What Is Atypical Autism?

Atypical autism or pervasive developmental disorder not otherwise specified (PDD-NOS) is a form of autism that was diagnosed from 1994 to 2013. It first appeared in the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV) to encompass the many children who had some, but not all, symptoms of autism.

The next edition, the DSM-5 published in 2013, updated the diagnosis of autism to include a broader range of symptoms that fall under autism spectrum disorder (ASD), along with three levels to indicate support needs. Level 1 requires the least support, while Level 3 requires the most support.

Most people who previously were given a diagnosis of PDD-NOS would likely receive a diagnosis of ASD under the new diagnostic criteria.

The DSM is the American Psychiatric Association's manual that lists all mental and developmental disorders. There are five versions of the DSM, dating back to 1952.

Unlike physical disorders, mental and developmental disorders are often based on social norms and each DSM update has new or revised diagnostic criteria and disorders.

Autism was first classified as its own diagnosis in the DSM-III, published in 1980. Prior to that, children with autism-like behaviors were given a diagnosis of childhood schizophrenia.

The DSM-IV was published in 1994 and divided autism into five separate diagnostic categories. Among these were autistic disorder, Asperger syndrome, and PDD-NOS.

The term autism spectrum in the DSM-IV referred to pervasive developmental disorders (PDDs), a category of diagnoses with certain symptomatic similarities. 

Atypical autism was another name for one of the five official autism spectrum diagnoses: pervasive developmental disorder not otherwise specified (PDD-NOS).

PDD-NOS referred to "presentations that do not meet the criteria for autistic disorder because of late age of onset, atypical symptomatology, subthreshold symptomatology, or all of these."

The two main diagnostic criteria include:

• 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.

Characteristics of PDD-NOS are similar to autism but tend to be milder and include:

• Atypical or inappropriate social behavior
• Uneven skill development of skills, such as fine or large motor skills, visual or spatial organization, or cognitive skills
• Delays in speech or language comprehension
• Difficulty with transitions
• Deficits in nonverbal and/or verbal communication
• Increased or decreased sensitivities to taste, sight, sound, smell and/or touch
• Repetitive or ritualistic behaviors

A PDD-NOS or "atypical autism" diagnosis meant there were too few symptoms to be diagnosed with autistic disorder or Asperger syndrome, and the wrong symptoms to be diagnosed with Rett syndrome or childhood disintegrative disorder.

When the DSM-5 was published 2013, it made the rather momentous decision to collapse all five of the autism diagnoses from DSM-IV into a single diagnostic category: autism spectrum disorder (ASD).

A diagnosis of PDD-NOS is under the autism umbrella in the DSM-IV, and, according to the DSM-5, a diagnosis made under DSM-IV cannot be revoked. However, a person diagnosed with PDD-NOS may or may not fit the current criteria for autism.

The study also found that most prior PDD-NOS diagnoses that don't meet the current ASD criteria fit a diagnosis of social communication disorder (32%). The remaining 2% could be diagnosed with attention-deficit/hyperactivity disorder (ADHD) and anxiety disorder.

Other terms to describe PDD-NOS include atypical autism, autistic tendencies, and autistic traits. But even among people with PDD-NOS or ASD can have a wide range of challenges.

While some people with PDD-NOS have milder autism symptoms, that does not necessarily mean it is any less disabling. It is, in fact, quite possible to have this diagnosis and be severely disabled. However, many children and adults diagnosed with PDD-NOS have relatively mild symptoms.

A study published in the Journal of the American Academy of Child and Adolescent Psychiatry compared individuals with PDD-NOS to individuals with autism and Asperger syndrome, and found children with PDD-NOS could be placed into one of three subgroups:

• a high-functioning group (24%) who resembled Asperger syndrome but had a transient language delay or mild cognitive impairment
• a subgroup resembling autism (24%) but who had late age of onset, too severe cognitive delays, or too young to potentially meet the full diagnostic criteria for autism
• a group (52%) not fulfilling the criteria for autism because of fewer stereotyped and repetitive behaviors.

In terms of the level of functioning measures, the PDD-NOS children had scores that were between those of the children with autism and those of the children with Asperger syndrome.

In contrast, the PDD-NOS group had fewer autistic symptoms, especially repetitive stereotyped behaviors, than the other groups.

Regardless of the subcategory autism spectrum diagnosis, the treatments recommended are likely to be very similar no matter what the official autism spectrum diagnosis: intensive behavioral and/or developmental therapy, along with speech, occupational, and physical therapies, and social skills classes for older children.

Autism presents differently in individuals and the autism spectrum covers a wide range of autism profiles and levels of disability associated with ASD. While autism is typically diagnosed in children, it is not a disorder you outgrow.

While early intervention and treatment can help to teach coping strategies, adults with autism and PDD-NOS can still struggle with social interactions, behavioral challenges, and perceptual problems.