Head-shaking nystagmus (HSN): the theoretical explanation and the experimental proof.
Head-shaking nystagmus (HSN) is induced by oscillating the head at high frequency in the horizontal plane. This test is used in the clinic to detect the presence of a unilateral loss of vestibular function. HSN has been described as monophasic with fast-phase direction towards either side, or biphasic with the direction of fast phases reversing after a few seconds. Loss of vestibular function amplifies existing non-linearities in the vestibular system, so that imposed sinusoids can induce biases which are the source of HSN. Fifty-one patients suffering from loss of peripheral vestibular function (43 partial, 11 total unilateral tests) were exposed to whole-body sinusoidal stimulation, with increasing head velocities (90-220 degrees/s) at 1/6Hz, to explore the consistency of per-rotatory induced biases. A bias was induced in all cases, but it wandered on either side, healthy or pathologic, unless test head velocities were larger than approximately 180 degrees/s. Given this condition, the slow-phase bias was located towards the pathologic side for all patients with significant bias ( > 5 degrees/s). These observations demonstrate that the sign and amplitude of the bias is variable and is not correlated with 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 labile. 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.