Clinical significance of biphasic head-shaking nystagmus

Biphasic head-shaking nystagmus (b-HSN) is a rather uncommon phenomenon which was first reported by Kamei in 1975 (Arch. Otorhinolaryngol. 209: 59-67, 1975). He pointed out that b-HSN appeared in cases of peripheral vestibular disturbance especially in the cases of unilateral disturbance, and that the direction of the second phase indicated the damaged side. While investigating cases of b-HSN, the author encountered some cases which did not correspond to b-HSN reported by Kamei. Nineteen patients in whom b-HSN was recorded by electronystagmography were investigated. These patients visited our clinic in the last two years and showed no spontaneous nystagmus in the narrow sense. Of these, 16 were cases of unilateral peripheral vestibular disturbance. In 13 of these (81%), as Kamei had described, the first phase beat toward the healthy side and the second phase beat toward the damaged side, while in the remaining 3 cases (19%), the first phase beat toward the damaged side, and the second phase toward the healthy side. This is contrary to Kamei's report. This time the b-HSN was also observed in 3 cases of central vestibular disturbance, which indicates that b-HSN occurs not only in cases of peripheral vestibular disturbance but also in cases of central vestibular disturbance.