Purpose of review: Patients presenting with features of Huntington's disease but lacking the causative genetic expansion can be challenging diagnostically. The differential diagnosis of such Huntington's disease phenocopy syndromes has not recently been reviewed.
Recent findings: Cohort studies have established the relative frequencies of known Huntington's disease phenocopy syndromes, whereas newly described ones have been characterized genetically, clinically, radiologically and pathologically.
Summary: About 1% of suspected Huntington's disease cases emerge as phenocopy syndromes. Such syndromes are clinically important in their own right but may also shed light on the pathogenesis of Huntington's disease. Huntington's disease produces a range of clinical phenotypes, and the range of syndromes that may be responsible for Huntington's disease phenocopies is correspondingly wide. Cohort studies have established that, while the majority of phenocopy patients remain undiagnosed, in those patients where a genetic diagnosis is reached the commonest causes are SCA17, Huntington's disease-like syndrome 2 (HDL2), familial prion disease and Friedreich's ataxia. We review the features of the reported genetic causes of Huntington's disease phenocopy syndromes, including HDL1-3, SCA17, familial prion disease, spinocerebellar ataxias, dentatorubral-pallidoluysian atrophy, chorea-acanthocytosis and iron-accumulation disorders. We present an evidence-based framework for the genetic testing of Huntington's disease phenocopy cases.