1. Overall, the recurrence risk was 6.9% among the sibling cohorts affected with autism spectrum disorders (ASDs).
2. Siblings of an affected older child were 7 times more likely to have an autism spectrum disorder.
Evidence Rating Level: 2 (Good)
Study Rundown: Over the last two decades, the prevalence of ASD has risen to approximately 1%, and perhaps even higher. It is likely that this increase is multifactorial including but not limited to: increased awareness and new diagnostic criteria. Known risk factors for ASD include genetic background and parental psychiatric history. The present study sought to assess the relative recurrence risk of ASD in children with an older full- or half-sibling with ASD compared to the background population. The study found a statistically significant increased recurrence risk in all groups except paternal half-siblings. The recurrence risk was higher in full-siblings than half siblings, supporting a genetic component. Furthermore, the finding of a statistically significant increased recurrence risk in maternal half-siblings but not in paternal half-siblings supports a role for the intrauterine environment in the development of ASD.
One drawback to the study is that parents who already have a child diagnosed with an ASD may be more adept at recognizing the symptoms in subsequent children, increasing the likelihood that a later-born sibling be diagnosed with autism. In contrast, parents of a child with autism may decide against having more children, especially those parents of children most severely affected. Overall, this study shows an increased risk of ASD in children with an older sibling with the disorder, and allows more in-depth parental counseling.
Relevant Reading: Sibling recurrence and the genetic epidemiology of autism
In-Depth [population-based cohort study]: This study cohort consisted of 1,546,667 children born in Denmark between January 1st, 1980 and December 31st, 2004, excluding multiple births. Overall, the prevalence of ASD in this population was 1.2% (95% CI, 1.2-1.2%), and the recurrence risk was 6.9% (95% CI 6.1-7.8), though it ranged from 4.5% (1990-1991) to 10.5% (1996-1997) depending on the year of birth. The recurrence risk for maternal full-siblings was 7.5% (95% CI, 6.3-9.0%) and 2.4% (95% CI, 1.4-4.1%) for maternal half-siblings. For paternal full-siblings it was 7.4% (95% CI, 6.2-8.9%), while for paternal half-siblings the recurrence risk was not significant. Siblings of an affected older child were 7 times more likely to have an autism spectrum disorder.
By Katherine Skosnik and Brittany Hasty
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