| Transcript |
When we're trying to look either to the right, or to the left, horizontal gaze, or up and down, we have supranuclear pathways for doing this (supra- above), nuclear pathways that live in your brainstem, and infranuclear pathway, which is below the nucleus and is composed of the peripheral nerve, the neuromuscular junction, and the muscle. Supra-, nuclear and infranuclear. So for horizontal gaze, the final common pathway for horizontal gaze lives in the pons, and that is mediated by the parapontine reticular formation and the sixth nerve nucleus. This is horizontal pathway. The vertical pathway lives at the junction of the thalamus and the midbrain, the thalamomesencephalic junction. The vertical gaze center lives in the midbrain and in the connection between the thalamus and the membrane, the thalamomesencephalic junction-- the thalamomesencephalic junction. These have various names in the Macaque monkey, including the area of Darkschewitsch, the rostral interstitial medial longitudinal fasciculus, and the interstitial nucleus of Cajal. You don't have to know any of that, but you do need to know that the vertical gaze center lives in the midbrain, and the horizontal gaze center lives in the pons. The supranuclear input to these brainstem nuclei comes from the frontal eye fields for saccades contralaterally so if I want to look horizontally to the right, my left frontal eye field has to fire. That tells my right pons to fire. That activates the right parapontine reticular formation and the right 6th nerve nucleus, and the horizontal gaze is mediated from the 6th to the contralateral 3rd via the medial longitudinal fasciculus. So, that is the nuclear and the internuclear portion of the horizontal gaze pathway. Then it travels down the nerve, to the junction, to the muscles, and that is the infranuclear pathway. The frontal eye fields are for saccades. The parietal occipital temporal region--ipsilateral-- is for the pursuit. But the same final common pathway applies. for pursuit to the right, my right parietal occipital temporal lobe fires, firing the right parapontine reticular formation, and the right 6th nerve nucleus, via the medial longitudinal fasciculus to the contralateral third, and that allows my eyes to look to the right, because they are yoked together, the muscles are yoked, and that means they go in the same direction at the same speed and in the same way. For vertical gaze, that's at the thalamomesencephalic junction, but the supranuclear input is the same: contralateral frontal eye field ipsilateral parietal occipital temporal. So when we're dealing with vertical or horizontal gaze palsy, the first question we'd like to know is: is it supranuclear or not? And the way to know that is, we have ways to input without the cortex, our vestibular system can talk to our ocular system via the vestibuloocular reflex. So, our ear, but not the hearing part of the ear, the balance part of our ear, if we rotate our head, the ear senses that rotation, and transmit the information to the vertical and horizontal gaze center, and that we call a Doll's Head Maneuver. So by moving the head passively, rotating horizontally or vertically, we can make the eyes move if the problem is supranuclear. Because the nucleus is intact, the infranuclear pathway is intact, so if it's a supranuclear problem in the frontal eye field or the parietal occipital temporal lobe, or before the thalamomesencephalic junction, we can use the Doll's Head Maneuver, and if the eyes move, that tells us that the nucleus and everything below the nucleus, the infranuclear pathway, is intact. Once we have decided that it is nuclear or internuclear, then you have to decide is it vertical or is it horizontal? If it's the vertical gaze center, that is in the thalamomesencephalic junction, at the level of the midbrain. So, patients who have for example the Dorsal midbrain syndrome, may not be able to look up, they have an up gaze palsy, because that's where the vertical gaze center is. If patients have progressive supranuclear palsies, the connections at that level are disrupted, and that causes PSP-- progressive supranuclear palsy. The up gaze pathway crosses at the level of the posterior commissure, and that means a single lesion of the posterior commissure can result in an up gaze paresis, but the downgaze innervation is bilateral, and so in the dorsal midbrain syndrome, only very late do we get a bilateral downgaze palsy. The up gaze palsy is way more common, because of a single crossing point at the level of the posterior commissure. Horizontal gaze can be effected on one side, a horizontal gaze palsy, or both sides, a bilateral horizontal gaze palsy, or it can involve the medial longitudinal fasciculus, so if you have a horizontal gaze palsy, and a medial longitudinal fasciculus lesion, the medial longitudinal fasciculus lesion produces an internuclear ophthalmoplegia, called an INO, so if you have a horizontal gaze palsy, that would be a 1, and you get a 1/2 from the inter nuclear ophthalmoplegia, and that we call a One-and-a-half syndrome. The One-and-a-half syndrome is one of the dorsal pontine syndromes. If however you are in the dorsal midbrain, you might get an up gaze palsy, and a vertical gaze palsy usually for up gaze is the thing we are looking for at the level of the dorsal midbrain, different than the dorsal pons. So in summary, you should know there are supranuclear, nuclear, and infranuclear pieces of the efferent pathway for motor control. That the vertical gaze center lives at the thalamomesencephalic junction, and it is the dorsal midbrain that we're worried about in the vertical gaze problems. The horizontal gaze is controlled at the level of the pons via the parapontine reticular formation, but the final common pathway is the 6th nerve nucleus for the horizontal gaze, the 6th nerve nucleus talks to the contralateral 3rd via the medial longitudinal fasciculus, resulting in a clinical finding called the internuclear ophthalmoplegia, and the combination of a horizontal gaze palsy and an INO is what we call the One-and-a-half syndrome. If you just know this basic anatomy, you can pretty much figure out the major disorders of horizontal and vertical gaze from the brainstem and also the supranuclear problems. |
