A 44-year-old female with known generalized epilepsy presented having fainted two hours after a mild allergic reaction. Following the syncopal episode she had fixed unilateral cerebellar signs consistent with stroke.

Examination revealed left sided cerebellar ataxia, past pointing and dysdiadochokinesis. MRI showed mild cerebellar atrophy with no focal lesions. EMG was unremarkable. CSF was normal.

Her daughter (age 7) has symptoms of progressive ataxia, morning tremor, and dysarthria. An ataxia 98 gene panel revealed an autosomal dominant mutation of ATP1A3, with a heterozygous variant for c.2267G>A p. (ARG756His). This pathogenic variant allowed a diagnosis of ATP1A3-related condition for the daughter with subsequent identification of the same variant gene mutation in ATP1A3 in our patient.

ATP1A3-related neurologic disorders represent a rare clinical continuum with three distinct phenotypes; rapid-onset dystonia-parkinsonism (RDP); alternating hemiplegia of childhood (ACH); and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS). Some individuals have intermediate phenotypes or limited features that do not fit a major phenotype.

Here the phenotype varies between mother and daughter as our patient has overlapping symptoms of CAPSOS and RDP. The hyperacute onset of a cerebellar syndrome is notable. The diagnosis was only possible with knowledge of her daughters’ condition.