The next autistic spectrum disorder I'll go over is Pervasive Developmental Disorder - Not Otherwise Specified, which is normally shortened to PDD-NOS and also known as atypical autism.
PDD-NOS is somewhere between Aspergers and classical autism. This is the classification given to those who don't fit into any other form of the disorder, who have symptoms which don't quite match anything else. It is also thought to be one of the more frustrating diagnoses a child can receive because there isn't as much information about it as there is about AS and autism.
Early on, children with PDD-NOS may show some similar symptoms to those with autism - they usually won't babble or speak on time as infants and have problems socializing. They're more likely to speak as time goes on, but their vocabulary is usually limited and nothing like that of a child with Asperger's.
The DSM-IV says this of PDD-NOS:
The essential features of PDD-NOS are severe and pervasive impairment in the development of reciprocal social interaction or verbal and nonverbal communication skills; and stereotyped behaviors, interests, and activities. The criteria for Autistic Disorder are not met because of late age onset; atypical and/or sub- threshold symptomotology are present.
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, Schizotypical Personality Disorder, or Avoidant Personality Disorder.This essentially means that the child isn't presenting normally, severely enough or at the correct age for autism and they don't have a specific diagnosis for the symptoms the child is displaying. The diagnosis is used if there is an obviously autism-related problem going on that doesn't fit other categories or other disorders such as those listed in the definition.