Question
When nystagmus is observed post head shake, can the side of peripheral lesion be determined based on the direction of nystagmus observed? Are there typical abnormal response patterns? What treatments or additional tests would be indicated when an abnormal response is observed?
Answer
Post head-shaking nystagmus (HSN) is induced by oscillating the head at high frequency in the horizontal plane, as when you shake your head to indicate a "No" response. This test is used in the clinic to detect the presence of a unilateral loss of vestibular function.
There have been two distinct types of HSN in the horizontal plane reported: monophasic with fast-phase direction towards either side, or biphasic with the direction of fast phases reversing after a few seconds. Peripheral vestibular loss of function intensifies existing non-linearities in the vestibular system, so that imposed oscillations of the head can induce biases which are the source of HSN.
Clinical and research observations demonstrate that the sign and amplitude of the bias is variable and does not lateralize the lesioned side, unless high head velocities are imposed. This explains why the direction of the initial phase of HSN in the clinic seems so prone to error. Subsequent monophasic or biphasic characteristics of HSN are simply the reflection of interactions between two main time constants associated with "velocity storage" and "gaze holding" in the vestibular central processes.
Other tests that might be helpful would be tests that measure VOR gain, such as the Vorteq, or possibly tests that help in identifying oscillopsia, such as the Dynamic Visual Acuity Test.
Henry P. Trahan, Au.D., is Assistant Professor of Audiology at the Arizona School of Health Sciences.
Henry Trahan, MCD
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