Clinicopathologic Correlates: Progressive Downbeat Nystagmus in Spinocerebellar Ataxia Type 27B.

A 61-year-old woman presented with episodic dizziness that met diagnostic criteria for vestibular migraine. Vestibular and ocular motor examination was normal, aside from a 1°/second downbeat nystagmus (DBN) that was only seen with removal of fixation (in the dark) using video oculography. Two years later, lithium was initiated for treatment of depression and visual bouncing and jumping developed even when her head was still. On repeat examination, she had clear spontaneous (20°/second) DBN even with fixation in room light. Lithium therapy was discontinued, and for days and weeks, there was subjective improvement in oscillopsia and objective lessening of her DBN. However, DBN remained significant and symptomatic after several months. A broad search for reversible nutritional, metabolic, and immune-mediated causes of DBN and cerebellar dysfunction was completed and was unremarkable. Therapy with 4-aminopyridine was initiated and she had a robust improvement in oscillopsia and DBN. During the following years, she developed mild gait ataxia, and genetic testing revealed GAA repeat expansions in 1 copy of the fibroblast growth factor 14 (FGF14) gene, consistent with a diagnosis of spinocerebellar ataxia type 27B (SCA27B).