To characterize genotype-specific ocular motor features in spinocerebellar ataxia (SCA) as a function of SCA genotype.

The spinocerebellar ataxias are a genetically diverse group of AD neurodegenerative disorders attributed to mutations in nearly 50 distinct genes. They exhibit unique clinical features, among which are varied eye movement abnormalities. Based on recent reports regarding down-beat nystagmus, associated with the newly described SCA27B, we reviewed bedside exams of a large cohort of genetically defined SCA patients.  

Findings from standardized oculomotor exams applied to a cohort of 312 genetically defined SCA patients (SCA1,2,3,4,5,6,7,8,11,15,17,26,34,36,49) were collected by retrospective chart review.  The comprehensive protocol included annotations of spontaneous movements in primary position, lateral/vertical gaze, estimated ocular versions, and speed/accuracy of saccades to finger targets positioned at 10 degrees.  Additional data included the neurological exam, scale for assessment and rating of ataxia (SARA), demographic information, and disease course. 

The most common oculomotor abnormalities were impaired smooth pursuit (68%), horizontal gaze-evoked nystagmus (64%), and saccadic overshoots (38.6%). Saccadic slowing occurred in 78.4% of SCA2 and gaze intrusions were common in SCA3 (horizontal nystagmus 66.7%, square-wave jerks 25.6%, and downbeat nystagmus 6.4%). Restricted versions were most common in SCA2 (40.5% horizontal, 35% vertical). SARA scores tended to be higher in patients with EOM abnormalities. Downbeat nystagmus was predominantly found in SCA6 (59%) and SCA 27B (50%).

1. No single ocular motor abnormality could define a specific SCA type.
2. Saccadic pursuit and horizontal nystagmus were common in several SCA types.
3. Downbeat nystagmus was predominantly found in both SCA6 and SCA 27B.
4. Saccade slowing and restricted versions were most common, but not unique to SCA2.
5. SCA3 exhibited the most diverse oculomotor abnormalities.
6. The presence of oculomotor abnormalities indicates disease progression due to propensity for higher SARA scoring. 
7. Distinct oculomotor features in SCA may aid in tailored clinical management.