| Identifier |
Fourth_Nerve |
| Title |
Fourth Cranial Nerve |
| Creator |
Andrew G. Lee, MD; Omar Ali |
| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (OA) Class of 2020, Baylor College of Medicine, Houston, Texas |
| Subject |
Neuroanatomy; Fourth Cranial Nerve |
| Description |
Dr. Lee lectures on the fourth cranial nerve. |
| Transcript |
So today we're going to be talking about the fourth nerve. The fourth cranial nerve begins like all the other cranial nerves as a nucleus. And when we're talking about the oculomotor system, it's the dorsal portion of the brainstem. So in this particular example, we're doing the fourth nerve nucleus, number four, and this is the dorsal portion of the midbrain at the level of the inferior colliculus. One of the important things you need to know right away is that the fourth nerve exits dorsally before traveling ventrally and it is crossed. So that means lesions in the dorsal midbrain, especially trauma, can cause a bilateral fourth because it's both crossed and dorsal and exits dorsal. It's also got a very long intracranial course which also makes it susceptible to trauma. So the fourth cranial nerve is the only cranial nerve that crosses, all the other cranial nerves are ipsilateral. And so when we're dealing with a dorsal brainstem lesion, you have to think about a contralateral fourth nerve palsy. The fourth nerve has only one job; it's innervating a single muscle, the superior oblique muscle. So its only job is to innervate the superior oblique. And so that means its primary symptom is going to be diplopia. The diplopia is going to have torsion associated with it, but the patient might not notice the torsion. The primary action of the superior oblique is intorsion and that's because of the anatomic arrangement of the muscle and its trochlea. So even though all the muscles start in the upper orbital apex, the effective origin of the superior oblique muscle is the trochlea. I will demonstrate now using my arm. This is the trochlea. This is the muscle and its tendon. This is the insertion and its insertion is at an oblique angle superior on the globe, thus the name superior oblique muscle. And so its primary action is intorsion when it fires. However, when the eye is in the adducted position, notice that the arrangement of the muscle plane and the pupil plane now align and so in the adducted position the superior oblique becomes a depressor. In the abducted position, the muscle plane is no longer parallel to the pupil plane and in fact is becoming perpendicular to the pupil plane, and therefore in the abducted position firing of the superior oblique causes more abduction and intorsion but not much depression. So when we're dealing with a fourth nerve palsy, we need to do a three-step test which is covered in a different video. The predominant symptom is diplopia with or without torsion. You need to know it's crossed, exits dorsally can be bilateral in trauma, and innervates only one muscle, the superior oblique. |
| Date |
2019-02 |
| Language |
eng |
| Format |
video/mp4 |
| Type |
Image/MovingImage |
| Collection |
Neuro-Ophthalmology Virtual Education Library: Andrew G. Lee Collection: https://novel.utah.edu/Lee/ |
| Publisher |
North American Neuro-Ophthalmology Society |
| Holding Institution |
Spencer S. Eccles Health Sciences Library, University of Utah, 10 N 1900 E SLC, UT 84112-5890 |
| Rights Management |
Copyright 2019. For further information regarding the rights to this collection, please visit: https://NOVEL.utah.edu/about/copyright |
| ARK |
ark:/87278/s66q695v |
| Setname |
ehsl_novel_lee |
| ID |
1403673 |
| Reference URL |
https://collections.lib.utah.edu/ark:/87278/s66q695v |
