Direction-reversing Nystagmus in Horizontal and Posterior Semicircular Canal Canalolithiasis.

OBJECTIVES: To investigate the incidence and characteristics of direction-reversing nystagmus in patients with horizontal (HSCC) and posterior semicircular canal (PSCC) canalolithiasis, and evaluate the effect of direction-reversing nystagmus on the treatment outcome. STUDY DESIGN: A retrospective study. METHODS: Between March 2014 and September 2015, 63 and 92 consecutive patients with HSCC and PSCC canalolithiasis, respectively, were enrolled. Positional nystagmus characteristics were examined using video-nystagmography. RESULTS: In HSCC canalolithiasis, direction-reversing nystagmus was observed in 73% of patients (46 of 63), of which 19 cases were bilateral and 27 unilateral. In patients with bilateral reversal, maximal slow-phase velocity (mSPV) was significantly greater when the head turned to the lesioned side than to the healthy side in both the first and second phase. In all patients with unilateral reversal, direction-reversing nystagmus always occurred in the side of stronger initial nystagmus in a supine roll test. The mean mSPV of first phase nystagmus was significantly greater on the side with reversal than without (p < 0.001). The duration of second-phase nystagmus exceeded 60 seconds in all patients with reversal. Although not statistically significant (p = 0.059), patients presenting with direction-reversing nystagmus required more repositioning maneuver sessions. In contrast to HSCC canalolithiasis, only 4% of patients (4 of 92) with PSCC canalolithiasis exhibited spontaneous reversal of initial nystagmus. CONCLUSION: The incidence of direction-reversing nystagmus was higher in HSCC canalolithiasis than in PSCC canalolithiasis, and second-phase (direction-reversing) nystagmus in HSCC canalolithiasis has a prolonged duration. Short-term adaptation of the vestibulo-ocular reflex may be responsible for the development of direction-reversing nystagmus.