http://www.mayoclinic.com/health/childhood-disintegrative-disorder/DS00801/DSECTION=symptoms
Children with childhood disintegrative disorder typically show a [1]dramatic loss of previously acquired skills in two or more of the following areas:

• Language, including a severe decline in the ability to speak and have a conversation
• Social skills, including significant difficulty relating to and interacting with others
• Play, including a loss of interest in imaginary play and in a variety of games and activities
• Motor skills, including a dramatic decline in the ability to walk, climb, grasp objects and perform other movements
• Bowel or bladder control, including frequent accidents in a child who was previously toilet trained

Loss of developmental milestones may occur abruptly over the course of days to weeks or gradually over an extended period of time.

When to see a doctor
Children typically develop at their own pace, but any loss of developmental milestones is cause for concern. If your child has suddenly lost previously acquired language, social, motor, play, thinking (cognitive) or self-help skills, such as toilet training and feeding, contact your doctor. In addition, if you suspect that your child has gradually shown a loss in any area of development, talk with your doctor.