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New terminology for Autism Spectrum Disorders
For many years, pervasive developmental disorders have been described as a spectrum of disorders, with severe, autistic disorder being at one end of the spectrum and higher functioning Asperger’s syndrome being at the other. In between these two extremes were childhood disintegrative disorder, and pervasive developmental disorder not otherwise specified (PDD-NOS).
Autism spectrum disorders is now the umbrella term for all of these conditions. However, it is helpful to understand the older terms because they are still widely used and found in most literature about ASD.
Autistic disorder is a brain development disorder that impairs social interaction and communication, and causes restricted and repetitive behaviors, all starting before a child is three years old (1). This term was used for the more severe expressions of these classic autistic symptoms. Many people with this diagnosis are nonverbal and may have intellectual disability or other issues that cause them to be lower functioning and dependent on others for some or all of their everyday needs. Repetitive behaviors like rocking or hand flapping are quite common and little or no interest is shown in socializing with others.
Childhood disintegrative disorder, also known as Heller’s syndrome and disintegrative psychosis, is a rare condition characterized by late onset, usually at three years of age onward. It is similar to autistic disorder but has a distinct period of fairly normal development before regression occurs (often quite suddenly) in language, social function, and motor skills. Some children describe or appear to be reacting to hallucinations, but the most obvious symptom is that skills apparently attained are lost. As is the case with all Autism Spectrum Disorders, there is considerable controversy around the right treatment for childhood disintegrative disorder (2).
Asperger's syndrome is on the milder end of the autism spectrum. It is characterized by significant difficulties in social interaction and restricted or stereotyped interests and activities. Asperger’s is different from other expressions of ASD in that there is no general delay in language or cognitive development. A person with Asperger's may be very intelligent and able to handle activities of daily life. Although not mentioned in standard diagnostic criteria, motor clumsiness and atypical use of language are frequently reported (1). Please click *** No item or category found for id='asperger_syndrome' *** for a fuller description of Asperger’s syndrome.
Pervasive Developmental Disorder-Not Otherwise Specified (PDD-NOS) became the diagnosis applied to children or adults who are on the autism spectrum but do not fully meet the criteria for another ASD such as autistic disorder or Asperger’s syndrome. Like all forms of autism, PDD-NOS can occur in conjunction with a wide spectrum of intellectual ability. Some developmental health professionals refer to PDD-NOS as “subthreshold autism." In other words, it’s the diagnosis they use for someone who has some but not all characteristics of autism or who has relatively mild symptoms. For instance, a person may have significant social deficits, but mild or no symptoms in restricted, repetitive behaviors (3).
Rett Syndrome. Children with Rett syndrome often have behaviors similar to autism and experts used to group it among the spectrum disorders. But now that it’s known to be caused by a genetic mutation, it is no longer considered an ASD (4).
(1) Morris, B.K., (2008). Introduction to Asperger’s and autism. Retrieved from 
http://www.autism-help.org/introduction-asperger-autism.htm
(2) Morris, B.K., (2008). Childhood disintegrative disorder. Retrieved from http://www.autism-help.org/childhood-disintegrative-disorder.htm
(3) What is PDD-NOS. (2018). Retrieved from https://www.autismspeaks.org/ppd-nos
(4) What are the types of autism spectrum disorders? (November 19, 2016). Retrieved from https://www.webmd.com/brain/autism/autism-spectrum-disorders