||Andrew G. Lee, MD, Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, TX; Professor of Ophthalmology, Weill Cornell Medicine; Matthew Miller, Baylor College of Medicine Class of 2023
||I just want to talk to you a little bit about degeneration, and in this case, Wallerian degeneration. And as you know, the axon is connected to the cell body, and then it's connected to a synapse, to another cell body. And so what we have is, whenever you have a disruption in the axon, you're gonna have disruption of intra-axonal plasmic flow, and that will kill the cell body unless it can recover. So if we have damage to the axon for whatever reason - trauma, demyelination, ischemia, a tumor-the dying-back will occur in both directions. And so we have this correct direction which is anterograde degeneration, and we have retrograde degeneration. So the cell body to the cell body is where the damage is going to occur. And the reason that's super important to know is, the axon of the optic nerve is very long, and so it starts as a cell body in the retina, which is the retinal ganglion cell layer, and then that axon enters into the optic nerve, and then the nasal fiber crosses and the temporal fiber doesn't cross, and to the genicuate light body. And what that means is if you have optic atrophy, which we can see inside the eye, that lesion could be in your retinal ganglion cell, like glaucoma; or it could be in the optic nerve, in the intraocular, intraorbital, intracanalicular, or intracranial portion; it can be at the chiasm; or can be in the optic tract. But once you get past the geniculate body, the cell body is in the geniculate, and therefore occipital lobe lesions and parietal lobe lesions and temporal lobe lesions do not result in Wallerian degeneration that we can see, even though there is transsynaptic degeneration that we can see when we do macular ganglion cell layers on patients with occipital lobe lesions. And that we call transsynaptic degeneration. So we have Wallerian and transsynaptic degeneration. But ophthalmoscopically the only thing we can see is if we're in front of the geniculate. And what that means for us clinically is, if I say to the resident this patient has optic atrophy, where's the lesion, they should not say optic nerve. They should say if you have optic atrophy Dr.Lee, that lesion could be from anterograde Wallerian degeneration from a lesion in the retina, or it can be retrograde anywhere from the geniculate body, tract, chiasm back to the optic nerve.And that is the basis of our superpower. We literally can look in people's eyes and see the nerve is pale and predict there's going to be a lesion somewhere in the pathway. And because the pattern of the optic atrophy is dependent on the nerve fiber layer, the type of optic atrophy that we're going to be looking for is called band atrophy. So this band optic atrophy results from nasal fiber loss. So in the optic nerve-superior temporal, inferior temporal fiber- the nasal fibers run like a band across the optic nerve. And you should watch the retinal nerve fiber layer video if you don't have that basis. But once you get that foundation then we know that if you have a lesion here, at the chiasm, that will produce a bi-temporal hemianopsia. And because the bi-temporal hemianopsiais from the nasal fiber, if we have atrophy, because we're in front of the geiculate body, that will cause band optic atrophy, in both eyes, in a bitemporal hemianopsia, from a chiasm lesion, due to involvement of the nasalcrossing fiber in both eyes. Likewise ifwe have a lesion in the optic tractrather than the optic chiasm- so in this example, right optic tract -right optic tract will produce a left incongruous homonymous hemianopsia. And if we have a left homonymous hemianopsia from a right optic tract lesion, the left eye has temporal field defect that is hemianoptic, and the right eye has nasal field loss, and the eye that has the temporal field defect will have the band atrophy of the nasal fiber. In the eye that has the nasal field defect, that will be the temporal fiber and it will look like an hourglass. So if we have band atrophy in only one eye and a homonymous hemianopsia, that lesion is in the tract. If we have band optic atrophy and a bi-temporal hemianopsia, that lesion is in the chiasm. And atrophy helps localize, and optic atrophy in general can be either anterograde or retrograde axonal loss from Wallerian degeneration. And knowing that information is a true superpower, because a look allows us to look in people's eyes and tell where the lesion is.