Autism is one of a spectrum, or range, of disorders that vary in severity and in terms of how the condition manifests itself. According to the National Association for Autism Research, the autism spectrum disorders include:

Autistic Disorder Also known as "classic autism," this disorder affects a person's ability to communicate, form relationships and respond appropriately to the environment. Some people with autistic disorder are high-functioning, and have the ability to speak and interact with others, while others are more severely affected and nonverbal and/or mentally retarded.

Asperger's Syndrome This neurobiological disorder differs from autistic disorder in that individuals with Asperger's syndrome do not have a delay in spoken language development. But they can have serious deficits in social and communication skills. People with Asperger's syndrome often have obsessive, repetitive routines and preoccupations with a particular subject matter such as trains or action figures.

Childhood Disintegrative Disorder Children with childhood disintegrative disorder, also known as regressive autism, typically develop normally for two to four years before developing this condition, which resembles autistic disorder. Typically, language, interest in the social environment and often toileting and self-care abilities are lost, and the child may lose interest in the general environment.

Rett Syndrome Rett syndrome is a complex neurological disorder that is genetic in origin. It is among the most common genetic cause of profound intellectual and physical disability in girls, occurring in one in 10,000 female births. Individuals with Rett syndrome develop normally until 6 to 18 months of age, then experience a developmental regression. This is followed by a deceleration of head growth and a loss of purposeful hand movements. A gene associated with Rett syndrome was identified in 1999.

Pervasive Developmental Disorder Not Otherwise Specified (PDD/NOS) Also known as atypical autism, PDD/NOS is a diagnosis often considered for children who show some signs of autistic disorder but do not meet the specific diagnostic criteria for the other Pervasive Developmental Disorders (PPD). PPD is a diagnostic category that refers to a group of disorders characterized by delays in the development of multiple basic functions including socialization and communication.

-- Suz Redfearn