Causes of Diplopia, Strabismus Patterns, and Ocular Motor Features in Patients With Spinocerebellar Ataxia Type 27B

Spinocerebellar ataxia type 27 B (SCA27B) caused by GAA trinucleotide repeats in the fibroblast growth factor 14 gene is emerging as a common cause of late-onset ataxia. Oscillopsia due to downbeat nystagmus (DBN) and diplopia are common symptoms, yet the causes of diplopia and strabismus patterns are poorly defined.

Retrospective chart review of 18 patients diagnosed with SCA27B over the past year.

Ten of 18 patients had episodic or persistent oscillopsia or diplopia at disease onset, neurologically isolated in 4. Seventeen had detectable DBN, although it was often delayed in onset and was clinically obvious in only 5. Diplopia was present in 14 patients: vertical due to skew deviation (static and or alternating on lateral gaze) (n = 8) and/or horizontal due to vergence dysfunction (n = 11). Symptomatic vergence dysfunction included convergence insufficiency (CI) (n = 4) and divergence insufficiency (n = 5). Thirteen of 16 patients experienced improvement in oscillopsia or imbalance on 4-aminopyridine (4-AP).

Strabismus patterns causing diplopia in patients with SCA27B are, not unexpectedly, largely attributable to cerebellar dysfunction and are not unique to SCA27B. The exceptions to cerebellar localization were CI, sixth nerve palsy, and slow saccades. Careful assessment for DBN in patients presenting with episodic or persistent diplopia from skew deviation or vergence disorders is important, as this may be key to confirming a cerebellar localization, subtle on examination, and guide toward genetic testing and 4-AP treatment.