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.
Autistic Disorder: European Description
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.
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 4th 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:
Autistic Disorder or classic autism, for children with severe qualitative deficits in all three areas
Rett’s Syndrome (see section below)
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
Asperger’s Disorder (see section below)
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.