Autistic disorder, as defined by the World Health Organization in the ICD-10 Classification of Mental and Behavioural Disorders (1992) is distinguished into two types. http://www.mentalhealth.com/icd/p22-ch06.html

1. Childhood Autism, a pervasive developmental disorder defined by the presence of impaired development before the age of 3 years, with abnormal functioning in all three areas of social interaction, communication, and restricted, repetitive behaviour.
2. Atypical Autism, which differs from Childhood Autism in later age of onset or in insufficient clear evidence of abnormalities in one or two of the areas of impaired development.

North American professionals follow the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders (DSM), with the 4^th edition (DSM IV, 1994) being the current authority. www.autism-biomed.org/dsm-iv.htm

DSM IV distinguishes five subtypes within the broader category it calls Pervasive Developmental Disorders, and prescribes the diagnostic criteria and associated features for each type:

1. Autistic Disorder or classic autism, for children with severe qualitative deficits in all three areas
2. Rett’s Syndrome (see section below)
3. Childhood Disintegrative Disorder (or Heller’s Syndrome) which affects previously normal children who undergo massive regression between 2 and 10 years, resulting in severe acquired autism, usually with loss of cognitive skills—but not because of schizophrenia or degenerative disease of the brain
4. Asperger’s Disorder (see section below)
5. PDDNOS (Pervasive Developmental Disorders Not Otherwise Specified) including Atypical Autism—the label for children with autistic behaviour who do not fit any of the other specific types in the spectrum.

The category term Pervasive Developmental Disorders has been adopted by some professionals in Ontario and Canada. But many professionals, parents and caregivers are not satisfied with the labels and language of DSM IV. Dr Bernard Rimland, founder of the Autism Research Institute in San Diego, CA and the most influential professional in the field of autism in the United States, prefers the term autism spectrum disorder, which is now more commonly used outside North America. Rimland bluntly declares that "the label PDD is poorly understood, uninformative, confusing, disliked, and should be abandoned." http://www.autism.org/pdd.html For another critique, based on social justice, see Autism National Committee http://www.autcom.org/statement.html.

Two useful diagnostic tools are noted. The first was developed by Rimland in the 1960s and helped in identifying autism in thousands of autistic children in North America. Participating families may respond to the Autism Research Institute’s Form E-2 Check List questionnaire, and receive a score that indicates their child’s general position on the autism spectrum; they thus also contribute to the largest database about autistic people. Form E-2 is posted at www.autism.com/ari/e23.html The Childhood Autism Rating Scale (CARS) was devised by Eric Schopler at the University of North Carolina; see details at www.parinc.com/achieve/CARS9h.html