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Word of the Day

Individual risk of ASD increases with increasing genetic relatedness

Clinical Question:
What is the likelihood of a child having autism or autism spectrum disorder when they have a relative with the same disorder?

Bottom Line:
Having a relative with autism or autism spectrum disorder (ASD) increases an individual’s risk of also having the disorder. The risk increases with increasing relatedness. See the synopsis for specific relative risk results. (LOE = 1b)

Sandin S, Lichtenstein P, Kuja-Halkola R, Larsson H, Hultman CM, Reichenberg A. The familial risk of autism. JAMA 2014;311(17):1770-1777.  [PMID:24794370]

Study Design:
Cohort (retrospective)



Discussing the risk of autism or autism spectrum disorder (ASD) for a child with relatives with the disorder is important and difficult. Existing studies show heritability, but do not describe risk well. Small sample sizes, limited follow-up, and nonrandom sampling have limited reliability and generalizability. This large cohort study of more than 2 million individuals included all live births in Sweden between 1982 and 2006. Follow-up occurred through 2009 and the only patients lost to follow-up were those who died or emigrated. The investigators followed up sibling and cousin pairs for the development of autism starting from age 1 (or from 1987, when the code for autism was first used). Stratification occurred for degrees of relatedness: monozygotic twins, dizygotic twins, full siblings, maternal half siblings, paternal half siblings, and cousins. Additional statistical analyses accounted for confounding factors, such as maternal and paternal ages, parental psychiatric history, birth year, and sex. Relative recurrence risk (RRR) is highest among monozygotic twins (RRR = 153.0; 95% CI, 56.7-412.8) and decreases as degree of relatedness decreases (RRR = 10.3 for full siblings, 3.3 for maternal half siblings, 2.9 for paternal half siblings, and 2.0 for cousins). In other words, a full sibling of a child with ASD is 10 times more likely to develop the disorder than a child who does not have an affected sibling (cumulative probability of an ASD diagnosis at age 20 years estimated to be 12.9% compared with 1.2% for individuals without an affected sibling). Heritability was 50%, meaning that genetic factors explain half the risk of autism. There is no statistically significant difference in risk between boys or girls or between participants with male or female siblings. This information will help clinicians counsel families with autism or ASD. Kathleen A. Barry, MD; Faculty Development Fellow, Department of Family Medicine, University of Virginia, Charlottesville, VA.


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