| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (BL) Class of 2022, Baylor College of Medicine, Houston, Texas |
| Transcript |
So we're going to talk today about visual field defects and specifically about homonymous hemianopsia. So if you got your eyeballs here they connect to the optic nerves and the crossing point is called the chiasm, which resembles the Greek letter chi, into the geniculate body, radiations, and finally to the cortex, the occipital cortex. So when we have a visual field defect that is behind this line, the chiasm, you are involving the crossing nasal fiber as well as the uncrossed temporal fiber, and that's what accounts for the field defect we call the homonymous hemianopsia. Homo means same, nymous means name, hemi means half, an is not, and opsia is see. So same side half no see - homonymous hemianopsia. And because, if you have a left optic tract lesion or any lesion in the retro-chiasm pathway, that will involve the nasal crossing fiber from the right eye as well as the uncrossed fiber from the left eye, and that's what produces this field defect and, in this case, a right homonymous hemianopsia. And we want to be looking at this field as if we're the patient. So the ride side half no see - a right homonymous hemianopia from the left optic tract lesion. But this same field defect could be caused by any lesion behind the chiasm in the retro-chiasm pathway including the optic radiations or the occipital cortex. All will produce a right homonymous hemianopia. There are some features that will separate out the homonymous hemianopia and we're going to go over those features now. The first is, in an optic tract lesion, not only is the visual pathway for the seeing involved in an optic tract lesion, but the afferent pathway for the pupil is also involved. So this afferent pathway for the pupil is trying to go from the optic tract to the efferent pathway for the pupil, which is the Edinger-Westphal nucleus, to the third nerve and then the ciliary ganglion to the iris. So in the dorsal midbrain we have what we call pretectal nuclei and these pretectal interneurons carry the afferent pathway for the pupil from the tract to the efferent pathway which is the third nerve via the Edinger-Westphal nucleus. And so if we find a right homonymous hemianopsia and have a relative afferent pupillary defect, that is a sign that the problem is in the tract. The other sign that suggests a right homonymous hemianopsia is a tract lesion is because we're in front of the cell body that this axon is trying to reach in the lateral geniculate body, any lesion that occurs before the geniculate body will cause retrograde degeneration, and that we'll be able to see in the eye nerve as optic atrophy. So the presence of optic atrophy and a relative afferent pupillary defect are the signs for the tract lesion and the specific name of the atrophy. we don't have time to cover right now, is called band atrophy. We'll save that for another day. If, however, you're behind the chiasm but you don't have a RAPD, and you don't have any optic atrophy, then we'll have to differentiate whether we're dealing with the radiations, the optic radiations of the temporal lobe or the optic radiations of the parietal lobe or the occipital cortex. And there are some features that help. So if this is a sagittal section of your brain, then from the geniculate body, the radiations travel superiorly in the parietal occipital region and inferiorly in the temporal lobe. These fibers of the temporal lobe, inferior fibers, are called Meyer's loop. So one of the features of homonymous hemianopia that would suggest it's temporal lobe is if the homonymous hemianopsia is denser superiorly. So in this particular example we have a right homonymous hemianopsia but it's denser superiorly, and it's not the same in the two eyes which we call incongruous. So radiation related field defects, because they're radiation out like this, are much more likely to be incongruous that is not the same in the two eyes. And the same thing for if you a hit a parietal radiation. The parietal radiations also will produce a homonymous hemianopsia, but the field defect will be denser inferiorly. So this we call pie in the sky in the temporal lobe, so right hemianopsia pie in the sky or it can be pie on the floor. They're incongruous if they're in the radiation. And finally the cortex, the occipital cortex, As opposed to the radiations where the fibers are very spread apart - that's why they're called the radiations - in the cortex the fibers are coming very, very close together and so we can make field defects that are very similar between the two eyes. And so that we cause congruous, when the field defect is the same between the two eyes we call it congruous. So if it's very congruous, that suggests its occipital cortex. In addition because a lot of real estate in the occipital cortex is being devoted to the central macular fibers if we have a field defect that spares the macula so this little bit of macular sparing here of the field that macular sparing means that the tip of the occipital cortex is spared and that is also a sign that it is an occipital lobe lesion if we have macular sparing or it's very congruous. And the last thing that you should know about occipital cortex lesion is that even though we've drawn the fields as a circle, the field is really an oval shaped field because the nasal field is smaller than the temporal field. That is, there's no correlate for this portion of the temporal field and the nasal portion of fellow eye. So when we have a right and left eye this portion of your left eye in this example this is a temporal crescent this temporal crescent is monocularly represented in your cortex. And because it's temporal field, you know that's a nasal fiber. So the nasal fiber carrying the temporal field is travelling across to the geniculate in the radiations, and that temporal crescent is represented right here in the anterior portion of the calcarine cortex. The most anterior portion of calcarine cortex in the occipital lobe is where the monocular temporal crescent lives. And so what that means is if you have a homonymous hemianopsia again on the right, if we have sparing of this temporal crescent, that temporal crescent sparing implicates occipital lobe. You could also have just selective involvement of this temporal crescent although that's rare. So the three features that suggests occipital congruous macular sparing temporal crescent. So if you just apply the following rules you'll get through homonymous hemianopsia. The legion is homonymous. It's on the other side. It's behind the chiasm. If you have a RAPD and optic atrophy that's the tract. If it's pie in the sky, it's Meyer's loop temporal lobe. If it's pie on the floor, parietal floor. They tend to be incongruous. But if it's congruous, macular sparing, or you have a temporal crescent involvement or sparing, think occipital. |
