Downbeating nystagmus and other ocular motor defects caused by lithium toxicity.

We report the clinical and neuropathologic findings of a 63-year-old woman who died following an accidental lithium overdose that produced coma, respiratory depression, horizontal gaze palsy, and downbeating nystagmus. She also had mild hypomagnesemia. The pathology was cytotoxicity, predominantly in the regions of the nuclei prepositus hypoglossi and medial vestibular nucleus. Damage to this area with kainate and ibotenate in rhesus monkeys has produced horizontal gaze palsy and downbeating nystagmus. In addition, we report our clinical experience during the past 6 years with other examples of downbeating nystagmus in patients receiving lithium.