Advanced Assessment of the Cranial Nerves I–VII

00:01 So let's start off with cranial nerve I.

00:04 This is the olfactory nerve.

00:06 And while historically, we haven't paid as much attention to the olfactory nerve, I think the COVID-19 pandemic brought to light the fact that there are certain conditions that do compromise the olfactory system.

00:16 And COVID-19 virus had a predilection for causing a nausea.

00:21 So it's useful to at the bedside quickly assess for olfaction.

00:25 A simple test at the bedside is simply to use 2 or 3 common sense.

00:29 And for me, I'll oftentimes use the following.

00:33 Alright, so Shawn, I'd like you to do is to close your eyes and occlude your right nostril.

00:38 Great.

00:41 What do you smell? Alcohol, for sure.

00:45 Perfect.

00:45 Okay, now occlude your other nostril.

00:52 What do you smell? Tea.

00:57 Perfect.

00:58 Simple bedside test of the olfactory system.

01:00 Alright, so now let's move on to assessing the second cranial nerve, the optic nerve.

01:05 For the purposes of the neurologic exam that we're covering here, we're going to be doing peripheral field testing, basic vision using a snellen chart.

01:13 And we'll also look at the pupillary afferent input.

01:17 In combination, we're looking at the third cranial nerve.

01:19 So first off, we want to assess grossly, can this person see or not? And so we use our handy snellen chart, I'm going to have you stand over there.

01:29 The snellen chart can be, you know, kept in your pocket.

01:32 And it's easy enough to use, you want the patient to be about 6 ft away from the chart.

01:36 And I'm going to have you cover up your right eye.

01:39 You can have a patient do this with glasses and then without glasses, depending upon the clinical circumstances that you're doing the assessment in.

01:47 And I just want you to start around here and tell me the letters that you can read from right to left.

01:54 O - L - C - F Great, let's skip down to the down here.

01:59 E - P - T - Z - L Great.

02:03 I would have him cover up his left eye and do the exact same thing and want to mark based on the snellen chart exactly what level of visual acuity that the patient has.

02:13 It can also usually with the snellen chart will include some color testing as well as needed.

02:19 And also there's a nice pupillary gauge on here for when we get to the part of assessing pupillary size.

02:23 Please have a seat again, Shawn.

02:26 Now we're going to do visual field testing, which is actually a very useful skill in terms of its likelihood ratios for detecting an actual visual field cut on peripheral testing.

02:38 But at the bedside, we can do a very good job.

02:40 So what I'm going to do is have you again cover up your right eye.

02:44 And I want you to look at my nose.

02:46 And the way that I do this, I'm just going to have you tell me whether the upper hand is moving or the lower hand is moving, okay.

02:54 This is called kinetic visual field testing because I'm moving something.

02:59 Upper.

03:00 Great.

03:01 Lower.

03:03 Upper.

03:05 Lower.

03:06 Very simple testing, very easy way to do that.

03:09 And you can move in slowly if you're finding the patient is not detecting things in just those quadrants.

03:15 But that's a quick and dirty way to assess visual field problems.

03:18 And of course, I would have him do the other side as well.

03:21 The next component of assessing the optic nerve is again pupillary size, and we'll jump into doing that.

03:27 Now keep in mind that the afferent input for pupillary constriction is the second cranial nerve.

03:35 But the efferent pathway which actually involves pupillary constriction happening is mediated through your third cranial nerve.

03:43 So let's take a look at that.

03:45 So for starters, you of course want to dim the lights when you're trying to assess pupillary size so you can maximally have the pupils dilated so you can best see them react when you expose them to light.

03:54 So first off, we're just going to have you look over here.

03:59 I'm looking at one pupil, looking for constriction.

04:02 Looking at the other pupil, looking for a good constriction.

04:05 I would document my exam that the pupils are going from 4 mm to 2 mm.

04:11 You want both numbers, both before you put the light in and then after you've exposed them to light.

04:17 And then an important test is the swinging flashlight test.

04:21 This is to test for a Marcus Gunn pupil also known as an afferent pupillary defect.

04:27 Oftentimes seen with, for example, optic neuritis from multiple sclerosis where the input from the optic nerve is not being collected in one eye.

04:37 And so there's no afferent information going back to the brain in one eye, but the efferent information from the third cranial nerve is functioning.

04:46 So if I shine light in this eye, not only does this I constrict but this one does as well and that's completely normal, that's called the consensual pupillary response.

04:55 If I go back to this eye on the right, where there's no afferent information it would actually dilate as this eye was relaxing and not getting input anymore.

05:04 So that's a classic sign of a Marcus Gunn pupil.

