Abstract
We report herein on a case of acoustic neuroma, which showed persistent geotropic positional nystagmus at the fourth time under clinical observation. This positional nystagmus was static direction-changing positional nystagmus (DCPN) canceled at 20 degrees yaw head rotation from the supine position. This nystagmus was also cancelled when the head was rotated 180 degrees from this position. We termed these head positions the “neutral point”. In addition, when the head was rotated to either side away from the neutral point, this case showed nystagmus beating away from the neutral point (as the geotropic direction). This positional nystagmus observed at all positions except for the neutral points was thought to occur due to “light cupula”, which may be determined by the specific gravity of its endolymph in the lateral semi-circular canal. The affected side was thought to be that to which the neutral point deviated from the supine position, so the affected side of this positional nystagmus corresponded to that of the acoustic neuroma. Acoustic neuromas localized in the internal auditory meatus at the early stage, may influence peripheral endo-organs such as the lateral semi-circular canal in any way and show symptoms associated with peripheral vestibular disorder before paralytic nystagmus will appear, derived from a retrolabyrinthine neural disorder caused by the progression of the tumor.
