A 59-year-old woman with late-onset diabetes mellitus presented with a 4-week history of oscillopsia and vertigo. Physical examination revealed downbeating nystagmus in the primary position that worsened on right gaze and left gaze. Magnetic resonance imaging of the brain and orbits showed enhancement and signal abnormality in the right temporal lobe without evidence of a cervicomedullary junction lesion. Serum anti-glutamic acid decarboxylase 65 (GAD65) antibody titer was markedly elevated. Given these findings, her vertigo and downbeat nystagmus were likely secondary to elevated anti-GAD antibodies.
