Written by Travis M. Moore
Last edited 04-Oct-2019
A good place to begin diagnostic testing is to determine whether the patient can make various types of eye movements at all. If there is an anomaly when inspecting the way the eyes should be able to move, that anomaly will show up in the rest of our tests and needs to be accounted for (if possible) and noted in the report. Apart from testing for spontaneous nystagmus with the eyes straight ahead, there are four main types of eye movements we test when assessing ocular motility:
The diagnostic version of the spontaneous nystagmus (SN) bedside exam uses similar procedures, with the addition of testing without fixation through the use of video goggles. How is this possible? Videonystagmography (VNG) goggles come equipped with cameras that can record eye movements, even in the dark. By putting a cover over the goggles, the patient can no longer fixate, but we can still see what's going on.
A reasonable protocol is to have the patient sit with the cover on the goggles and record for 30 seconds. It is important that you occupy the patient's mind during this test. If there is some small nystagmus present, we don't want the patient to be able to focus and potentially override the eye movements. "Occupying the patient's mind" is referred to as tasking. Tasking can take the form of asking your patient to come up with a girl's name that starts with the letter "A" (and move down the alphabet). The same can be done for boys' names and animals.
After testing in the vision denied condition, remove the cover from the goggles and ask the patient to fixate on a point straight ahead of them. Now we want to see if the patient is able to override any sort of eye movements by concentrating, so we do not need to task.
The purpose of this test is to determine whether a patient can move just the eyes to the left, right, up, and down, while holding each position for at least 20 seconds. The target is typically a dot presented on a light bar or monitor. Holding gaze at an off-center angle requires constant activity from the ocular motor muscles (EOM), referred to as tonic firing. Tonic activity is necessary to hold the eyes at an angle because the EOMs naturally want to pull the eye back to center. Nystagmus during gaze holding can be due to both central and peripheral pathologies.
For horizontal testing, the target should start at 40 degrees from midline (straight head). If there is no nystagmus present, no further eccentricities (positions away from midline) need to be tested. However, if nystagmus is present, testing eccentricities of 30, 20, and 10 degrees should be done to find the angle (if any) where nystagmus is no longer present.
For vertical testing, the target should start at 20 degrees from midline. If there is no nystagmus present, no further eccentricities need to be tested; otherwise, test again at 10 degrees to determine whether the nystagmus goes away.
Assessing smooth pursuit also makes use of a dot on a light bar or monitor. For this test, the patient is instructed to follow the motion of the dot, moving just the eyes. The dot proceeds to move back and forth (sinusoidal motion), starting slowly (0.2 Hz) and slowly increasing in speed to around 0.8 Hz. The localization ability of smooth pursuit is poor, with test abnormalities indicating a lesion in the brainstem or cerebellum.
The ideal optokinetic stimulus fills the patient's entire visual field, and rotates around the patient. The closest we can get is an enclosed rotary chair. This type of chair is literally put inside a cylindrical enclosure with a door built into the curve of the "room." Once inside, the enclosure is dark, which makes testing "vision denied" easy. The benefit for measuring optokinetic nystagmus (OKN) is that these enclosures have a projector built in, which projects a near full-field stimulus (e.g., light bars) onto the wall of the enclosure, and makes the stimulus appear to be rotating around the patient. Due to the cost of these devices, you may not come across them except at hospital clinics, research institutions, and the like.
So what is the alternative if we probably won't have access to an enclosed chair? Some chairs come with a ceiling-mounted projector that will shine the stimulus on the walls of the exam room. Oftentimes though, we just use a monitor displaying moving light bars (or similar). The worst case scenario is to use a series of dots on a light bar appearing to travel to the right or left. Not very full field or rotatory.
The good news is that a large part of what we refer to as the optokinetic response overlaps fairly heavily with the smooth pursuit system. All of the methods mentioned above are capable of measuring smooth pursuit. Let's think about this. If the visual field starts to move, how do you move your eyes? You start out by slowly following the scene. It's only after you make a saccade back to center that OKN is beginning to occur. In fact, it takes a few seconds before the optokinetic system kicks in. So at best we are measuring smooth pursuit and OKN. The only way to measure OKN in isolation is to turn off the stimulus suddenly, and record the nystagmus that is slowly released from the neural integrator. This is called optokinetic after-nystagmus (OKAN), and is variable across patients so not collected clinically very often. In short, it seems like we're ok to use a less-than-perfect stimulus to measure the combination of pursuit and optokinetic system response.
The morphology (shape) of OKN varies depending on how you instruct your patient. Asking the patient to choose one particular dot/light bar and follow it until the eyes naturally go back to center (repeating for 40 seconds) will produce a large slow phase in the nystagmus. This is called look nystagmus, because it's similar to looking out a car window at the passing scenery. As you might expect, this type of OKN has a large smooth pursuit component to it.
Asking the patient to stare straight ahead and count each dot/light bar that passes through the middle of the bar/screen emphasizes the optokinetic system response; however, the overall response is still mostly from the pursuit system. This type of OKN is called stare nystagmus, because it involves staring straight ahead. The patient follows the stimulus over a much shorter distance, attempting to keep the eyes centered to count each stimulus, and the nystagmus has an accordingly small slow phase component.
The stimuli can cross the visual field at different speeds (e.g., 60 and 90 deg/s). The dots/bars should be presented "moving" to the right and to the left.
That was a lot of information, but it's important to understand what you are (and are not) measuring with tests of ocular motility. While peripheral vestibular nystagmus can interfere with these tests simply due to the unwanted eye movement, keep in mind smooth pursuit and saccades are exclusively central tests. Gaze-evoked nystagmus can arise from either peripheral or central structures, and OKN is rarely assessed without a large component from smooth pursuit.
REFERENCESBaloh, R. W., & Honrubia, V. (2001). Clinical Neurophysiology of the Vestibular System (Third ed. Vol. 18): Oxford University Press, Inc.