During the period of regression, mental retardation sets in and autistic behavior begins to develop. Associated symptoms often include emotional lability, fits of anger, aggression, sleep disorders, hyperactivity, clumsy walking, abnormal reactions to sound stimuli, and clinging to stereotypes. The regression phase gradually turns into the stagnation phase. With age, symptoms may become milder, but never completely disappear. This is a very rare disorder, which is more often diagnosed as autism. Prior to the outbreak of the disorder, increased anxiety and tension, increased activity, turning to stereotypical movements, and compulsive behavior may be observed in the child.