Three-dimensional eye position and slow phase velocity in humans with downbeat nystagmus.
Downbeat nystagmus (DN), a fixation nystagmus with the fast phases directed downward, is usually caused by cerebellar lesions, but the precise etiology is not known. A disorder of the smooth-pursuit system or of central vestibular pathways has been proposed. However, both hypotheses fail to explain why DN is usually accompanied by gaze-holding nystagmus, which implies a leaky neural velocity-to-position integrator. Because three-dimensional (3-D) analysis of nystagmus slow phases provides an excellent means for testing both hypotheses, we examined 19 patients with DN during a fixation task and compared them with healthy subjects. We show that the presentation of DN patients is not uniform; they can be grouped according to their deficits: DN with vertical integrator leakage, DN with vertical and horizontal integrator leakage, and DN without integrator leakage. The 3-D analysis of the slow phases of DN patients revealed that DN is most likely neither caused by damage to central vestibular pathways carrying semicircular canal information nor by a smooth pursuit imbalance. We propose that the observed effects can be explained by partial damage of a brain stem-cerebellar loop that augments the time constant of the neural velocity to position integrators in the brain stem and neurally adjusts the orientation of Listing's plane.
Glasauer
S
S
Department of Neurology with Center for Sensorimotor Research, Klinikum Grosshadern, Ludwig-Maximilians-University, 81377 Munich, Germany. sglasauer@nefo.med.uni-muenchen.de
Hoshi
M
M
Kempermann
U
U
Eggert
T
T
Büttner
U
U
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001933:Brain Stem / Q000166:cytology / Q000503:physiopathology; D002531:Cerebellum / Q000166:cytology / Q000503:physiopathology; D005133:Eye Movements / Q000502:physiology*; D005403:Fixation, Ocular / Q000502:physiology; D006801:Humans; D008875:Middle Aged; D008959:Models, Neurological; D009434:Neural Pathways; D009759:Nystagmus, Pathologic / Q000503:physiopathology*; D012044:Regression Analysis