Autism risk in younger siblings higher than previously thought: study
TORONTO - Parents of autistic children have long been told there's a roughly one in 10 chance a younger sibling may also develop the disorder, but a large international study suggests the actual risk is substantially higher.
The study by the international Baby Siblings Research Consortium, published Monday in the journal Pediatrics, estimates the overall risk at almost 19 per cent after studying 664 infant siblings with at least one older brother or sister with autism spectrum disorder.
Researchers found the risk was even higher in some cases: 26 per cent for male infants and more than 32 per cent for youngsters with more than one older sibling with autism.
"This is the largest study of the siblings of children with autism ever conducted," lead author Sally Ozonoff of the University of California at Davis, said in a statement. "There is no previous study that identified a risk of recurrence that is this high."
But she said it's important to recognize the estimates are averaged across all the families that participated in the study, a collaborative effort involving researchers from the U.S., Canada and other countries.
"So for some families, the risk will be greater than 18 per cent and for other families, it would be less than 18 per cent," said Ozonoff, a professor of psychiatry and behavioural sciences at the university's MIND Institute. "At the present time, unfortunately, we do not know how to estimate an individual family's actual risk."
Autism spectrum disorder (ASD) is a life-long neurodevelopmental condition that typically appears before age three and significantly affects a child's ability to think, communicate and interact with others. The disorder is often marked by repetitive behaviours and activities.
There is strong evidence that genetic factors play a critical role in the development of autism, which affects an estimated one in 110 children — far more of them boys than girls. Because it is a spectrum disorder, symptoms may cover a wide variety of combinations and range from mild to severe.
To conduct the study, researchers tracked the development of younger siblings of children with ASD from about six months of age to 36 months, then ran a battery of tests. Of the 664 participants, 132 met criteria for an autism spectrum disorder.
But the recurrence rate differed considerably depending on whether a child had one sibling or more than one sibling with autism. In families with one older child with autism, the rate of ASD in younger children was about 20 per cent; for families with more than one child with autism, the rate was 32 per cent.
"The importance of this finding is that this actually is considerably higher than previous studies have shown," said Dr. Lonnie Zwaigenbaum, a developmental pediatrician at the University of Alberta who co-led the Canadian arm of the research.
Those earlier studies concluded that the risk of a couple having a second child with ASD ranged from three to 10 per cent.
"In fact, the number from previous studies are what pediatricians and genetic counsellors quote to families when they're looking at having subsequent children after having a child with autism," Zwaigenbaum said from Edmonton.
Among later-born siblings diagnosed with ASD in the new study, 26 per cent were male and nine per cent were female.
"And this is consistent with what we see in children with autism generally, that the ratio is roughly three or four to one, with more boys being diagnosed than girls," he said.
The findings underscore what doctors have long known — that younger siblings of children diagnosed with ASD are a high-risk group that need extra attention.
"Both families and clinicians need to be quite vigilant to early siblings," Zwaigenbaum said. "Clinicians, including community physicians, need to take parents' concerns very seriously, and they need to be particularly attuned to families who have a child with autism and now are raising a younger infant."
Ozonoff said the study's large size, the geographic diversity of participants, their young age at enrolment and the gold-standard assessment methods used reinforce the accuracy of its findings.
The study design also minimized the effects of other factors such as "stoppage," the tendency of families with an autistic child to stop having children, which would lead to an underestimate of potential recurrence rates. The study accounted for stoppage by studying only families with later-born siblings.
Its findings have significant family-planning and genetic-counselling implications, she said.