Conclusions: The co-existence of cupulolithiasis and canalolithiasis might be a possible mechanism for the spontaneous inversion of positional nystagmus.
Objective: To investigate the mechanism of spontaneous inversion of nystagmus direction without a positional change in experimental models of co-existing cupulolithiasis and canalolithiasis.
Methods: Co-existing canalolithiasis and cupulolithiasis models were prepared using the bullfrog posterior semicircular canal (PSC). Ten bullfrogs were used. The ampullary nerve discharges were recorded as compound action potentials (CAPs). First, an otoconial mass was placed on the cupula to produce cupulolithiasis. Subsequently, another otoconial mass was introduced into the canal lumen to produce canalolithiasis. Decremental time constants for cupulolithiasis and incremental time constants for canalolithiasis were determined.
Results: At first the CAPs increased and continued for a long time when the cupulolithiasis was generated. Subsequently the CAPs were suppressed by creating canalolithiasis. Finally, the CAPs increased again after the motion of otoconia inside the canal lumen stopped. The decremental time constant for cupulolithiasis was significantly longer than the incremental time constant for canalolithiasis.
Keywords: BPPV; Horizontal canal; benign paroxysmal positional vertigo; lateral canal; positional nystagmus.