Biphasic nystagmus in horizontal semicircular canal benign paroxysmal positional vertigo: A case report

Benign paroxysmal positional vertigo (BPPV) involving the horizontal semicircular canal (HSC) typically presents with unidirectional nystagmus during positional testing. However, biphasic nystagmus, characterized by a spontaneous reversal of direction without head movement, can be observed. We report two cases of HSC BPPV (canalithiasis) in which biphasic positional nystagmus was observed during diagnostic positional testing. In both cases, the initial nystagmus spontaneously reversed after a brief latency, without any additional head movement. Notably, one case exhibited reversal on the ear-down side with weaker nystagmus—a pattern not previously described. We hypothesize that free-floating otoconia distributed in both anterior and posterior segments of the HSC may produce sequential excitatory and inhibitory responses, resulting in biphasic nystagmus. These cases may represent an atypical presentation of HSC BPPV and highlight the need for further investigation into otoconial dynamics and their influence on nystagmus patterns.