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Last updated on April 20, 2014 at 21:20 EDT

Siblings Of Autistic Children At Higher Risk For Disorder

August 15, 2011

A new study has found that if a child has autism, the risk of a younger sibling also developing the disorder is higher than previously thought.

Researchers from the University of California, Davis studied 664 infants up to the age of three during the study.

They found the average risk was 18 percent for siblings who have other siblings with autism.  Previous research says the average risk was 3 to 10 percent.

The study involved infants from 20 different places across the U.S. and Canada who were six to eight months old at the start of the research.  Each of them had older siblings with autism.

The researchers followed the children’s development up to the age of three years, when they were tested or autism.

A total of 132 children of the 664 participants were found to meet the criteria for an autistic spectrum disorder.  Of the children who met this criteria, 54 received a diagnosis of autistic disorder and 78 were considered to have a milder form of autism.

All of the children were tested using a tool that measures non-verbal cognitive, language and motor skills.

About 26 percent of male infants were diagnosed with a form of autism, compared to 9 percent of female infants.

Previous studies showed that autism is more common in males than females and that 80 percent of all affected children are male.

The average risk in families with more than one older sibling with autism in the study was 32 percent.

Sally Ozonoff, lead study author and professor of psychiatry and behavioral sciences at the Mind Institute, University of California, said in a statement: “Previous studies used different diagnostic criteria. This is the largest study of the siblings of children with autism ever conducted, the first to follow families up to the time of diagnosis as opposed to looking back once they have been diagnosed.”

“So for some families the risk will be greater than 18%, and for other families it will be less than 18%. At present we do not know how to estimate an individual family’s actual risk.

“Genes is a large part of autism, but it’s not the whole story. Non-genetic factors are also important, but we don’t know exactly what they are.”

The Centers for Disease Control and Prevention says about 1 in 110 U.S. children have an autism-spectrum disorder.

The study will be published in the September issue of the journal Pediatrics.

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