| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (ZF) Class of 2020, Baylor College of Medicine, Houston, Texas |
| Transcript |
So today we're going to be talking about the dorsal midbrain syndrome. I'm going to be just drawing a drawing here of the midbrain, and we're going to be paying attention to the dorsal part of the dorsal midbrain. This is ventral and dorsal, so the aqueduct cerebral peduncles, and for our purposes we're going to be talking about the pretectal nuclei at the level of superior colliculus the tectum, which is the roof, and the tegmentum, which is the floor. This composes those cerebral peduncle, and all of our action in ophthalmology lives in the dorsal midbrain. So when we have a dorsal midbrain lesion we can have involvement of the Edinger-Westphal nucleus of three, and that third nerve passes between the peduncles in the interpeduncular cistern, and it is the input of supranuclear input from above and input to both sides in the pretectal nuclei that produce the constellation of findings that are associated with the dorsal midbrain syndrome. ; So the first thing that we might see is lid retraction, which is called the Collier's lid retraction sign. And the reason you have lid retraction bilaterally in a dorsal midbrain lesion is the supranuclear input to the third nerve nucleus tells the lid to fire or not fire, and so if you inhibit the inhibitory fiber, the lid will retract. So lid retraction bilateral and symmetric the Collier's lid retraction sign is a feature of the dorsal mid brain syndrome. ; You might also see a problem with the pupil called light-near dissociation: basically the light and the near pathway are different, I.E. dissociated, at the level of the pretectal nuclei input to the Edinger Westfall nucleus. So we can have an interruption that pretectal signal (in this case, the light pathway coming from the optic tract) that will disrupt the light pathway, but will spare the near pathway, because the near pathway is more rostral and can talk directly to the third nerve nucleus and the Edinger Westfall from above. So even if we have a disruption of the light pathway, we can have preservation of the near pathway in a lesion in the dorsal midbrain, which we see as light-near dissociation of the pupils. ; In addition you might see a very unusual movement called convergence retraction, and even though it's called nystagmus, it really isn't rhythmic, so it's really a "nystagmoid" movement and not a "nystagmus". And what we see is, again, because we have innervation to the third nerve nucleus, we'll see firing of the muscles innervated by three. So when that happens the medial rectus will fire - the eyes will converge - and we'll have retraction of the globe because both the superior rectus and the inferior rectus muscle will fire. So when both muscles fire, the globe retracts. So it converges and retracts because all of the muscles of three are firing and that produces the Convergence Retraction Nystagmus. It's often apparent in attempted up-gaze. The reason it converges is because the medial rectus is unopposed. The lateral rectus is innervated by the sixth nerve and that lives in the pons, and this is a midbrain problem. The third is midbrain, the sixth is in the pons. So converges retraction nystagmus occurs from dorsal midbrain lesions and that's what this is. ; And then we might see up-gaze paresis. So as with the other findings, innervation to the third nerve to look up is mediated by the vertical gaze center, which lives in the thalamo-mesencephalic junction, so both looking up and looking down come from a vertical gaze center that lives in the rostral thalamo-mesencephalic junction. Because that fiber that innervates both third nerve nuclei crosses in the posterior commissure, a single lesion in the dorsal midbrain can produce an up-gaze palsy. To get a down gaze palsy, it usually requires a bigger lesion because the down gaze pathway is bilateral. In addition the eyes could be forced into the down position - sun setting of eyes - especially with hydrocephalus, because you have unopposed down gaze innervation with no up gaze. ; So the dorsal midbrain anatomy basically centers on the ocular motor nucleus of three (Edinger Westfall for the pupil). If we fire things that aren't supposed to fire in the third, we get lid retraction rather than ptosis, we get disruption to the light pathway but preservation of the near pathway, we get a convergence retraction nystagmoid movement because of convergence and retraction from firing of the third nerve muscles from three, and we might get an up gaze, or down gaze palsy, but usually up gaze, because it crosses in the posterior. The constellation of findings? the Perinaud's dorsal midbrain syndrome. |
