One in five children with autistic older sibling will have disorder, study shows

NEARLY one in five children with an autistic older sibling will also develop the disorder — a much higher incidence than previously thought, a study has found.

Researchers followed 664 babies who had at least one older brother or sister with autism. Overall, 132 infants, or about 19%, were diagnosed with an autism by their third birthdays. Previous smaller or less diverse studies reported a prevalence of between 3% and 14%.

“We were all a bit surprised and taken aback about how high it is,” said lead author Sally Ozonoff, a psychiatry and behavioural sciences professor with the Mind Institute at University of California at Davis.

The highest incidence was in infants with at least two older siblings with autism — 32% also developed autism. Among boys with autistic siblings, 26% developed autism compared with 9% of girls. Autism is known to be more common in boys.

The study involved 12 US and Canadian sites and was published in Paediatrics. Earlier studies were more local or involved fewer sites.

Ms Ozonoff said parents of autistic children often ask, “How likely am I to have another child” with autism? She said her study provides a more up-to-date answer.

However, she noted that 80% of siblings studied did not develop autism, and that the prevalence rate was an average. It may be different for each family, depending on other risk factors.

Autism has no known cause but experts believe that genetics and external influences are involved. Research is examining whether these could include infections, pollution and other non-inherited problems.

Ms Ozonoff noted that siblings often are exposed to similar outside influences, which could partly explain the study results.

Infants in the study were enrolled before they showed any signs of autism, such as poor eye contact and little social interaction.

The study “has critical implications for families who are deciding whether they’ll have another child”, said Catherine Lord, director of the Institute for Brain Development at New York-Presbyterian/Weill Cornell Medical Centre, who was not involved in the study.

Ms Ozonoff said the study should prompt families and their children’s doctors to be vigilant with infants whose older siblings have autism. Early diagnosis is important as experts say behavioural treatment has the best chance of working if started early. “Paediatricians need to listen and make a very focused plan for how to monitor those things, rather than taking a wait-and-see attitude” toward children with autistic siblings, she said.

Alycia Halladay, a research director at the advocacy group Autism Speaks, said the study provides a more robust, accurate prevalence estimate than previous studies, and strengthens the idea that family history is a risk factor. Her group, the National Institutes of Health and the Canadian Institute for Health Research, are among those who paid for the study.