Childhood Disintegrative Disorder (CDD), also known as Heller's syndrome dementia infantilis and disintegrative psychosis:
Childhood Disintegrative Disorder is characterized by a late onset of developmental delays, usually after three years old but before ten years old, the child will start to regress or lose language and previously mastered skills. This may happen as quickly as a few days or within a few weeks. This is one of the saddest and most unfortunate disorders on the Autism Spectrum. There is no cure or medication that can treat CDD, but with behavior therapy, a child can relearn some of the skills that were lost.
Working With Autism Behavior Management Reward Systems