An atypical clinical course of a 71-year-old man with right arm weakness and ataxia

•71-year-old simplex case with a diagnostically challenging neurological disorder.•Complex course with phenotypic overlap between MSA-C and spinocerebellar ataxias.•Investigation excluded MSA-C while clinical signs of SCA3 prompted genetic testing.•SCA3, presenting with an atypical course, was confirmed by genetic tests.•Test results discordant with the clinical course warrant cautious interpretation.•Lack of family history should not deter genetic testing in ataxic disorders.