Written by Michelle Petrak, Ph.D.
The optokinetic reflex allows the eyes to follow objects in motion while the head remains stationary.
This function is performed within the central vestibular system.
The inability to produce symmetric optokinetic nystagmus implies a dysfunction of the central vestibular system.
It should be noted that to have a valid optokinetic nystagmus test, you must stimulate the patient’s entire visual field.
That is why a video projector or large LCD screen is necessary for this test.
If the test is set up to maximize the patient’s visual field, then very little instruction is needed since the response is a ‘reflex’ and should occur on its own with little intervention.
Instruct the patient to “simply look ahead and watch the pattern in front of you.”
Optokinetic responses are measured for both rightward-moving and leftward-moving visual fields at varying velocities.
A patient with the ability to perform the optokinetic nystagmus test normally will produce tracings that have symmetrical slow phase velocity for leftward and rightward eye movements and gain responses that are within normal limits for both directions (Figure 1).
Figure 1: Normal optokinetic test results.
An abnormal response seen in one direction but not the other is suggestive of a vestibular disorder.
If not accompanied by spontaneous nystagmus, the disorder is likely central in origin.
When the maximum speed of the slow phase velocity is less than 60% of the stimulus speed, it is outside of threshold limits.
When the asymmetry between rightward-moving and leftward-moving eye movements is greater than 25%, it is outside of threshold limits.
Figure 2 shows an abnormal response.
Figure 2: Optokinetic test showing abnormal gains and an asymmetrical response.
It is possible that the patient can track the target in one direction, but not in the other direction.
In most cases, the abnormality will present itself toward the side that has the lesion.
Also, it is critical that the examiner is aware of whether the patient is actively watching the screen.
The optokinetic nystagmus test is often disturbing to the patient’s vestibular system.
To compensate, the patient might try to ‘stare through’ the screen.
The result will appear as a ‘flat-line’ on the tracing.
This is good information in that it may show that the patient has adopted this ‘stare through’ strategy in learning to compensate for the vestibular disorder.
Optokinetic tests are used to determine whether there is a central pathology that is precluding a reflexive response to targets moving across the field of vision while the head remains immobile.
This is intended only as a guide; official diagnosis should be deferred to the patient’s physician.
Michelle Petrak, Ph.D., is the Director of Clinical Audiology and Vestibular Research for Interacoustics. Her primary role is development and clinical validation of new technologies in the vestibular and balance areas. She is located in Chicago where she is a licensed private practice clinical audiologist at Northwest Speech and Hearing (NWSPH). Dr. Petrak received her doctorates in Electrophysiology (1992) and Biomolecular Electronics (1994) from Wayne State University and her Masters in Audiology in 1989. Her special areas of expertise include vestibular and balance assessments and management of the dizzy patient. Dr. Petrak is involved with new innovative product developments, clinical evaluations of new protocols, and publishing, teaching, and training on the management of patients with dizziness. She continues to lecture extensively nationally and internationally, and she has numerous articles published in the hearing industry journals. She also participates on the committees for several doctoral students as support for the research projects.