| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (PT) Baylor College of Medicine, Houston, Texas |
| Transcript |
"So today, we had a question about third nerve palsy versus isolating Edinger-Westphal nucleus. It's a very interesting question and I would encourage you to put any questions you have into the comments, so if you want me to do a video for you, I'll do one just for you as long as it's appropriate for neurop, and this is an interesting question. So as you know, the third nerve nucleus arises from the brainstem and it starts in the midbrain. So the third nerve, just like all the cranial nerves, has a nucleus. That nucleus starts in the midbrain. And then its fascicle is the third, that's the peripheral nerve, and then it gets to the ciliary ganglion, and then to the iris and the eye. So that's pretty much the pathway of the third nerve. However, the Edinger-Westphal nucleus is inside of the third nerve nucleus, and it is receiving two types of input: input for the light pathway and input for the near pathway. They are separate, but combine and have a final common pathway along the Edinger-Westphal nucleus and the third nerve. So the light pathway - as you know, the light hits the retina and then you've got the crossing and the optic chiasm, and off the tract before the lateral geniculate body, that pretectal nucleus is receiving pretectal input. And the tectum is this roof of the fourth ventricle, so that input goes to both Edinger-Westphal nuclei. So there's a afferent fiber right here in the pretectal area; that afferent fiber carries the light signal to both pupils, and that accounts for the direct response - you shine a light to the right eye, the right pupil constricts - but also the consensual response, because both Edinger-Westphal nuclei are receiving the pretectal input from the pretectal afferent pathway. So we have a direct response to the pupil and a consensual response. In addition, there's a way that we can activate this Edinger-Westphal nucleus from above, and so if we're looking at the midbrain here - here's my third nerve, superior colliculus, inferior colliculus, tectal plate, Edinger-Westphal, third nerve - we can talk to this Edinger-Westphal nucleus from the near pathway, and I don't need the light pathway. I can just tell the Edinger-Westphal nucleus to constrict the pupil for near. That's part of the accommodative convergence miosis pathway that is syn-kinetic when we look at something at near. The accommodative pathway. And so lesions right here can cause a dissociation between the light and the near pathway. So if we have a lesion right here in the dorsal midbrain, it can disconnect the light pathway, but the near pathway is preserved. So that is called light-near dissociation. And because you have the dissociation only to the Edinger-Westphal nucleus, that will not have all the other features of third nerve palsy. There'll be no ptosis, there'll be no motility deficit. It's an isolated pupil problem. So the way to tell the difference between third nerve palsy and Edinger-Westphal is not the reaction of the pupil. The way to tell those apart is the other features, the fellow travelers, the lid and the motility. If you have just the pupil involved, it'd be very, very difficult to make that a third nerve palsy. It'd be hard to just get the pupil. The dissociation has to occur in the pretectal nuclei, and that can be from a dorsal midbrain lesion, Parinaud dorsal membrane syndrome, some pressing on there, tumor, hydrocephalus, aqueductal hydrocephalus, or syphilis. So syphilis affects that little pretectal afferent. We call that the Argyll Robertson pupil. But in the Argyll Robertson pupil, there's no other evidence of third nerve palsy. There's no ptosis, there's no motility deficit. This is a small pupil that doesn't react to light, but reacts to near. The second place you get the light-near dissociation is here, at the ciliary ganglion; that's a totally different video. And you should watch that video. But if the question is how do you determine pupil involvement from third nerve and Edinger-Westphal, the answer is don't look at the pupil. Look at everything else." |
