Heller's syndrome, also known as Childhood Disintegrative Disorder (CDD), is a rare Pervasive Developmental Disorder (PDD) which involves regression of developmental ability in language, social function and motor skills.
It affects around 140,000 children per year worldwide, usually between the ages of 2 and 10, and can occur gradually or suddenly.
The cause(s) are unknown, but are thought to possibly include environmental factors, and possible links with epilepsy.
“Comprehensive medical and neurological examinations in children diagnosed with childhood disintegrative disorder seldom uncover an underlying medical or neurological cause. Although the occurrence of epilepsy is higher in children with childhood disintegrative disorder, experts don't know whether epilepsy plays a role in causing the disorder.” Source: Mayo Foundation for Medical Education and Research
Further reading : UK Patient Info