Autism risk in siblings of autistic children is higher: Study

By Dr. Ananya Mandal, MDAug 17 2011

A new study reveals that parents with an autistic child have a 19% increased chance of getting another one with a similar disorder. Experts said the study, published Monday in the journal Pediatrics, is the most comprehensive of its kind and should be used to counsel families.

Study leader Sally Ozonoff, a professor of psychiatry and behavioral sciences at the UC Davis M.I.N.D. Institute in Sacramento said, “Parents are concerned — could this happen again?” Although researchers have long known that autism runs in families, most previous estimates of the risk of recurrence range from 3% to 10%. Those estimates have been based on small numbers of families and an older, narrower definition of autism than the one used here she explained. “We were all a bit surprised and taken aback about how high it is,” she added.

This new study was based in 12 locations across the U.S. and Canada and followed 664 infants who had at least one older sibling with autism. The infants, on average, were 8 months old when their parents enrolled them in the study — before the signs of autism are usually apparent. At age 3, each child was evaluated.

Among the boys, 26% were found to have some form of autism. The rate for girls was 9%. The rates were the same regardless of the gender of the older sibling and the severity of that child's case. Having more than one older sibling with autism further increased the chance of diagnosis to 32%. The calculations are averages, and the risk for any given family could be higher or lower, experts said. Dr. Fred Volkmar, a child psychiatrist and autism expert at Yale University who was not part of the study said, “I have seen families with five kids with autism.”

The scientists have collected DNA samples from many of the children in the study to look for genetic differences between the sibling pairs who had autism and those who did not. The aim is to develop a clearer understanding of the genes involved in autism — and potentially develop tests that would measure risk in individual families.

To a limited extent, scientists can already predict risk from DNA, said Dr. Daniel Geschwind, an expert on autism genetics at UCLA who was not involved in the study. Up to 15% of autism cases have been linked to specific genetic mutations. If one of those mutations is present in both an autistic child and a parent, the risk to the next child is considerably higher than the rate found in the study. But if the mutation occurs only in the autistic child and not in a parent, that means it arose spontaneously, and future siblings are probably in the clear. Geschwind and other outside experts said the study provided further evidence for the strong role of genetics in autism.

The study is an important addition to autism research and “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 Center. Lord was not involved in the study.

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 funded the study.