Examination of the Cranial Nerves I – VI

00:00 Alright, now we're going to take a deep dive into one of the larger systems that we cover which is the neurologic exam. Now we've already previously talked about the neuromuscular component of the neurologic exam. In this lecture, we're going to cover everything else.

00:15 We're going to start off with the cranial nerves, we're going to go through the assessment of the cerebellum with ataxia, etc. We're going to talk about the assessment of tremor.

00:23 We're going to look for cortical sensation and higher cognitive processing. And we're just going to wrap things up by looking at some of the tests we use to assess for meningitis. So, let's start off by jumping into the cranial nerve exam. So let's start off with cranial nerve number I. This is the olfactory nerve 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 and COVID-19 virus had a predilection for causing anosmia. So, it's useful to, at the bedside, quickly assess for olfaction.

00:57 A simple test to the bedside is simply to use 2 or 3 common sense and for me I will oftentimes use the following. Alright. "So Shawn what I would like you to do is to close your eyes and occlude your right nostril." "Great." "What do you smell?" "Alcohol for sure." "Perfect." "Okay now occlude your other nostril." "What do you smell?" "Tea." "Perfect." Simple bedside test of the olfactory system. Alright, so now let's move on to assessing the second cranial nerve, the optic nerve. We're going to focus on the retina during the fundoscopic exam on the eye exam lecture, but 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 and we'll also look at the pupillary afferent input in combination with looking at the 3rd cranial nerve. So first off, you 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." The Snellen chart can be kept in your pocket and it's easy enough to use. You want the patient to be about 6 feet away from the chart.

02:15 "And I'm going to have you cover up your right eye." You can have the patient do this with glasses and then without glasses depending upon the clinical circumstances that you're doing the assessment in. And I just want you to start around here and tell me the letters that you can read from right to left. "O, L, C, F." "Great. Let's skip down here." "E, P, T, Z, O." "Great." I will 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. You could also usually with the Snellen chart would include some color testing as well as needed. And also there's a nice pupillary gauge on here for when we get to the part of assessing pupillary size.

03:03 "Please have a sit again Sean." 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, but at the bedside we can do a very good job. "So, what I'm going to have you do is have you again cover up your right eye. And I want you to look at my nose. 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." This is called kinetic visual field testing because I'm moving something. "Upper." "Great." Lower. Upper. Lower." Very simple testing, very easy way to do that. And you can move in slowly if you're finding the patient is not detecting things in just those quadrants, but that's a quick and dirty way to assess visual field problems. And of course I would have him do the other side as well. The next component of assessing the optic nerve is again pupillary size and we'll jump in to doing that now. Keep in mind that the afferent input for pupillary constriction is the 2nd cranial nerve, but the efferent pathway which actually involves pupillary constriction happening is mediated through your 3rd cranial nerve. So let's take a look at that. 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. So, first off, we're just going to have you look over here. I'm looking at one pupil, looking for constriction, looking at the other pupil, and looking for a good constriction. I would document my exam that the pupils are going from 4 mm to 2 mm, you want both numbers, both before you put the light in and then after you've exposed them to light. And then an important test is the swinging flashlight test. This is the test for a Marcus Gunn pupil, also known as an afferent pupillary defect oftentimes seen with, for example, optic neuritis from multiple sclerosis where the input from the optic nerve is not being collected in one eye and so there's no afferent information going back to the brain in one eye, but the efferent information from the 3rd cranial nerve is functioning so if I shine light in this eye not only that this eye constrict but this one does as well. And that's completely normal. That's called the consensual pupillary response.

05:34 If I go back to this eye on the right where there is no afferent information, it would actually dilate as this eye was relaxing and not getting input anymore. So that's a classic sign of a Marcus Gunn pupil. 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 specifically mediated by the 3rd cranial nerve due to the problems with sympathetic or parasympathetic input to the pupillary constrictor muscles.

06:06 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, it's not quite a full circle if they have had cataract surgery or some other kind of surgery and there's nothing to be alarmed about if there's a known history and if it has been documented in the past. Alright, so we've already started testing the 3rd 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 3rd cranial nerve. Now we're going to look at the role of the 3rd cranial nerve in terms of oculomotor function, and this is of course going to also include the 4th cranial nerve and the 6th cranial nerve since all 3 of them mediate movement of the eyeball. The 6th cranial nerve, to start off with, is the abducens nerve and that is operating the lateral rectus muscle, the 4th cranial nerve is the trochlear nerve and that's operating the superior oblique muscle and then the 3rd cranial nerve mediates all the other movements of the eye. So the superior and inferior rectus, the medial rectus, and the inferior oblique. "So, to test those things we'll start off by having you look at my nose and just follow my finger with your eyes. Perfect." So, abnormalities that we want to be paying attention to.

07:38 A patient with a lateral rectus palsy will come out when I have him look to one side. If he had a lateral rectus palsy or 6th nerve palsy involving his left eye, his left eye would not move over and only his right eye would move over. Instantly just again with nomenclature, the person who reports double vision is called diplopia and it's important right at the beginning of the exam to really make sure this is true diplopia because of a problem with your oculomotor muscles versus a monocular diplopia which is where you're seeing double vision out of one eye typically due to a displaced contact lens or potentially a problem with your lens. "So, quick and dirty way to do that is simply to cover up your right eye" and ask the patient "Are you still having double vision?" If they have double vision with one eye closed, you know that there is a problem with the lens either contact lens or the actual lens of your eye. So, next up we'll look at the consequence of the 4th cranial nerve. So folks with a superior oblique palsy, it can be a little bit trickier to detect even after we do that grid of movement with the eye.

08:46 The real subtle finding that can help to identify a trochlear nerve palsy which, to be clear, is fairly rare is that the patient will be compensating for trochlear nerve palsy by tilting the head toward the affected side. Since the superior oblique muscle is responsible for intorsion of the eye and also depression of the eye they try to compensate for that by tilting toward that side which will elevate the eye and allow them to still see things in front of them. So just making sure when you're looking at the patient's head that they really do have their head squarely on their shoulders. And lastly, a patient with a 3rd cranial nerve palsy, the oculomotor nerve since you've lost all of the other muscle groups and you have unopposed action of the lateral rectus muscles, a patient with a 3rd nerve palsy will have the affected eye down and out. So heading off towards the lateral corner of their vision. So that would be readily detected as well. So having said that, we'll move on to the 5th cranial nerve, the trigeminal nerve. Now, I always get mixed up with which nerve is the 5th cranial nerve because the trigeminal nerve, the sensory input for the face is called V1, V2, and V3 and so I hear the letter V and I start thinking about the vagus nerve, but the V refers to the roman numeral V. So it's roman numeral V section 1, section 2, and section 3. So, always the nomenclature can throw me off as much as anybody else. So the trigeminal nerve has sensory and motor function. The sensory function I've just eluded to V1 is the ophthalmic branch of the trigeminal nerve then we have a maxillary branch and then a mandibular branch and you can simply test for the function of that nerve as follows. "You feel me on both sides?" "Um-hum." "Does it feel the same on both sides?" "Yes." "Perfect." So that's the testing for V1, V2, and V3 of the trigeminal nerve and then motor function the trigeminal nerve works on the masseter muscle and the pterygoid muscles. "Clench your teeth. Good." I should be able to feel his temporalis muscles up here and the masseter muscles which are keeping his bite nice and tight. "And then I want you to move your jaw side to side." So those are the pterygoid muscles and so by showing evidence that his jaw can move and then he can have a tight clenched jaw tells me that his trigeminal nerve's motor function is functioning.