Childhood disintegrative disorder
Childhood disintegrative disorder (CDD), also known as Heller syndrome and disintegrative psychosis, is a rare condition characterized by a late onset of developmental delays—or severe and sudden reversals—in language (receptive and expressive), social engagement, bowel and bladder, play and motor skills. Researchers have not been successful in finding a cause for this disorder.
Source: Wikipedia — Childhood disintegrative disorder (CC BY-SA 4.0)