Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Autism

What Is It?

Autism is a developmental disorder of the brain. People with autism have problems communicating and interacting socially with other people. They also may have unusual patterns of behavior, interests and activities.

There are five kinds of autism, which is why doctors use the term autism spectrum disorder. The three main types of autism include:

• Classic autism

• Asperger's syndrome — A milder form

• Nonspecific pervasive developmental disorder (PDD-NOS) — Used to describe a group of children who don't quite fit the criteria for the other types

There are also two rare autism disorders:

• Rett syndrome — A neurodevelopmental disorder that affects mostly girls. It includes problems with movement and speech, along with autistic features

• Childhood disintegrative disorder — A severe type in which the child loses more physical, language and social skills than in classic autism

Signs of autism spectrum disorder are typically first seen in toddlers before a child's third birthday, but only half the children with it are diagnosed before kindergarten. Rett syndrome appears to be caused by a genetic mutation. The causes of the other types of autism remain a mystery despite many studies. Some studies suggest that other forms of autism may be inherited (genetic), while other evidence points to infection or the effects of an environmental toxin (poison). Some doctors believe autism may result from a brain injury or brain abnormality that occurs during development in the womb or in early infancy. Others have reported evidence that the disorder is a result of abnormal levels of chemicals called neurotransmitters, such as dopamine and serotonin, which send messages between cells in the brain and nerves.

Autism spectrum disorder affects about 6 out of 1,000 children, from all racial, ethnic and social backgrounds, according to the Autism Society of America. All the types except Rett syndrome are three to four times more common in boys than in girls.