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.