Research Update: Risk of Autism in Siblings Nearly Double Prior Estimates

Siblings of children with autism are known to be at increased risk for autistic spectrum disorder (ASD), but now researchers from 12 sites in the United States and Canada report the risk is substantially higher than previously believed and much higher than the risk for children in the general population.

Past studies have estimated the risk for those siblings at 3 percent to 10 percent, but new results, published in the September issue of the journal Pediatrics, show that 19 percent of infant siblings develop ASD by age 3. In families where more than one child was affected, an infant sibling’s risk topped 32 percent.

Autism is a neurobiological disorder that inhibits a person’s ability to communicate and develop social relationships. Currently, ASD is diagnosed in one in 110 children in the United States, and in one in 70 boys. The number of children diagnosed with the condition has increased by 600 percent in the past two decades.

For this study, researchers in the High Risk Baby Siblings Research Consortium followed more than 650 infant siblings of children with an ASD diagnosis. Most were enrolled by six months of age and followed until age 3. About 50 families participated through Washington University School of Medicine in St. Louis. Child psychiatrist John N. Constantino, MD, directs the St. Louis site.

“The risk of autistic spectrum disorders in this group is nearly twice the prior estimates,” says Dr. Constantino, the Blanche F. Ittleson Professor of Psychiatry and Pediatrics and director of the William Greenleaf Eliot Division of Child and Adolescent Psychiatry at Washington University. “The good news is some of these children may outgrow their diagnosis. The proportion of school-aged siblings who carry autism diagnosis among the 15,000 families affected by autism participating in the national volunteer registry called the IAN, or Interactive Autism Network, is actually much lower, in the 10 to 12 percent range. So some children identified very early may actually improve over time.”

Dr. Constantino says the investigators plan to follow these children as they grow up to determine if there is a subset that improves. Following such a group over time may provide researchers with clues about how to treat ASD or whether certain symptoms may be less troublesome in the long term than others.

Although the study found higher-than-expected risks for ASD among infant siblings, the researchers continued to find that the condition affects male children at much higher rates than females. Male infants with an older sibling with ASD had almost three times the risk of female infants (26.1 percent compared to 9.1 percent). The study did not find any increases in risk associated with the gender of the older sibling, the severity of the older sibling’s symptoms or with parent characteristics, such as age, socioeconomic status or race/ethnicity.

Dr. Constantino says it’s clear genetic and familial factors influence ASD risk, but it remains less clear which specific genes or environmental factors are causing problems.

Knowing that the risk is higher provides parents of children with ASD with knowledge about the statistical risks involved in having more children. But the study’s first author, Sally Ozonoff, PhD, says these findings tell families relatively little about risks for an individual child.

“It’s important to recognize that these are estimates averaged across all of the families,” says Dr. Ozonoff, professor of psychiatry and behavioral sciences at the University of California Davis M.I.N.D. Institute in Sacramento. “For some families, the risk may be greater than 19 percent, and for others it will be less. At the present time, however, we do not know how to estimate an individual family’s actual risk.”

But Dr. Constantino adds that there are new clinical tests that now can identify such risks for about 10 percent of the families affected by autism.

For now, the investigators say the findings emphasize the importance of family history as a risk factor, and they say parents and clinicians should pay close attention and track these infants from an early age for signs of ASD because early interventions may help improve the long-term outcome.