Handbook of Genetic Counseling/Autism-1
< Handbook of Genetic CounselingAutisim
What is autism?
- Disorder of brain function that appears early in life -- before the age of 3
- Autism belongs to a family of related brain conditions affecting behavior early in life, which are often referred to as pervasive developmental disorder
- Cause of autism is unknown
Traits associated with autism
- social detachment, e.g., failure to smile at parents and an unawareness of events around them
- abnormal language development, such as repeating phrases in a mechanical voice
- unusual repetitive movements, e.g., rocking and flicking fingers
- mental retardation (usually in more severe cases)
- Traits vary in severity and persist into adulthood
- Some individuals with autism have amazing skills (e.g. Rainman)
Incidence
- Savant syndrome is quite rare (only about 200 cases ever recorded)
- childhood autism is more common, affecting about one in 700 children
- Boys are affected four times more often than girls
Can autism "run in families"?
- If a child has autism, each of the parent's later children has a 3% to 9% chance of having autism.
- This is 100 to 200 times greater than in the general population.
- Believed to be multifactorial inheritance
- Genetic component supported by twin studies
| Type of twin pair | Number of twin pairs | Twin pairs in which both twins had autism ("concordance rate") |
|---|---|---|
| Identical twins | 25 | 60% (15 of 25 pairs) |
| Non-identical twins | 20 | 0% (0 of 20 pairs) |
- The second study used the same twins, but changed the definition of autism
- new definition was looser -- people with milder behavioral problems were counted as autistic
| Type of twin pair | Number of twin pairs | Twin pairs in which both twins had "expanded" autism ("concordance rate") |
|---|---|---|
| Identical twins | 25 | 92% (23 of 25 pairs) |
| Non-identical twins | 20 | 10% (2 of 20 pairs) |
Genes involved in autism
- no gene found yet
- Several candidate regions on chromosomes have been identified through linkage studies
Differential Diagnosis
(known genetic conditions that may share some symptoms of autism)
- Fragile X syndrome (the FMR1 gene on the X chromosome).
- Tuberous sclerosis (the TSC1 gene on chromosome 9).
- Duplications of part of the long arm of chromosome 15.
- Untreated phenylketonuria (the PAH gene on chromosome 12)
Environmental factors
- UNKNOWN
- Autism was thought to result from certain events before or shortly after birth
- Infection of the brain with herpes virus and infection of the pregnant mother with rubella virus
- Problems with brain structure are associated with autism, including hydrocephalus and, very rarely, a brain tumor
- 1993 British study was reviewed by both the Medical Research Council in Great Britain and the Centers for Disease Control in the United States
- The Centers for Disease Control concluded that: "to date there is no conclusive evidence that any vaccine can cause autism"
Web sites resources
- The autism page at the National Institute for Neurological Disorders and Stroke:
- The MedlinePlus web site for autism:
- The February 2000 issue of Scientific American has an article entitled "The Early Origins of Autism" (full text not on-line).
- More about autism from the National Immunization Program at the Centers for Disease Control:
- Diagnosis, assessment and interventions for Autistic Spectrum Disorders:
Notes
The information in this outline was last updated in 2002.
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