ATP1A3 and alternating hemiplegia of childhood: ATP1A3 mutations have been recognized in infants and children presenting with a diverse group of neurological phenotypes, including rapid-onset dystonia parkinsonism (RDP), alternating hemiplegia of childhood (AHC), and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) syndrome.