ATP1A3 and alternating hemiplegia of childhood: The allelic disorders rapid-onset dystonia-parkinsonism (RDP), alternating hemiplegia of childhood (AHC), and CAPOS/CAOS syndrome (cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural deafness) are caused by ATP1A3 mutations.1, –, 3 Intermediate RDP-AHC phenotypes are emerging.