| 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 |
Today we're going to be talking about optic tract syndrome. So the optic pathway, as you know, starts in the eyeballs and then crosses in the chiasm-the nasal fiber crosses, but the temporal fiber remains uncrossed. The optic tract is a combination of the crossing fiber from one eye and the uncrossed temporal fiber from the fellow eye, and that travels through the geniculate body and then from the radiations to the occipital cortex. But today, we're only going to be talking about the optic tract. So as you know, any lesion behind the body of the chiasm produces a visual defect that is a homonymous hemianopia. In a patient who has a right homonymous hemianopia, that would indicate the left retrochiasmal pathway. So when you have a homonymous hemianopia, especially when it's incongruous, we know that we are further forward in the retrochiasmal pathway, and because it's a right homonymous hemianopia we're worried about a left retrochiasmal lesion. So now what we're trying to with the optic tract syndrome is figure out: are we in the radiations, the cortex, the geniculate body, or the optic tract? The two features that would help us determine whether we are in the tract are: number one because the lateral geniculate body is where the axon is heading to synapse, any lesion in front of the geniculate body will cause optic atrophy. The special form of optic atrophy that we're looking for in patients who have optic tract lesions is correlated to the homonymous hemianopia. So in the retina, this is the optic nerve, the fovea, the temporal fiber-in this case a superior temporal fiber, which doesn't go straight into the optic nerve. It goes all the way around the fovea and inserts into the top of the disc, so these fibers insert into the top of the disc. The inferior temporal fibers likewise go all the way around the fovea and insert into the bottom of the disc. Because the fovea is the center of the eye, anything on this side of the fovea is actually temporal and everything on this side is nasal. This means practically that this papillomacular bundle and these nasal fibers are both nasal. If we look at an optic nerve at the top, it's temporal fiber, the bottom is temporal fiber, this is papillomacular bundle, but nasal, and this is also nasal. So T, T, N, N-this is the fiber distribution of the optic nerve, based on this nerve fiber layer distribution inserting into the optic nerve. So in a patient with a right homonymous hemianopia-the right eye has the temporal field loss and the left eye has nasal field loss. In the eye with the temporal field defect, that's going to be nasal fiber loss, so the temporal field is coming from the right eye (which is the nasal fiber). In the right eye, atrophy will involve the nasal fibers-that is a bowtie or a band of atrophy across the right optic nerve in an optic tract lesion. Likewise in the left eye, which has the nasal field defect, the nasal field defect in the left eye is going to affect the temporal fiber in the left eye. It'll be the reverse pattern of atrophy-the temporal fiber will be out. In addition, because the tract carries the pupil fiber to the Edinger-Westphal nucleus in the midbrain, that pretectal nucleus is an afferent fiber. So we might get a RAPD-a relative afferent pupillary defect if we have a tract lesion because the pupil fiber is in front of the geniculate body. If we have a right homonymous hemianopia, because the temporal field is bigger than the nasal field, usually the RAPD and the tract lesion are going to be in the eye that has the temporal field defect-that came from the nasal fiber which came from the contralateral eye. In this particular example, it would be a RAPD because the temporal visual field is bigger than the nasal visual field. These comprise the three key elements of the optic tract syndrome. A homonymous hemianopia on the contralateral side that tends to be incongruous. There is going to be an associated relative afferent pupillary defect because we have a defect in the pupil pathway on the afferent side and because it's relative to the fellow eye the eye with the temporal field defect has bigger field loss-and that's the nasal fiber which is coming from the contralateral eye, in this example a right relative afferent pupillary defect. In addition there's going to be band atrophy. That atrophy band across the optic nerve is the nasal fiber, which also represents the temporal field, which in this example in the right homonymous hemianopia is the right eye. So if we have a RAPD, band atrophy, and a homonymous hemianopia you should be thinking the optic tract syndrome. |
