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As is the case with many other families of diagnoses in the DSM, a "Not Otherwise Specified" (NOS) diagnostic category is provided so as to account for cases that present with symptoms characteristic of pervasive personality disorders, but which cannot be definitively identified as any particular pervasive developmental disorder at the time of diagnosis. The NOS diagnostic category is used to make provisional diagnoses of PDD when cases first come to the attention of doctors. It is also used to diagnose cases that are atypical and do not fit within normal diagnostic categories, for example, atypical autism cases that present with the symptoms of autism, save for an onset after age 3. According to the DSM-IV-TR:
This category should be used when there is a severe and pervasive impairment in the development of reciprocal social interaction or verbal and nonverbal communication skills, or when stereotyped behavior, interests, and activities are present, but the criteria are not met for a specific pervasive developmental disorder, schizophrenia, schizotypal personality disorder, or avoidant personality disorder. For example, this category includes "atypical autism" --presentations that do not meet the criteria for autistic disorder because of late age of onset, atypical symptomatology, or subthreshold symptomatology, or all of these.
