Alternating hemiplegia of childhood (AHC) is an ultra-rare neurological disorder named for the transient episodes, often referred to as "attacks", of hemiplegia (weakness or paralysis) that those with the condition experience. It typically presents before the age of 18 months. These hemiplegic attacks can cause anything from mild weakness to complete paralysis on one or both sides of the body, and they can vary greatly in duration. Attacks may also alternate from one side of the body to the other, or alternate between affecting one or both sides during a single attack. Besides hemiplegia, symptoms of the disorder include an extremely broad range of neurological and developmental impairments which are not well understood. Normally, hemiplegia and other associated symptoms cease completely with sleep, but they may recur upon waking. Most frequently AHC is caused by a spontaneous mutation in the ATP1A3 gene. It is an extremely rare disorder ¨C approximately one in one million people have AHC. It was only recently discovered, having first been characterized in 1971.

Reference
Wiki: Alternating hemiplegia of childhood