal field), also suggests that this is occipital. And as opposed to a lesion in your retina, number one you would see a lesion in the retina in that temporal crescent lesion and in the temporal crescent from the eye there's going to be an RAPD in this involved eye. So, a temporal crescent with no correlate in the nasal retina on the fundus exam and no RAPD strongly suggests that it is occipital and in the most anterior at portion of the occipital cortex on the contralateral side. And that temporal crescent has no correlate in the nasal field because the temporal field is bigger and it's bigger by exactly the temporal crescent. So, in summary, in retrochiasmal lesions you're going to get a contralateral homonymous hemianopsia. You'd like to know if it's denser superiorly or inferiorly ("pie-in-the-sky", "pie-in-the-floor"). You'd like to know whether the lesion is congruous or incongruous, whether there's macular sparing or macular splitting, and whether you have preservation or involvement of the temporal crescent. |
