We report a 43-year-old man who developed pure acquired downbeat nystagmus with increasing velocity slow phase. This downbeat nystagmus continuously appeared at vertical eye positions of ±20°, while showing nystagmus intensity-vertical eye position relationship contra to Alexander's law. The patient had ischemic lesions at the left caudal paravermis-hemisphere following acute bacterial mengitis that included the paraflocculus. Simulation analysis showed that slow phase position curves of the nystagmus were approximated by both exponential and quadratic curves with a high correlation coefficient. However, differential curves were close to a linear slope rather than an exponential one. There was a linear correlation between vertical eye position and maximum velocity (
p<0.01) of slow phase. The neutral point obtained from the regression line was down 32.6°which was extremely shifted from the primary position. GABAergic agonists were applied orally to evaluate their effects on the nystagmus. We found that clonazepam combined with gabapentin significantly reduced maximum slow phase velocity and the neutral point obtained from the regression line was shifted to down 18.6°. Furthermore, increasing velocity slow phase of downbeat nystagmus changed to linear upward drift in waveform during fixation at the primary to down positions. The present study indicates that our acquired downbeat nystagmus with increasing velocity slow phase resulted from the deficit of inhibitory input from the paraflocculus to the vestibular nuclei and would coexist with vestibular upward drift.
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