To describe an atypical presentation of spinocerebellar ataxia type 8 (SCA8) with chorea, and how this impacts workup of Huntington’s disease (HD)-like syndromes.

SCA8 is associated with CTG repeat expansions in ATXN8OS and inherited in an autosomal dominant pattern. Common manifestations include abnormal eye movements, extrapyramidal signs, ataxia, and cognitive impairment.

Our patient was adopted at age 4 with unknown family history. In childhood, she was diagnosed with mild intellectual disability. At age 24, she started exhibiting choreiform movements in her face and mouth, with less involvement of the hands and feet. On exam, she had jerky, incomplete ocular pursuit and difficulty initiating saccades in all directions. No deficits were elicited in speech, muscle tone, reflexes, finger-nose-finger, and gait. Brain MRI, CRP, SPEP, ANA, ceruloplasmin, and blood smear were unremarkable. Genetic testing showed 27 and 25 CAG repeats in her HTT alleles, as well as 16 and 10 in ATN1, ruling out HD and DRPLA respectively. Movement disorders panel revealed 82 repeats of a CTG expansion within one ATXN8OS allele.

Several SCA subtypes constitute HD-like syndromes and can manifest as chorea, with SCA17 by far the most common. Cohort studies of SCA8 patients indicate that when hyperkinesia is present, the typical phenotype involves dystonia and tremor. However, a 2012 report of HD phenocopies described one patient with 81 CTG repeats in ATXN8OS, presenting at age 58 with chorea but no cerebellar findings. Our patient thus constitutes the second known case of SCA8 manifesting as an HD-like syndrome. These two patients suggest that the relationship between trinucleotide repeat size and age of hyperkinesia onset may not be as clear in SCA8 compared to HD. Moreover, extraocular movements should be carefully examined, since truncal and limb ataxia may be absent.

The evaluation of HD-like syndromes should include screening for all SCA subtypes.