Suspected autoimmune cerebellar ataxia: Predictors of immunotherapy response and diagnosis reclassification to neurodegenerative disease

Background Autoimmune cerebellar ataxia (ACA) is potentially treatable; however, some patients initially suspected of having ACA are later diagnosed with neurodegenerative diseases. We examined clinical features, immunotherapy response, and diagnostic reclassification in patients with suspected ACA. Methods We retrospectively reviewed 130 consecutive patients admitted for cerebellar ataxia between January 2015 and December 2023. At admission, 24 were classified as clinically suspected ACA and 81 as SCA/MSA (spinocerebellar ataxia or multiple system atrophy). The 24 cases were subclassified as definite, probable, or non-criteria ACA according to the Dalmau–Graus criteria. Immunotherapy response was defined as a ≥ 3-point improvement in the Scale for the Assessment and Rating of Ataxia (SARA). Results Compared with the SCA/MSA group, the suspected ACA group had lower frequencies of cerebellar atrophy, brainstem atrophy, and cerebellar hypoperfusion (all p < 0.005). Among 20 evaluable treated patients, 11 responded; responders presented earlier than non-responders (median onset-to-presentation interval, 12.0 vs 24.0 months; p = 0.006). Based on the ACA criteria, 5/24 were definite and 6/24 probable. During follow-up through December 2025, 8/24 patients (33.3%) were reclassified as having neurodegenerative diseases. Conclusions Approximately one-third of patients initially suspected of having ACA were later reclassified as having neurodegenerative diseases. Although fulfillment of the Dalmau–Graus criteria was less frequent among reclassified patients, routine baseline clinical and imaging features did not clearly distinguish them. Some patients with early neurodegenerative cerebellar ataxia may initially present with clinical features overlapping those of ACA, underscoring the importance of longitudinal follow-up and periodic diagnostic reassessment.