Diagnostic redirection in dementia-first spinocerebellar ataxia type 17: a family-based case report and focused literature review

BackgroundSpinocerebellar ataxia type 17 (SCA17) is an autosomal dominant repeat-expansion disorder with marked phenotypic heterogeneity. Cognitive and neuropsychiatric symptoms may dominate early recognition and initially suggest a primary dementia syndrome. We aimed to illustrate diagnostic redirection in dementia-first SCA17 through integrated clinical, imaging, familial, and molecular assessment.Case presentationWe describe the clinical course, neurological findings, cognitive and functional assessments, ancillary investigations, neuroimaging, pedigree information, and molecular genetic findings of a proband with SCA17 and one tested at-risk adult relative. To contextualize the family, we conducted a focused literature review of genetically confirmed SCA17 case and family reports identified through PubMed, Web of Science, Embase, China National Knowledge Infrastructure (CNKI), and Wanfang up to April 16, 2026.FindingsRepeat-expansion testing established SCA17 in a proband who had initially presented through a dementia-first clinical pathway, with TATA-box binding protein (TBP) alleles of 37/51 repeats. Targeted presymptomatic cascade testing identified the same expanded 51-repeat allele in her asymptomatic adult daughter. Review of 25 published studies showed broad variation in age at onset, TBP repeat size, family context, presenting syndrome, and cumulative phenotype, including cognition-dominant, behavior-dominant, Huntington disease-like, parkinsonian, dystonic, choreic, seizure-associated, and atypical neuroimaging presentations.ConclusionThe present family illustrates a dementia-first route to SCA17 recognition, in which the initial syndrome-based dementia interpretation remained etiologically provisional as cerebellar signs, cerebellar-predominant atrophy, autosomal-dominant family context, and TBP expansion were integrated. This case-based perspective is intended to support etiological reconsideration in selected dementia-first presentations, rather than to serve as validated clinical criteria.