Evolution of the vestibular function during head impulses in spinocerebellar ataxia type 6.

Evolution of vestibular function requires further elucidation in spinocerebellar ataxia (SCA). This study aimed to determine temporal evolution in the findings of head impulse tests (HITs) in SCA type 6 (SCA6). We serially evaluated HITs in 12 patients with SCA6 using video-oculography for 3 months to 5 years [median = 12 months, interquartile range (IQR) = 9-50] at two university hospitals in South Korea. Patients (8/12, 67%) usually showed abnormal responses at least for one semicircular canal during video-HITs. The gains of the vestibulo-ocular reflex (VOR) for the anterior canals (ACs) were larger than those for the posterior canals (PCs, p = 0.005) at initial presentation. During the follow-up, the VOR gains decreased for the horizontal canals (HCs, p = 0.008) and ACs (p = 0.021), but those for the PCs remained unchanged (p = 0.212). Perverted HITs were observed in seven patients (7/12, 58%). The differences in the head impulse VOR gains were larger between the ACs and PCs (ΔACs - PCs) in those with perverted HITs than in those without (p = 0.003). The gains for each semicircular canal showed a negative correlation with the Scale for the Assessment and Rating of Ataxia (HCs, Spearman's coefficient = - 0.675, p = 0.003; ACs, - 0.637, p = 0.006; PCs, - 0.605, p = 0.010). The head impulse gain of the VOR may serve a marker for clinical decline in SCA6. The dissociation in the temporal evolution of the VOR gain indicates dissimilar cerebellar modulation of the vestibular signals from each semicircular canal.