PDD-NOS is not a current part of the
Diagnostic and Statistical Manual of Mental Disorders typology (as of the
DSM-5), which is the most common diagnostic typology used in the United States. In the still used (though deprecated)
ICD-10, it is considered "atypical autism" and "pervasive developmental disorder, unspecified". The diagnosis of a pervasive developmental disorder not otherwise specified was given to individuals with difficulties in the areas of social interaction, communication, and/or stereotypic behavior patterns or interests, but who did not meet the full DSM-IV criteria for autism or another PDD. This does not necessarily mean that PDD-NOS is a milder disability than the other PDDs. It could simply mean that individuals who receive this diagnosis do not meet the diagnostic criteria of the other PDDs, but that there is still a pervasive developmental disorder that affects the individual in the areas of communication, socialization, and behavior. Description of PDD-NOS merely as a "subthreshold" category without a more specific case definition poses methodological problems for research regarding the relatively heterogeneous group of people who receive this diagnosis. While it is true that children diagnosed with PDD-NOS, as a whole, show fewer intellectual deficits and are higher-functioning than autistic children, many others who fit the criteria for PDD-NOS have some autistic features but also have intellectual deficits that are so severe that it is difficult or impossible to tell whether some of the deficits come from the autism or from the severe to profound degree of
intellectual disability itself. Furthermore, some others who fit the criteria for PDD-NOS come to professional attention at a later age, compared to those diagnosed with autism.
Subgroups In 2004, Walker et al. found that persons with PDD-NOS belonged to one of three very different subgroups: • A high-functioning group (around 25 percent) whose symptoms more or less overlap with that of what was
Asperger syndrome, while also not meeting the then current criteria for
autism spectrum disorder, but who completely differ from those with Asperger syndrome in terms of having a lag in language development and/or mild cognitive impairment.) • Another group (around 25 percent) whose symptoms more closely resemble those of autism, but do not fully meet all its diagnostic signs and symptoms. This is because either the symptoms were recognized at a later age or because they were too young or have cognitive deficits that are too severe to properly identify all the symptoms of autism that they may have. • The biggest group (around 50 percent) consists of those who met all the diagnostic criteria for autistic disorder or Asperger's disorder but whose stereotypical and repetitive behaviors were noticeably mild. ==Treatment==