Reversal of initial positioning nystagmus in benign paroxysmal positional vertigo involving the horizontal canal.

In benign paroxysmal positional vertigo (BPPV), spontaneous reversal of the initial positioning nystagmus rarely occurs without further position changes. We analyzed the characteristics of spontaneous reversal of the initial head-turning nystagmus in 21 patients with BPPV involving the horizontal semicircular canal. All patients showed initial geotropic nystagmus (first-phase nystagmus) on head turning to either side while supine, which was followed by spontaneous reversal (second-phase nystagmus). The reversal was either unilateral (n = 16) or bilateral (n = 5). The maximal slow phase velocity (SPV) and duration of the first-phase nystagmus were greater and shorter than those of the second-phase nystagmus. The reversal group showed greater maximal SPVs of the initial nystagmus in either ipsi- or contralesional direction than the control group. BPPV resolved after particle repositioning maneuver (PRM) in most patients. However, one patient showed persistent apogeotropic nystagmus after PRM. Short-term adaptation of the vestibulo-ocular reflex seems to be the main mechanism of spontaneous reversal of the initial positioning nystagmus. However, coexistence of canalo- and cupulolithiasis should be considered in the patient showing bilateral spontaneous reversal.