ATP1A3 and alternating hemiplegia of childhood: ATP1A3-related neurologic disorders have been shown to cause multiple distinct phenotypes: rapid-onset dystonia-parkinsonism (RDP), alternating hemiplegia of childhood (AHC), and cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss (CAPOS) syndrome [8–11].