Abstract

Horizontal canal benign paroxysmal positional vertigo (HC-BPPV) is characterized by direction-changing nystagmus of the horizontal component induced by lateral head turning in supine position. Yet, some patients with HC-BPPV show nystagmus inversion during head turning to one side. And, mostly, the nystagmus started in a geotropic direction and the inversion was observed only when the head is turned to the lesion side. Reversal of the clot movement in the canal and sensory adaptation were suggested as the hypothesis for this finding. However, bilateral inversion of geotropic nystagmus into apogeotropic nystagmus in HC-BPPV has seldom been described before. In this paper, the authors presented the characteristic features and progression during reposition of a patient with bilateral nystagmus inversion and speculated its mechanisms.