Mechanism of Downbeat Nystagmus While Normal Subjects HaveHead Upside-down Position in Darkness

ABSTRACT

BACKGROUND: Bohmer and Straumann have proposed that downbeat nystagmus is due to an asymmetry of the geometry of the vertical semicircular canals of the vestibular labyrinth. This hypothesis does not predict any sustained change in vertical nystagmus when subjects are placed in a head-upside-down position. METHODS: Using the magnetic search coil technique, I measured vertical eye movements in darkness in three normal human subjects while they maintained head-erect or head-upside-down positions. RESULTS: Two subjects had upbeat nystagmus and one downbeat nystagmus with their heads erect (slow-phase velocity < 1 deg/sec). All developed sustained nystagmus with quick phases directed towards their chins while in the head-upside-down position. CONCLUSIONS: These findings suggest that factors other than canal imbalance - otolithic or ocular - may also cause downbeat nystagmus.