Vestibular assessment in definite cerebellar Ataxia, Neuropathy, Vestibular Areflexia Syndrome (CANVAS): A case of siblings study.

Cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome (CANVAS) is a rare neurodegenerative disorder characterized by a triad of cerebellar dysfunction, bilateral vestibular hypofunction, and somatosensory deficits. Here, we report two Japanese siblings with CANVAS caused by biallelic RFC1 repeat expansions, representing the first familial cases identified in Japan. Case 1, a 68-year-old man, and Case 2, his 73-year-old sister, presented with progressive sensory neuropathy and gait disturbances. Magnetic resonance imaging revealed cerebellar and spinal cord atrophy, while vestibular function testing demonstrated progressive declines in vestibulo-ocular reflex gains and compensatory saccades over time. Notably, Case 1 retained otolith function, whereas Case 2 exhibited absent vestibular-evoked myogenic potential responses, reflecting interindividual variability in disease progression. A comprehensive diagnostic workup, including video head impulse test and genetic analysis, was essential for confirming the diagnosis. This report underscores the importance of long-term follow-up and multidisciplinary collaboration in the diagnosis of CANVAS and emphasizes the role of vestibular function testing in identifying bilateral vestibular hypofunction, which may manifest later in the disease course. Given the absence of curative treatments, rehabilitation remains paramount, while ongoing research is crucial for elucidating the underlying pathophysiology and developing effective therapeutic strategies for CANVAS.