2017 Volume 76 Issue 6 Pages 684-691
Positional down-beating nystagmus of peripheral origin (p-DBN) has been considered to be caused by Anterior-canal Benign Paroxysmal Positional Vertigo (A-BPPV). However, recently p-DBN was also described to be caused by a variant of Posterior-BPPV (v-P-BPPV). It has been difficult to make a differential diagnosis between A-BPPV and v-P-BPPV based on positional nystagmus findings alone, but this could be confirmed by the conversion from p-DBN to P-BPPV (canalolithiasis) presenting on the affected side. In this study, we examined a total of 18 conversion cases from P- or Lateral-BPPV (canalolithiasis) to p-DBN, in which the affected side was already identified. The conversion to p-DBN occurred within an average of 3 days after the repositioning maneuver. Based on the existence and the direction of torsional nystagmus, 12 cases were diagnosed as having v-P-BPPV and 6 cases were diagnosed as having probable A-BPPV. Hypothetically, free otoliths in the posterior- or lateral-canal may move towards the posterior-canal near the common crus, generating an inhibitory discharge of the posterior-canal, leading to p-DBN with torsional nystagmus. Furthermore, free otoliths may move through the common crus to the ampulla of the anterior-canal, touching or adhering to the cupula, generating an excitatory discharge of the anterior-canal, and leading to p-DBN with or without torsional nystagmus.
