Head-shaking nystagmus.

Head-shaking nystagmus (HSN) is provoked by rapid side-to-side movements of the head (head-shaking test: HST). This paper reviews the literature on HSN and HST. 1) HSN is always pathological, as long as it is observed under Frenzel's glasses in a dark room. Many authors assume that it is identical to latent spontaneous vestibular nystagmus.2) HST is the most suitable test for screening purposes in clinical vestibular examination because of its simplicity in manoeuvre and capability in provoking nystagmus. Usually, 20 to 30 cycles of head-shaking horizontally (with an excursion of about +/- 45 degrees and a frequency of about 2 Hz) are a suitable stimulus.3) HSN appears as monophasic or biphasic nystagmus, and as horizontal or vertical nystagmus. A monophasic vertical HSN strongly suggests a lesion in the posterior fossa.4) In monophasic horizontal HSN, the nystagmus is directed in most of the patients with unilateral labyrinthine lesions to the contralateral side of the lesion. This nystagmus (deficiency-type HSN) follows immediately after the head-shaking as a briskly beating nystagmus and regresses rapidly, usually in 10 to 20 seconds. In a minority of the patients with unilateral peripheral lesions, the nystagmus is directed ipsilateral to the site of lesion. This nystagmus (recovery-type HSN) occurs after a short latency (several seconds) following the head-shaking as a lightly beating and somewhat irregular nystagmus, which generally lasts for about 30 seconds or more. The latter type of monophasic HSN is frequently encountered in patients with Menière's disease.5) Biphasic HSN is mainly found in connection with peripheral vestibular disorders and particularly with unilateral disorders. The first phase nystagmus, which appears immediately after the head-shaking, generally represents a deficiency nystagmus. The second phase nystagmus, which appears in the opposite direction after a short latency following the disappearance of the first phase, represents a recovery nystagmus, the direction of which indicates the side of the lesion. This rule is widely applicable except in patients with additional central vestibular imbalance or in patients with Menière's disease in which a reversed response of biphasic HSN may occur.