What Is Atypical Autism?

Why PDD-NOS Is No Longer an Official Diagnosis

Atypical autism, also known as pervasive developmental disorder not otherwise specified (PDD-NOS), was a diagnosis previously used to classify children who had some (but not all) of the traits of autism. The terms have since been phased out as the definition of autism has changed in recent years.

The "Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition" (DSM-5) updated the definition of autism in 2013 to include a broader range of traits that fall under the larger umbrella of autism spectrum disorder (ASD).

Under this new definition, autism is regarded as a broader spectrum of behaviors and characteristics whose intensity is classified by three levels of support. As such, people previously diagnosed with PDD-NOS would now be diagnosed with ASD.

This article explains how and why PDD-NOS was combined into the diagnosis of ASD and what this means to autistic children and adults.

Boy, 3, eye close-up
Charly Franklin / Getty Images

How the Definition of Autism and PDD-NOS Evolved

The DSM, published by the American Psychiatric Association (APA), outlines the criteria by which psychiatric and mental disorders are diagnosed. There are five versions of the DSM, dating back to 1952.

Autism was first classified as its own diagnosis in the DSM-3, published in 1980. Prior to that, children with autism-like behaviors were given a diagnosis of childhood schizophrenia.

The DSM-4, published in 1994, divided autism into five separate diagnostic categories. Among these were autistic disorder, Asperger syndrome, and PDD-NOS.

For autism to be diagnosed under the DSM-4, a child had to meet three diagnostic criteria:

For PDD-NOS to be diagnosed, a child only needed to meet two criteria, one of which must be impairments in social interactions. On top of this, schizophrenia, schizotypal personality disorder (STPD), and avoidant personality disorder (APD) had to be ruled out before the diagnosis could be considered definitive.

PDD-NOS, also known as subthreshold autism or atypical autism, was used when some but not all autism traits were present. It suggested a milder form of autism, which some today might refer to as autism with low support needs or high-functioning autism (though this term itself is controversial).

What Are Pervasive Developmental Disorders (PDD)?

Pervasive developmental disorders are those characterized by impaired communication and socialization skills. These not only included PDD-NOS and Asperger syndrome but also Rett syndrome and childhood disintegrative disorder (CDD).

Changing Definitions: DSM-5 and Autism Spectrum Disorder

When the DSM-5 was released in 2013, the APA made the decision to fold all five autism categories into a single diagnostic category known as autism spectrum disorder (ASD).

This means that PDD-NOS has been folded into the larger category of ASD. ASD is diagnosed based on a broader range of traits and classified based on intensity and the level of support a person needs in everyday life.

Rather than describing autism as a condition bound by a relatively narrow list of characteristics and behaviors, the DSM-5 acknowledges that ASD comes in many different forms.

Traits of Atypical Autism

Today, there is a single diagnosis of ASD that replaces the different subcategories used in the DSM-4. The DSM-5 criteria are based on difficulties in two specific areas:

Social Communication Skills

Signs of impairment of social communications skills in autistic children include:

  • Rarely or never using language to communicate with others
  • Rarely or never responding when spoken to
  • Rarely or never using gesturing like waving or pointing
  • Limited facial expressions
  • Rarely or never engaging in imaginative play
  • Not sharing interests or recalling events or achievements
  • Not showing interest in making friends or interacting with others

Restricted, Repetitive, and Sensory Behaviors or Interests

These are behaviors in which an autistic child has a limited range of focus or interest, and an often obsessive attachment to objects or sensations.

Examples include:

  • Lining up toys in a specific way over and over again
  • Repeatedly flicking switches or spinning objects
  • Speaking in a repetitive way
  • Having very narrow and/or intense interests
  • Needing things to happen in the same way every time
  • Difficulty switching out of patterned behaviors or changing schedules
  • Intense reactions to certain sounds, textures, tastes, smells, or sights

To be diagnosed with ASD, children must have difficulties with both social communications skills and narrowly focused, repetitive, or sensory behaviors. Moreover, they must have had these issues from early childhood.

Treatment of PDD-NOS vs. ASD

Another way that PDD-NOS stands outside of the current definition of ASD is the way in which mental health professionals address the intensity of the condition. In the past, the treatment of PDD-NOS was largely the same as for Asperger's syndrome (autism with low support needs) given that both were regarded as "milder" forms of autism.

It is true that people with PDD-NOS often have milder traits but that doesn't necessarily mean that it is any less disabling. In fact, some people with PDD-NOS face intense challenges.

As such, the DSM-5 has established three levels of ASD that reflect the amount of support that a person needs irrespective of the exact features of their condition:

  • Level 1 ASD indicates low support needs, with assistance that may be needed to overcome challenges with social connections, schedules and organization, and nuanced communication.
  • Level 2 ASD indicates the need for structured support to overcome problems with spatial awareness, sensory hypersensitivity, thought fixation, and gaps in social skills, among others.
  • Level 3 ASD indicates high support needs, with intense autism traits that limit a person's ability to communicate, avoid repetitive behaviors, or deal with sensory overload.

Regardless of the category of diagnosis—whether PDD-NOS or ASD—the treatment approach will likely involve behavioral and/or developmental therapy along with speech, occupational, and physical therapies, and social skills classes for older children.

As of 2022, PDD-NOS was removed from the International Statistical Classification of Diseases and Related Health Problems, Version 11 (ICD-11), which is used to classify diseases for surveillance reasons as well as to facilitate medical billing and reimbursements.

Summary

Pervasive developmental disorder not otherwise specified (PDD-NOS)—also known as atypical autism—is an outdated term used when a child has some autism traits but does not meet the criteria listed in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-4).

The next edition, or DSM-5, established three levels of ASD to reflect the amount of support that a person needs. Regardless of whether a diagnosis is ASD or the previous PDD-NOS, treatment typically involves a combination of therapies, including behavioral and/or developmental therapies.

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Lisa Jo Rudy

By Lisa Jo Rudy
Lisa Jo Rudy, MDiv, is a writer, advocate, author, and consultant specializing in the field of autism.