Sibling autism risk rates quantified

Boys with an autistic older sister have a one in six chance of also being diagnosed with the condition

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Boys with an autistic older sister have a one in six chance of themselves being diagnosed with the condition, according to research that quantifies, for the first time, how the sex of siblings relates to intra-generational recurrence of autism spectrum disorder (ASD).

US researchers, who analysed more than 1.5 million two-child families (37,507 of which had at least one ASD diagnosis among the children) found boys with an autistic older sister had the highest risk of being diagnosed (16.7%), while girls with an autistic older brother had the lowest risk (4.2%).

Thirteen out of every 100 boys with an autistic older brother were diagnosed, representing around a 1.3-fold increase in the risk of recurrence if a boy’s older sibling was female.

The results, published in JAMA Pediatrics last month, rely on data from a database of the US private health insurer Aetna, with which two of the researchers were affiliated.

Corresponding author Dr Isaac Kohane, of the Department of Biomedical Informatics at Harvard Medical School, said in light of the findings, GPs need to lower the threshold of considering a diagnosis of autism when there was an older sibling with the condition – particularly when the older sibling was female and their patient male.

Dr Kohane told The Medical Republic this would allow children with ASD to be targeted for early intervention with cognitive-behaviour therapy, which he said was an effective treatment for many.

“The earlier the better,” he said.

The research supports previous findings that ASD tends to cluster in families and is more common in men, and also that the disorder is rare.

The researchers said further investigation was needed into the environmental and genetic factors that conferred a risk of ASD.

The researchers stressed the risk should be kept in perspective. ASD only affects about 1% of the population – 2% of boys and 0.5% of girls.

“Even for the group at highest risk, males with an older female sibling with autism, the odds are still about five to one that the child will be unaffected,” study lead Dr Nathan Palmer, of the Department of Biomedical Informatics at Harvard Medical School, said in a statement.

“What we have provided here is context for families who already have children with autism or another similar disorder and who need a clearer perspective on the recurrence risk.”