There's no known cause of childhood disintegrative disorder. The disorder may occur abruptly over the course of days to weeks or gradually over an extended period of time.
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.
Childhood disintegrative disorder has also been associated with:

• Tuberous sclerosis. In this condition, noncancerous (benign) tumors grow in the brain.
• Lipid storage diseases. In this rare group of inherited metabolic disorders, a toxic buildup of excess fats (lipids) occurs in the brain and nervous system.
• Subacute sclerosing panencephalitis. This chronic infection of the brain is caused by a form of the measles virus that results in brain inflammation and the death of nerve cells.

Experts also suspect there may be a genetic basis or that an autoimmune response may play a role in the development of childhood disintegrative disorder. In an autoimmune response, your body's immune system attacks itself.
Further studies are needed to investigate the causes of childhood disintegrative disorder, but this is difficult due to the limited number of children diagnosed with the disorder.