title: Differential diagnosis of true and pseudo-bilateral benign positional nystagmus.
abstract: CONCLUSIONS
  Two patients showing two rotational axes of their positional nystagmus had canalolithiasis in bilateral posterior semicircular canals (PSCCs), leading to the diagnosis of true bilateral benign paroxysmal positional nystagmus (P-BPPN). Another 18 patients had a single rotation axis of their positional nystagmus with short time constant (TC) on one side and long TC on the other. Since canalolithiasis in the uppermost PSCC may be transient cupulolithiasis, evoking positional nystagmus with long TC, the diagnosis of the latter patients should be pseudo-bilateral P-BPPN.
  
  
  OBJECTIVE
  To differentiate the true bilateral posterior canal type of P-BPPN from pseudo-bilateral P-BPPN.
  
  
  METHODS
  The rotational axis and TC of positional nystagmus were three-dimensionally analyzed in 20 patients showing geotropic torsional nystagmus on the left and right Dix-Hallpike (D-H) maneuvers.
  
  
  RESULTS
  Two patients showed two rotational axes of their positional nystagmus, which were perpendicular to the plane of the PSCCs. There were no differences in TCs of their positional nystagmus between bilateral D-Hs. Another 18 patients showed a single rotational axis of their positional nystagmus, which was perpendicular to the plane of either the left or right PSCC. TCs of their positional nystagmus were short on one side and long on the other.
authors.affiliation: Department of Otolaryngology, Osaka University Graduate School of Medicine, Osaka, Japan. imaitakao@hotmail.com
authors.forename: Takao
authors.initials: T
authors.lastname: Imai
authors.forename: Noriaki
authors.initials: N
authors.lastname: Takeda
authors.forename: Go
authors.initials: G
authors.lastname: Sato
authors.forename: Kazunori
authors.initials: K
authors.lastname: Sekine
authors.forename: Mahito
authors.initials: M
authors.lastname: Ito
authors.forename: Koji
authors.initials: K
authors.lastname: Nakamae
authors.forename: Takeshi
authors.initials: T
authors.lastname: Kubo
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003937:Diagnosis, Differential; D004584:Electronystagmography; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D008875:Middle Aged; D009760:Nystagmus, Physiologic / Q000502:physiology*; D010037:Otolithic Membrane / Q000503:physiopathology*; D012665:Semicircular Canals / Q000503:physiopathology*; D014724:Vestibular Function Tests; D014743:Videotape Recording