Akihide Ichimura
In horizontal semicircular canal benign paroxysmal positional vertigo (HSC-BPPV) due to canalolithiasis, biphasic nystagmus is typically observed in the affected ear down or in both ear-down positions during the supine head-roll test. This report describes a case of HSC-BPPV with biphasic nystagmus observed only in the healthy ear-down position. A 59-year-old male had a one-day history of rotational vertigo when turning to the right. The supine positional nystagmus test produced left-beating horizontal nystagmus in the face-up position. In the left-ear-down position, there was biphasic nystagmus (i.e., left-beating horizontal nystagmus followed by right-beating horizontal nystagmus). In the right-ear-down position, there was right-beating horizontal nystagmus. The first-phase nystagmus observed in the left-ear-down position was stronger than the right-beating nystagmus in the right-ear-down position. The patient was initially diagnosed with left-sided HSC-BPPV (canalolithiasis), and the Lempert roll maneuver was performed. Re-examination the following day revealed findings consistent with right-sided HSC-BPPV (cupulolithiasis). Biphasic nystagmus can occur with the presumed healthy ear-down during the supine head-roll test. Misidentification of the affected side in HSC-BPPV (canalolithiasis) may lead to an inappropriate therapeutic maneuver and suboptimal treatment. In the present case, the affected side was initially misidentified, and persistent direction-changing apogeotropic nystagmus consistent with HSC-BPPV (cupulolithiasis) was observed after the maneuver. Although the mechanism underlying this change remains speculative and is based on a single case, clinicians should recognize that biphasic nystagmus during the head-roll test does not necessarily identify the affected ear. Careful interpretation of all positional nystagmus findings is warranted before performing a therapeutic maneuver.