ATP1A3-related disorders in the differential diagnosis of acute brainstem and cerebellar dysfunction.

Alternating Hemiplegia of Childhood (AHC), Rapid-onset Dystonia-Parkinsonism (RDP), and CAPOS syndrome (Cerebellar ataxia, Areflexia, Pes cavus, Optic atrophy, and Sensorineural hearing loss) are all caused by mutations in the same gene: ATP1A3. Although initially they were considered separate disorders, recent evidence suggests a continuous clinical spectrum of ATP1A3-related disorders. At onset all these disorders can present with acute brainstem dysfunction triggered by a febrile illness. An infectious or autoimmune disorder is usually suspected. A genetic disorder is rarely considered in the first acute episode. We present three patients with ATP1A3 mutations: one patient with AHC, one patient with RDP, and one patient with CAPOS syndrome. We describe the acute onset and overlapping clinical features of these three patients with classical phenotypes. These cases highlight ATP1A3-related disorders as a possible cause of acute brainstem dysfunction with normal ancillary testing.