Understanding Hereditary Ataxias: A Historical Quest for Definition and Classification.

The classification of hereditary ataxias has evolved over two centuries, from anatomical descriptions to genetically informed frameworks. Initially grounded in clinicopathological observation, early models emphasized familial patterns and structural lesions. Key figures, including Gordon Holmes, Joseph Greenfield, and Anita Harding, shaped the nosology, culminating in Harding's inheritance-based classification in the 1980s. This model enabled significant genetic discoveries, including the identification of spinocerebellar ataxia type 1 (SCA1) and the emergence of the SCA nomenclature. However, despite advances in molecular genetics, current classifications remain limited in clinical practice. They offer little guidance for diagnosis or therapy and often function as expanding gene catalogues rather than coherent clinical tools. Recent efforts to standardize nomenclature for autosomal recessive cerebellar ataxias have improved clarity but have not resolved broader challenges. The overlap between hereditary and secondary forms, phenotypic variability, and inconsistent terminology continue to complicate classification. This review retraces the historical trajectory of hereditary ataxia classification to provide insights for future developments. © 2026 International Parkinson and Movement Disorder Society.