||Andrew G. Lee, MD, Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, TX; Professor of Ophthalmology, Weill Cornell Medicine; Jonathan Go, Baylor College of Medicine Class of 2021
||So, we're going to talk about infranuclear lesions today. And "infra" means "below", as opposed to "supra" which means "above". And it's "below" the nucleus. So, each of our cranial nerves (cranial nerves three, four, and six) that control the eye movement have nuclei. The nuclei of cranial nerves three and four live in the midbrain, and the nucleus of six lives in the pons. And you need to know that because there are control mechanisms that are above the nucleus. Those include your cortical mechanisms: So, if you want to look to the right or the left or up or down, your cortex has to tell your brainstem nuclei to fire. And these nuclei are connected by inter-neurons that allow the two nerves to fire at the same time with the same velocity, same speed, and in the same direction. And so the muscles are yoked together, not like an egg yolk, but like an ox yolk, where two oxen's heads are connected by a stick of wood so that they move their heads together. And so, it's critically important that you understand both the supranuclear component of the visual efferent pathway, but also the nucleus.And today, we're going to be talking about the infranuclear pathway. So once the nerve leaves the nucleus either in the midbrain or pons, that piece of the nerve that is inside the brainstem itself is called the fascicle. And that fascicle means "bundle": So, it's a bundle, it's white matter-encased, and it's [considered part of the] central nervous system inside the brainstem. But once it leaves the root exit zone of the midbrain or the pons, and all of these nerves exit ventrally except #4 which exits dorsally, once it leaves, either ventrally or dorsally, then it becomes a peripheral nerve - And that is the beginning of the infranuclear pathway in the peripheral system. The infranuclear pathway (peripheral nerve of cranial nerve three, four, and six) then travels to the target organ. And for the third nerve, that's the lid, the pupil, and also the extraocular muscles innervated by three. The fourth nerve only has one job, so it's super easy: it only innervates the superior oblique muscle. And the sixth nerve is also super easy to get out because it only does the lateral rectus muscle. So, everybody else is three. So: "SO4, LR6"…everybody else is three. So, when we get to the target organ, the muscle, there's a junction: A neuromuscular junction between the nerve and the muscle. And the neuromuscular junction disease that we're interested in is ocular myasthenia gravis. But obviously, it can be a part of generalized myasthenia, or be botulism or other things that can affect the junction. So whenever we have a patient with diplopia, we'd like to know if the problem is supranuclear, nuclear, or infranuclear. If it's infranuclear, it's below the nucleus (midbrain = three and four, pons = six). Once it leaves the brain stem, it [becomes] peripheral nerve and then it's going to reach the junction - the neuromuscular junction - that's where the myasthenia can affect it. And then it could be muscle itself, and the most common muscle problem that we see in the infranuclear pathway is thyroid eye disease. So thyroid eye disease, myasthenia, and peripheral infranuclear cranial neuropathies are all in the differential diagnosis of infranuclear causes of diplopia.