Ocular Motor and Vestibular Profile in Spinocerebellar Ataxia Type 27B: Toward a Practical Bedside Diagnostic Framework.

Abstract

Spinocerebellar ataxia type 27B (SCA27B), caused by GAA repeat expansions in FGF14, is an increasingly recognized form of late-onset cerebellar ataxia. However, early diagnosis remains challenging due to mild or absent cerebellar motor signs and often normal brain magnetic resonance imaging (MRI). Oculovestibular abnormalities, although prevalent, are frequently overlooked and not captured by standard clinical scales such as the Scale for the Assessment and Rating of Ataxia (SARA). This study aimed to perform a detailed and dedicated evaluation of vestibulo-ocular function in patients with SCA27B, and to develop a practical diagnostic framework that highlights the most prevalent findings and their anatomical correlates. We retrospectively analyzed 20 patients with genetically confirmed SCA27B who underwent structured bedside and quantitative neuro-visual assessments, including video-oculography (VOG) and video head impulse testing (vHIT). As a comparison group, we included patients with genetically confirmed SCA1, SCA2, SCA3, SCA6, and SCA8, who had undergone the same VOG protocol at our center. All SCA27B patients exhibited cerebellar ocular motor abnormalities, including downbeat, gaze-evoked, and rebound nystagmus. Compared to other SCAs, spontaneous and positional downbeat nystagmus was significantly more frequent in SCA27B (p < 0.001), whereas gaze-evoked and rebound nystagmus and impaired smooth pursuit occurred at similar rates, particularly in SCA6, which showed a partially overlapping profile. Quantitative vHIT revealed bilateral vestibular hypofunction, with lower vestibular-ocular reflex (VOR) gain most pronounced in the posterior canals (mean VOR gain: 0.44), followed by anterior (0.54) and horizontal canals (0.83; p < 0.001). Brain MRI was normal in two-thirds of patients, and SARA scores indicated only mild ataxia, underscoring the diagnostic limitations of conventional tools. Our results emphasize the value of oculovestibular evaluation as a sensitive disease marker and support its integration into future composite diagnostic scales for cerebellar ataxias.Trial Registration Information: Not applicable (retrospective study).