| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (AM) Class of 2023, Baylor College of Medicine, Houston, Texas |
| Transcript |
Ok today we are going to be talking about fascicle, and "fascicle" comes from "fasci-." Fasci- is a bundle, so it comes from a bundle, so you can tie these sticks all together and make a bundle and it makes it stronger. So that's what a fasci is. And so now you can imagine instead of sticks that these are axons, and so it's a white matter bundle that makes up the fascicle. And the fascicles that matter to us in neuro-op are the medial longitudinal fasciculus, and that is also sometimes called the MLF. So that is the medial longitudinal fasciculus. But all the cranial nerves that we deal with have a fascicle. So, for example, if you're in the pons you might have a sixth nerve and once it crosses out of the root exit zone of the pons, that becomes a peripheral (cranial) nerve. But this portion, after it leaves the nucleus but inside of the pons, is called the fascicle. And what that means is that this portion of the nerve is central nervous system, and that means it's going to be vulnerable to central nervous system disorders, including demyelination. So, in the sixth nerve nucleus as you know, it crosses over and talks to the medial rectus muscle which is the midbrain, and that's the third nerve of some nucleus, and that thing is called the medial longitudinal fasciculus. So, the two fascicles that we're interested in for ocular motor and diplopia are the sixth nerve fascicle and the medial longitudinal fascicle, called the medial longitudinal fasciculus or MLF. And so, when you have a demyelinating lesion in the pons, it can cause a unilateral sixth. But because the fascicle is relatively short, relative to the longitudinally running fascicle of the MLF, you are much more likely when you have diplopia in multiple sclerosis to have an internuclear ophthalmoplegia. And so that's internuclear ophthalmoplegia. It's an ophthalmoplegia: an adduction deficit, the eye can't AD-duct associated with nystagmus, which is a dissociated horizontal AB-ducting nystagmus. It's internuclear because it is traveling between cranial nerve nuclei from number six in the pons rostrally to cranial nerve nuclei number three for the medial rectus in the midbrain. And that means it's longitudinally long because it has to travel from the pons to the midbrain. And so as opposed to the fascicle of six where you really only have one demyelinating chance at hitting the fascicle with a demyelinating lesion in the pons, in an MLF lesion we have multiple locations that we can hit the longitudinally oriented MLF. And so, the most common cause that we see for acute binocular horizontal diplopia in an MS patient, is a demyelinating plaque in the MLF, the medial longitudinal fasciculus, however the demyelinating plaque could also affect the fascicle of six. Four is in the midbrain, as you know, and four is the only cranial neve that exits dorsally. And so, the chance of getting a fascicular, little tiny piece there, fourth, is so low from MS. So, it's unlikely to get a fourth. In a third nerve palsy, now you could get a third from demyelinating disease because it's also got a fascicle, but statistically it's going to be way more likely to be MLF even if it looks like a third superficially because there's an adduction deficit. So, you need to know a little bit about fascicles, a bundle of white matter. In the central nervous system, that's what we call this piece of the nerve- the sixth nerve, the third nerve, and the fourth nerve - fascicle. And one of these fascicles is called the fasciculus, it is longitudinal and medial. It runs from the pons, sixth nerve nucleus, all the way rostrally to the third nerve nucleus in the midbrain. And because there are so many shots at this MLF, an INO is a very common cause of acute diplopia in MS. |
