Subsequent children born to families that already have one child with an autism spectrum disorder (ASD) are at greater risk for developing a disorder than previously estimated, data from a prospective multicenter study indicate.

The ASD recurrence rate among 664 children who were born into families with an affected child was 18.4% (95% CI 13.34% to 25.5%). Those rates were even higher among boys and children with more than one affected sibling, Sally Ozonoff, PhD, of the University of California Davis in Sacramento, and colleagues wrote in Pediatrics.

These estimates are much greater than those from previously published studies involving siblings, which put recurrence rates between 3% and 10%.

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“Given the higher-than-expected recurrence rates, particularly for male infants and multiplex families, it is critical that primary care professionals closely monitor the development of infants who have older siblings with ASD, screening them routinely at well-child visits using a tool appropriate for infants,” the researchers wrote.

They attributed the differences to limitations in earlier study designs, including small sample sizes, potential patient selection bias and failure to account for the tendency of families of children with an ASD to stop having children.

To address these limitations, Ozonoff and colleagues prospectively analyzed data from a large sample of children from 12 sites in the United States and Canada, who participated in the Baby Siblings Research Consortium, an international research network supported by the advocacy organization, Autism Speaks.

All infants were enrolled by age 18 months and were evaluated at no younger than 3 years for an ASD using the Autism Diagnostic Observation Schedule. Among infants diagnosed with an ASD (n=139), 40.9% were diagnosed with an autistic disorder and 59.1% received a diagnosis of pervasive developmental disorder not otherwise specified.

ASD recurrence risk was three-times higher for boys than girls (26.2% vs. 9.1%; relative risk= 2.8; 95% CI: 1.9-4.0), the researchers determined. Furthermore, children with more than one older sibling with an ASD were twice as likely to be diagnosed than those with only one (32.2% vs.13.5%; RR=2.2; 95% CI: 1.4-3.3).

Primary care clinicians who notice signs and symptoms of an ASD among children who fit this profile are advised to refer the child for an infant intervention immediately, “rather than adopting a ‘wait and see’ attitude,” the researchers wrote, “because early specialized intervention is considered best practice for ASD and may represent the best hope for reducing symptoms and overall disability.”

The elevated risk for ASDs among siblings may also have important implications for genetic counseling. “If families base reproductive decisions on perceived risk of recurrence, it is important that they receive updated information about these risks,” the researchers wrote.

Ozonoff S et al. Pediatrics. 2011; doi: 10.1542/peds.2010-2825.