Autism among Siblings Is More Prevalent than Originally Suspected

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There are many factors that could contribute to a diagnosis of autism, including both genetic and environmental. Researchers focusing on modifiable autism causes hope that the condition can be prevented with some simple changes. Genetics plays a large role in autism spectrum disorders (ASD), however, and researchers are learning now that this may have a greater impact that previously thought. Sally Ozonoff PhD of the MIND Institute at the University of California Davis and colleagues have found that among families who have a child with autism, the risk of a sibling developing ASD is higher than originally suspected.

Siblings With Autism at 20 Times Greater Risk

Although previous studies have been conducted into sibling recurrence risk of ASD, those trials have been limited by biases related to patient selection, a failure to account for the tendency of families with an autistic child to stop having children, and over-reporting.

Dr. Ozonoff studied 664 infants born into families with a child affected with an autism spectrum disorder from 12 sites across the United States and Canada. The participants were part of the Baby Siblings Research Consortium, supported by the advocacy organization Autism Speaks. All infants were enrolled before age 18 months and were evaluated for an ASD no earlier than 3 years. Diagnosis for an ASD was made using the Autism Diagnostic Observation Schedule plus a clinical diagnosis from an expert healthcare provider.

Read: Environmental Factors May Significantly Impact Autism Risk

Of the 132 infants who screened positive (103 boys and 29 girls), 40.9% had autistic disorder and 59.1% had a pervasive developmental disorder not otherwise specified (PDD-NOS) which is used to describe individuals who do not fully meet the criteria for autistic disorder or Asperger syndrome.

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Overall, infants born into families who already had a child affected with autism had an 18.7% recurrence rate – in other words, a risk of autism roughly 20 times greater than children in the general population. Previous research had originally estimated the recurrence rate to be between 3% and 10%.

Read: Gene Found to Explain Why Boys Have Autism More than Girls

As already proven in other studies, males were found to have nearly three times the rate of autism than females. When an older child was diagnosed with autism, the recurrence rate for younger male siblings was even higher – 26.2%. Infants who had more than one affected older sibling were more than twice as likely to be diagnosed with an ASD as those with only one affected brother or sister.

It is possible that even this study underestimates the sibling recurrence rate for autism, says Dr. Ozonoff. Diagnoses were made at an age when milder forms of ASDs are not accurately diagnosed.

"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," Ozonoff and colleagues conclude.

Source reference:
Ozonoff S, et al "Recurrence risk for autism spectrum disorders: a baby siblings research consortium study" Pediatrics 2011; DOI: 10.1542/peds.2010-2825.

Image Credit: Autism Speaks

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