2014 年 73 巻 6 号 p. 533-537
We report herein on the clinical features in ten patients with positional down-beat nystagmus (p-DBN) without any sign and symptom of central nervous system (CNS) disorder. Brain MRI was normal in all subjects, except in one patient who refused the examination. Gaze nystagmus was not present in any patient. Down-beat nystagmus was observed only in the supine or head-hanging position: it was never observed upon returning to a sitting position. The patients' p-DBN had no torsional component, but had some latency. All patients felt dizziness when p-DBN was observed. In 8 patients (80%), benign paroxysmal positional vertigo (BPPV) preceded the incidence of p-DBN. Three patients (30%) were diagnosed as having posterior semicircular canal BPPV before the occurrence of p-DBN. Dizziness disappeared in 8 patients (80%) within 6 months, while p-DBN remained in 4 patients (40%). Although p-DBN without any CNS disorder was suspected to be a symptom of BPPV associated with the anterior semicircular canal, we question the strength of this hypothesis mainly because no upward-beat nystagmus is seen by reversing the position.