05:07 The last thing to take note of when you're looking at the pupils is the pupillary symmetry between one eye and the other folks with anisocoria which is where one pupil is larger than another.

05:19 It's typically mediated by the third cranial nerve due to problems with sympathetic or parasympathetic input to the pupillary constrictor muscles.

05:27 Oftentimes folks, if that's not the problem that's causing anisocoria a so-called surgical pupil is where one pupil is a different size or even a different shape.

05:36 It's not quite a full circle if they've had cataract surgery or some other kind of surgery.

05:41 And it's nothing to be alarmed about if there's a known history and if it's been documented in the past.

05:49 Alright, so we've already started testing the third cranial nerve while we were assessing pupillary size and we talked about an afferent pupillary defect versus a problem with parasympathetic or sympathetic information through the third cranial nerve.

06:01 Now we're going to look at the role of the third cranial nerve in terms of ocular motor function.

06:07 And this is of course going to also include the fourth cranial nerve and the sixth cranial nerve since all three of them mediate movement of the eyeball.

06:16 The sixth cranial nerve just to start off with is the abducens nerve, and that is operating the lateral rectus muscle.

06:23 The fourth cranial nerve is the trochlear nerve and that's operating the superior oblique muscle and then the third cranial nerve mediates all the other movements of the eye.

06:34 So the superior and inferior rectus, the medial rectus and the inferior oblique.

06:40 So to test those things, we'll start off by having you just look at my nose and just follow my finger with your eyes.

06:55 Perfect.

06:56 So having said that, we'll move on to the fifth cranial nerve, the trigeminal nerve.

07:01 Now, I always get mixed up with which nerve is the fifth cranial nerve, because the trigeminal nerve, the sensory input for the face is called V one, v two and V three.

07:12 And so I hear the letter V and I start thinking about the vagus nerve.

07:15 But the V refers to the Roman numeral five, so it's Roman numeral five, section one, section two, and section three.

07:22 So always the nomenclature can can throw me off as much as anybody else.

07:27 So the trigeminal nerve has sensory and motor function.

07:29 The sensory function I've just alluded to V one is the ophthalmic branch of the trigeminal nerve.

07:35 Then we have a maxillary branch and then a mandibular branch, and you can simply test for those for the function of that nerve as follows You feel me on both sides? Does it feel the same on both sides? Yes. Perfect.

07:48 So that's the testing for V one, v two and v three of the trigeminal nerve and then motor function.

07:53 The trigeminal nerve works on the master muscle and the triggering muscles.

07:59 Clench your teeth good.

08:00 I should be able to feel his temporal muscles up here and the mass of the muscles which are keeping his his bite nice and tight.

08:08 And then I want you to move your jaw side to side.

08:10 So those are the triggered muscles.

08:12 And so by showing evidence that his jaw can move and then he can have a tight clenched jaw, it tells me that his trigeminal nerve motor function is functioning.

08:22 All right.

08:22 So having completed the fifth cranial nerve, we already talked about the sixth cranial nerve.

08:26 So we're now moving on to the seventh cranial nerve.

08:29 The facial nerve.

08:30 The facial nerve has motor sensory and even autonomic nervous system function.

08:35 But the only parts that we can really assess on physical exam is the facial function.

08:38 So let's jump into that.

08:41 The facial nerve when it exits, just interior to the ear and goes to the product gland.

08:45 There's a number of different branches working on different muscles in the face.

08:49 Even before it exited interior to the ear, it went off and innervated the speed's muscle within the middle ear.

08:58 So to test the facial nerve, I'm going to have you first.

09:00 Lift up your eyebrows.

09:02 Great. That's testing the frontal US muscle vigorously.

09:05 Lift up your eyebrows. Great. Now close your eyes really tightly.

09:08 This is the optic hilarious axillary muscles that are going to have him puff out his cheeks.

09:13 Great. And don't let me push them. Perfect.

09:16 And now have you smile. Great.

09:18 So this is a quick way to test for the different innovations of the facial nerve to the muscles of the face.

09:24 Importantly, patients who have a peripheral lesion with a facial nerve palsy, the classic one being Bell's palsy, will have weakness to the entire half of their face.

09:34 In contrast, a patient who's had a stroke, it turns out that the front tallis muscle is duly innervated by both hemispheres.

09:42 So if you had a stroke on the left, yes, you'll have weakness on the left side of your face.

09:47 It's the stroke was on the right side of your in your right cerebral cortex.

09:51 But because the left cerebral cortex also innovates the frontal us muscle, you would still have intact movement of the frontal us muscle.

10:00 So that can be a telltale sign to identify a central lesion versus a peripheral lesion.

10:06 And of course, those are very different approaches that you'll take based on which of those you've you've identified.