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Friday, May 9, 2008

What are Autism Spectrum Disorders?

Terms and Definitions

Terms used to refer to various Autism Spectrum Disorders (ASDs) can be very confusing at first: one will hear terms such as:

  • autism, classic autism, high functioning autism

  • Autistic Disorder

  • Asperger Syndrome or Asperger's Disorder

  • Pervasive Developmental Disorder (PDD), and/or Autism Spectrum Disorder (ASD)
All are commonly used terms and may be interpreted differently in various settings. The term "autism" is often used in two different ways. It is used to refer specifically to Autistic Disorder and it is also used more generally to refer to all ASDs.
The term "spectrum" refers to a continuum of severity or developmental impairment. Children and adults with ASDs usually have particular communication and social characteristics in common, but the conditions cover a wide spectrum, with individual differences in:

  • Number and particular kinds of symptoms

  • Severity - mild to severe

  • Age of onset

  • Levels of functioning

  • Challenges with social interactions
There are in fact five Autism Spectrum Disorders described under this diagnostic category of Pervasive Developmental Disorders (PDD) that appear in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) of the American Psychiatric Association.
When speaking of ASDs, most people are referring to three of the PDDs that are most common:

  • Autistic Disorder (also called autism, classic autism and AD)

  • PDD-NOS (Pervasive Developmental Disorder - Not Otherwise Specified)

  • Asperger's Disorder (also called AS, Asperger's Syndrome and Asperger Syndrome)
Individuals with ASDs have varying degrees of difficulty in social interaction and communication and may show repetitive behaviours and have unusual attachments to objects or routines.
Autism is the most common neurological disorder affecting children ASDs change the way the brain processes information and can affect all aspects of a person's development. Classic autism usually appears during the first three years of life. Autism is four times more common in boys than girls.
Autism Spectrum Disorder is a neurological disorder resulting in developmental disability. This affects:

  • Communication

  • Social understanding

  • Behaviour, activities &; interests
Each individual is unique - no one person with an ASD responds or behaves exactly like another with the same diagnosis.
There are Five Pervasive Developmental Disorders (PDDs)

  • Childhood Disintegrative Disorder (CDD)

  • Rett's Disorder (RD)

  • Autistic Disorder (AD)

  • Pervasive Developmental Disorder - Not Otherwise Specified (PDD-NOS)

  • Asperger's Syndrome (AS)
Childhood Disintegrative Disorder (CDD)

  • Rare: ~0.2 in 10,000

  • Significant losses in social behaviour, language, play and adaptive behaviour after period of normal development of 3 or more years

  • Severe deficits in cognitive ability
Rett's Syndrome

  • Rare: ~1 in 10,000

  • Found almost exclusively in girls

  • Significant regression after a period of normal development

  • Severely impaired language and psycho-motor skills

  • Severe deficits in cognitive ability
Autistic Disorder (AD)

  • Most common: ~20 in 10,000

  • Cognitive impairments

  • Deficits in verbal and non-verbal communication

  • Deficits in social understanding

  • Unusual behaviours, restricted activities
Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS)

  • Common: ~15 in 10,000

  • Also called atypical autism

  • Severe and pervasive impairment in some areas such as reciprocal social interaction, or restricted activities and interests but the strict criteria for autistic disorder are not met
Asperger Disorder (also called Asperger's Syndrome, Asperger Syndrome, and AS)

  • Common: ~5 in 10,000

  • Mild to severe impairments in social interaction and understanding

  • Restricted and repetitive activities and interests

  • Language and cognitive development not delayed

  • Deficits in communication
Reference for prevalence:
Fombonne, E., 2003: Modern Views of Autism, Can. J. Psychiatry, 48:503-505.
Fombonne, E., 2003: Epidemiology of autism and other pervasive developmental disorders: an update. J. Autism.Dev. Disord. 33:365-381

Related Conditions


  • Several other medical conditions also include some features of PDD, such as Down Syndrome, Prader-Willi Syndrome, Fragile X Syndrome, Landau-Kleffner Syndrome, William's Syndrome or Tourette's Syndrome but PDD does not actually include these medical conditions.
A note on terms and labels

It is important to distinguish between the clinical terms and descriptions of ASD and our understanding and knowledge of people who live with ASD. We need to understand clinical terms used in medical settings, during diagnosis, and in certain treatment or intervention settings. However, it is wise to keep in mind that these terms may also be seen as limiting labels to some people with autism who feel that ASDs have been "medicalized" to the point where individuals who are unique in their skills, abilities and value to their communities, have been forgotten or eclipsed by the "disorder".