| Transcript |
We were asked to talk about vertical gaze and, by the way, it's great that you asked about it, because I don't know what you don't know and I don't know what you want to hear about. So, tell me if you want to hear about something. So, somebody asked about vertical gaze. We're going to talk about it. So, when we have vertical gaze, you're talking about looking up and looking down. Both eyes have to move together in the same direction at the same time and the same speed, and that we call conjugate. So, we're talking about conjugate movements vertically, and those movements can be fast movements, which is the saccade, or it can be a slow movement, which is the pursuit. The saccades and pursuit are controlled by the frontal lobe for saccades and the parietal lobe for pursuit. Once the eyes have been told to look up or down, that information from the cortex (the supranuclear control) has to go to the brainstem; and so, in our brainstem, the up and down is controlled in the midbrain. So just like the horizontal gaze center, which is in the pons, the six-nerve nucleus in the para pontine reticular formation, it's for horizontal gaze; we have a vertical gaze center (VGC), and that vertical gaze center is for looking up and down. In the vertical gaze center, which is at the junction between the thalamus and the midbrain, the thalamo-mesencephalic junction, there are a number of nuclei there that are thought to characterize this vertical gaze center. It goes by various names, including the interstitial nucleus of Cajal (INC), the area of Darkshewitsch (AD); and these vertical gaze center nuclei are analogous to the horizontal gaze nuclei (the para pontine reticular formation and the six-nerve nucleus). So, when we have the supranuclear input to the vertical gaze center at the thalamo-mesencephalic junction, that is what is telling your eyes to look up and to look down. And so, the muscles that bring the eye up - superior rectus and the inferior oblique - and the muscles that move the eye down - inferior rectus and superior oblique. And so, the third nerve (CNIII) is in charge of three of these muscles and the fourth nerve (CNIV) is in charge of one of these muscles, the superior oblique. But the signal must be received from the supranuclear pathway to the thalamo-mesencephalic junction and then talk to the nuclei: the nuclei of three (CNIII), and for down gaze, the nuclei of three (CNIII) and four (CNIV). And those nuclei live in the dorsal portion of the brainstem in the dorsal midbrain. For three (CNIII), that's at the level of the superior colliculus, and for four (CNIV) that's at the level of the inferior colliculus. For up gaze, the fiber innervation is crossed, and that crossing point is called the posterior commissure. So, at the level of the posterior commissure, we have the crossing of the up-gaze fiber and that's why dorsal midbrain lesions often have a bilateral up-gaze paresis, like the Parinaud's dorsal midbrain syndrome. In order to get a down-gaze palsy, however, that is bilaterally innervated without a crossing; and so it's much harder to get a down-gaze palsy from a supranuclear or even a midbrain lesion than an up-gaze palsy, because that requires it to be bilateral. A bilateral down-gaze palsy can be supranuclear, or it can be nuclear, or infranuclear, or it can involve the thalamo-mesencephalic junction - that vertical gaze center which includes the rostral interstitial medial longitudinal fasciculus - and that's why we can get skewed deviations in patients who have MLF lesions because it's actually got to communicate with the vertical gaze center as well. So, when we're dealing with an up-gaze or down-gaze paresis, the main thing you want to be able to do is determine whether it is supranuclear or infranuclear; and the way to do that is with the doll's head maneuver. So, if the patient can't look up or can't look down, we're going to move their head passively. That will allow the rotation of the eyes to be stimulated by the vestibular system. And so, if we have a down-gaze palsy the patient cannot look down, but we can overcome that down-gaze palsy with the doll's head maneuver. That's a supranuclear palsy, and the reason you need to know that is because the most common cause of a progressive and supranuclear palsy in an elderly patient is progressive supranuclear palsy (PSP), and that is a super important thing to know. If, however, the doll's head maneuver cannot overcome the vertical gaze palsy, then it's infranuclear, and we'd be worried about the vertical gaze center, the thalamo-mesencephalic junction, or the dorsal membrane. All of the rest of the pathway is what you're already used to, the nerves - three (CNIII) and four (CNIV) - the junction, myasthenia, and the muscles - superior rectus, inferior rectus, inferior oblique, and superior oblique. So, in summary, the vertical gaze center (cortical input from your brain, frontal lobe and parietal lobes), the brainstem (vertical gaze center), crossing fiber posterior commissure for up-gaze, uncrossed bilateral innervation for down-gaze, to the midbrain and the third and fourth nuclei at the superior and inferior colliculus level, and then passing down the infranuclear pathway down the nerves to the junction and the muscles. Do the doll's head maneuver to see if it's supranuclear, and if it's not supranuclear you have to work it up for every piece of the pathway. |
