Cranial Nerve Three Palsy

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Identifier Cranial_Nerve_3_Palsy
Title Cranial Nerve Three Palsy
Subject Neuroanatomy; Nerve Palsy
Creator Andrew G. Lee, MD, Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, TX; Professor of Ophthalmology, Weill Cornell Medicine; Peter Ugoh, Baylor College of Medicine Class of 2022
Description Dr. Lee lectures medical students on the third cranial nerve and third nerve palsy.
Transcript So today we're going to be talking about the third nerve and third nerve palsy. So, the number three nerve is in charge of the lid and so we might get a ptosis either a complete or partial ptosis. It's in charge of the pupil and the pupil might be dilated or it might be normal and it might be partially reactive, poorly reactive, or normally reactive depending on how much pupil involvement we have, and because it's in charge of several muscles: medial rectus, inferior rectus, the superior rectus, the inferior oblique, your eye will be down and out so it's going to be exotropic and hypotropic, so we're going to have an eye that is down and out the lid will be down the pupil will be dilated. However, you might not have every branch of the third involved, you might have only partial involvement, you might have just divisional involvement of just the superior division with the lid and the superior rectus, and so you can have partial or complete palsies. The third nerve like all the cranial nerves has it start in the brainstem and in this case in the midbrain. It travels as a fascicle after leaving its nucleus and exits the subarachnoid space in this location. It is parallel to the posterior communicating artery (the PCOMA). The posterior communicating artery communicates between the anterior and the posterior circulation and an aneurysm that occurs in the posterior communicating artery can hit the number-3 nerve, and so the most dangerous thing in a new acute third nerve palsy with or without pupil involvement, with or without partial or complete ptosis with or without partial or complete involvement of the muscles, the thing we're worried about is a PCOMA aneurysm. So, when we're confronted with a third nerve palsy we need to make sure there's no other cranial nerve involved that means testing the other nerves that live near three: number four, number five, number six. In order to test the integrity of four, which innervates the superior oblique muscle we'd like the patient to look down and we're going to see the torsion because the primary action of the superior oblique is intorsion so every third that's complete we want to see if there's intorsion and down gaze because the fourth could be involved and that would place the lesion somewhere else like the cavernous sinus or in the brainstem rather than just an isolated third. So, once you've made sure it's an isolated number three palsy we're going to try and figure out what to do with this. Some people would say that if you have a complete palsy in a vasculopathic patient who has no pupil involvement, complete pupil sparing that you could observe that. That's probably okay but most people would image third nerve palsy regardless of the pupil involvement regardless of the presence of pain and regardless of the; Completeness of the palsy or the partial or complete nature of the ptosis and so our first imaging study is non-contrast CT scan to look for subarachnoid hemorrhage if we're worried about aneurysm and in most places a CTA is the imaging study that we're going to do if we look for aneurysm so you need an a, an angiogram of some kind to find, an a. The a we're looking for is aneurysm. Different types of angiograms: CTA, MRA, DSA (digital subtraction angiography) but because in the emergency room it's faster to get a non-contrast CT followed by a contrast CTA of the head this is usually our go-to imaging in the emergency room. So, all the third nerve palsies need to have a CT/CTA if their urgent or emergent or you're worried about subarachnoid hemorrhage and if you're worried about aneurysm. If the CT and the CTA are both negative you still have to do an MRI and the reason is CT/CTA is good for aneurysmal causes of third nerve palsy but MRI is better for soft tissue and all the other causes of third nerve palsy that have nothing to do with aneurysm like stroke and tumors and demyelinating disease and a whole host of other things that a CT scan is just not good enough. So normally I will just tack on the MRA onto the MRI because sometimes the CTA can miss thrombosed aneurysm or might not be able to see as well as an MRA because of bone hardening artifact at the skull base or technical difficulties. So, CT/CTA followed by MRI/MRA is our procedure of choice in that order for the evaluation for third nerve palsy, and make sure it's not an aneurysm. If everything's negative then you're going to look for the usual suspects: ischemia would be the number one cause and vasculopathic age patients, giant cell arteritis has to be considered in every elderly patient who has new onset diplopia even if it looks like a third nerve palsy and then you could test for the infectious and inflammatory etiologies but the main point is making sure it's not an aneurysm. CT/CTA followed by MRI/MRA is the preferred imaging for third nerve palsy.
Publisher Spencer S. Eccles Health Sciences Library, University of Utah
Date 2019-02
Type Image/MovingImage
Format video/mp4
Rights Management Copyright 2019. For further information regarding the rights to this collection, please visit:
Holding Institution Spencer S. Eccles Health Sciences Library, University of Utah, 10 N 1900 E, SLC, UT 84112-5890
Collection Neuro-ophthalmology Virtual Education Library: NOVEL
Language eng
ARK ark:/87278/s6qp0kqc
Setname ehsl_novel_lee
Date Created 2019-03-01
Date Modified 2020-01-13
ID 1403669
Reference URL
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