Cranial Nerve Palsies

00:01 So we've talked about amblyopia, which is where the central processing of the input coming from the eye and the brain gets turned off and that can be for the causes that we talked about, including cataracts and abnormal kind of rotation of the eye.

00:20 We talked about strabismus so that the eye is not there, the axes are not aligned in the various reasons that that happens.

00:28 Now we're going to talk about how we actually have extraocular movement, how the muscles actually make the eye go and the consequences of having injury to the nerves that innervate the specific muscles and what that does in terms of malrotation of the eye.

00:47 We've seen this briefly before.

00:49 And again, we're going to have some creative redundancies, so you're going to see it again.

00:53 So eye muscles, you can see where the nose is, you can see where the medial canthus would be.

00:58 And you're looking at the green arrow indicating the medial rectus.

01:02 And when the medial rectus pulls, tugs on its insertion into the sclera, turns the eye towards the nose, that's the medial rectus.

01:10 The superior rectus will make the eye look up, pulling it up.

01:15 The inferior rectus pulls it down, the lateral rectus points it, pulls it laterally.

01:20 The superior oblique pulling from the top and kind of rotating it will actually make it the eye look down and out.

01:28 And the inferior oblique will make it look up and out.

01:32 Okay, so those are the muscles, let's talk about the nerves that are innovating these.

01:37 The oculomotor nucleus, oculomotor nerves, cranial nerve number III is going to innervate as shown here, the superior rectus, the inferior rectus, the medial rectus and the inferior obliques.

01:50 So four muscle bundles are going to be innovated by the oculomotor The superior oblique is innervated by the trochlear nucleus, cranial nerve number IV.

02:01 And the lateral rectus is going to be innervated by the abducens or the cranial nerve number VI.

02:07 So let's look at what happens if we start injuring either the nucleus or the nerve fibers running from that nucleus to those muscles.

02:17 If we injure the oculomotor nucleus, so cranial nerves III, we're going to affect 4 muscle bundles, right? The superior rectus, medial rectus, the inferior rectus, and inferior oblique.

02:29 All of those are going to be affected.

02:31 So that means that we can't pull inwards.

02:34 We can't pull up, but we can pull laterally and down, down and out.

02:38 So the abducens is still going to be acting on the lateral rectus and the trochlear nerve is still going to be acting on the superior oblique.

02:48 So in patients that had cranial nerve palsies where one eye is looking straight ahead, the other one can only be pulled down and out.

02:59 And that's typical for a cranial nerve III palsy.

03:03 Perfect. Okay, let's injure another one separately.

03:07 So now the trochlear nerve or the trochlear nucleus, cranial nerve number IV, that's going to be innervating the superior oblique.

03:13 Remember, the superior oblique's job is to pull the eye down and out so we have a little X down there, it cannot go that direction.

03:20 It can only go the other green arrows because those muscles are all intact.

03:25 Now, normally, you wouldn't see much of an effect.

03:28 So if the patient were to stare directly ahead at you, then both eyes would probably work appropriately in the middle.

03:35 But if we have lateral gaze on the non-affected eye and we have the eye trying to rotate towards the medial canthus, it's incomplete.

03:46 In fact, because I cannot pull down and out, there's excess force going up and in.

03:52 So the eye deviates up and in when you have a cranial nerve IV palsy.

04:00 And the last one, so the lateral rectus, if it cannot work, we cannot pull the eye laterally.

04:07 Okay, so injury to the abducens nucleus, cranial nerve number VI will give us an inward deviation of the eye and that's what happens.

04:16 So when we're staring straight ahead, now, the affected eye cannot be pulled outwards, it goes inward.

04:26 With that, we've covered some interesting motion abnormalities and some interesting processing abnormalities of the eye.