Written by Travis M. Moore
Last edited 11-Oct-2019
There are many different types and patterns of involuntary movements that the eyes can make. For example, some movements indicate underlying pathology, some signal the limits of the extra ocular muscles, and some help with our vision. The implication for diagnostic tests that record eye movements is that we must be able to differentiate between movements that are normal or of no concern, and those that help to identify the presence (and sometimes the location) of a lesion.
When we assess patients in the balance clinic, we are largely looking for nystagmus (an involuntary, rhythmic oscillation of the eyes; see Figure 1). Jerk nystagmus is the prototypical form of nystagmus. It is characterized by having a fast phase and a slow phase. This just means that the eyes move quickly in one direction (fast phase) and slowly in the opposite direction (slow phase). This jerky motion is referred to as beating, with the direction of the beating specified by the direction of the fast phase. Figure 1 below shows left-beating jerk nystagmus.
What you might not realize is that in certain situations, nystagmus is a good thing. In the clinic, we test for both good and pathologic nystagmus, and sometimes the only difference between "healthy" and "unhealthy" is under what conditions the nystagmus is present. When nystagmus occurs because the vestibular system is functioning normally, it is referred to as physiologic nystagmus. Abnormal nystagmus is referred to as pathologic nystagmus.
This type of nystagmus occurs when the eyes are moved to the extreme right/left/up/down (i.e., greater than 40 deg). Endpoint nystagmus can also occur if an eccentric gaze is held for more than 60 seconds.
If you turn around and around, the vestibular system will move your eyes in a way that prevents the entire scene from becoming a giant blur. Rotational nystagmus is initiated to keep part of the scene around you in focus. That means a slow eye movement in the opposite direction in which you are turning. Why? If your body is spinning to the left, your eyes would have to move to the right just to stay in the same place (i.e., to offset the motion of the body toward the left). However, at some point your body spins further away than your eyes can move (i.e., the eyes are at the extreme right corners and can go no further). When this point is reached, the eyes snap back to the direction of the rotation (left corner of the orbit), where they can focus on a new target. This process happens repeatedly during rotation, and is an effective method of helping you keep your bearings through clear glimpses of the environment as you spin.
Optokinetic nystagmus (OKN) is related to rotational nystagmus. This time, however, instead of your body rotating, the world is rotating around you (just like your mom always told you). Well, the scene doesn't even need to be rotating, per se. OKN is elicited whenever at least 90% of the scene in your field of vision is moving. Think of this like when you look out the window in the car. The landscape passes by, and as it does so, you focus on a part of the scene by making slow eye movements in the opposite direction of travel. Once your eyes reach their limits in the orbit, they snap back to the far left (i.e., the direction of travel) to find a new object to focus on and the process repeats.
Peripheral spontaneous nystagmus (PSN) is a type of jerk nystagmus that is primarily horizontal, but can have some vertical and torsional components as well. This makes sense if you recall two things: (1) PSN suggests there is an imbalance in tonic firing from the two labyrinths, and (2) the vertical semicircular canals and otoliths are arranged in a way that their tonic firing somewhat cancels out. The horizontal canal, however, has no opposing tonic signal on the contralesional side. The result is that the vertical and torsional components of the nystagmus are decreased, but the horizontal component is unopposed.
Central spontaneous nystagmus (CSN) is a type of jerk nystagmus that is typically purely horizontal, vertical, or torsional. The central vestibular system processes the incoming signals from the various direction-sensitive structures in the peripheral labyrinth, and begins to separate that information into separate pathways. That means if a pathway is damaged, the other pathway is unopposed. For example, if a lesion occurs in the central vestibular system and only affects the horizontal pathway, then only vertical information will be sent and only vertical eye movements will happen.
Gaze-evoked nystagmus (GEN) is elicited when the patient moves the eyes away from primary gaze (i.e., looking straight ahead). GEN can be a sign of either a peripheral or central pathology. GEN is often a jerk nystagmus, with a slow and fast phase. It is important to note that GEN can occur in healthy patients if the eyes are moved too far in one direction (i.e., more than 40 deg), and/or if eccentric gaze is held for longer than 30 seconds.
When gaze-evoked nystagmus is present in an eccentric eye position (e.g., to the far right), it is possible to see rebound nystagmus when the eyes are returned to center. Rebound nystagmus is a short-lived jerk nystagmus that beats in the opposite direction of the eccentric gaze. For example, if the patient held gaze to the far right, when the eyes returned to the center there would be a few seconds of nystagmus beating to the left. Note that one or two beats can be seen in a healthy patient if eccentric gaze is held for a long time (see also end-point nystagmus).
There are other types of involuntary eye movements apart from nystagmus. While nystagmus can serve an extremely beneficial purpose in a healthy patient, the following types of eye movements do not aid in vision or balance.
A square wave jerk is when the eyes make a small, quick movement away from fixation (around 0.5 to 4 deg), pause briefly (around 200 - 400 ms), then make a quick movement back to fixation. Note that square wave jerks occur in healthy patients, especially in the elderly. They are only considered abnormal when the jerks are larger than 5 deg and/or occur more than ~30 times per minute.
The more extreme version of square wave jerks (see above) are macro square wave jerks. The eye movements exceed 5 deg, and only pause for roughly 80 ms before making another quick movement. These movements are rare, and tend to happen in short bursts.
A healthy patient can keep the head still and find a target off to one side just by making a single quick eye movement. A disordered patient will sometimes overshoot the target, and have to make a corrective eye movement back. Patients displaying macro saccadic oscillations also overshoot the target when making the corrective eye movement, with the effect that the eyes oscillate back and forth around the target several times.
Ocular flutter consists of quick back-and-forth movements of the eyes in a single plane (usually horizontal). As many as 10 - 25 oscillations can occur per second. This condition is easy to notice, and often occurs during intentional quick eye movements.
Opsoclonus is a more extreme form of ocular flutter, that includes movements in multiple directions: horizontal, vertical and torsional. The movements can sometimes be quite large, and can interfere with smooth pursuit and fixation. To give you an idea of how marked these movements can be, another name of opsoclonus is "saccadomania."
REFERENCESBaloh, R. W., & Honrubia, V. (2001). Clinical Neurophysiology of the Vestibular System (Third ed. Vol. 18): Oxford University Press, Inc.