# Handbook of Genetic Counseling/Autism-1

Autisim

## 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.