The causes of and risk factors for autism spectrum disorders (ASD) are not fully understood. But, new data reports that the risk of recurrence among siblings with autism is substantially higher than previously believed.

Until recently, the risk of recurrence of ASD was estimated to be between 3% and 10% for children with a sibling diagnosed with any type of autism. Most of these studies were small and affected by selection bias or reporting limitations. A new longitudinal study published by Pediatrics is the largest study of this type to date and used a long follow-up period and prospective investigation techniques to mitigate these limitations.

For the study, the investigators enrolled 664 infants who had at least one sibling diagnosed with ASD. Six percent of the infants had more than one affected sibling. Eighty-four percent of the siblings were male. The average age of the infants at the time of enrollment was 8 months. Slightly more than half (56%) the infants were male and 40% of the infants were third-born or later in their respective families.

The study reported that nearly 19% of infants with a sibling with ASD will also be diagnosed with ASD by age 36 months, an estimate that is much higher than previously believed. Male infants were nearly 3 times as likely to develop ASD as females. Infants with multiple affected siblings were more than twice as likely to develop ASD compared to those with only one affected sibling. The risks were not affected by age at enrollment, gender or functioning level of the older sibling, or other demographic factors.

The authors conclude that these higher-than-expected rates of recurrence have implications for infant screening and genetic counseling. Pediatricians, the authors assert, should be more vigilant about screening for ASD in children with affected older siblings, and parents with children already diagnosed with ASD should be advised of the risk of recurrence if planning on having more children. However, a clear gene-based link to autism is likely a long way from being identified.

The characteristics of ASD are highly variable among children, as are the rate and severity of development. Though some risk factors and markers of future disability are present in children younger than 1 year old, identification of ASD in infants is extremely difficult. Special assessment tools specific to this population provide the best chance for early identification and intervention, which are critical to successful ASD therapy. Children at high-risk for ASD should be monitored early and often and referred for treatment at the first identification of signs of ASD.

References

Muhle R, Trentacoste SV, & Rapin I (2004). The genetics of autism. Pediatrics, 113 (5) PMID: 15121991

Ozonoff S, Young GS, Carter A, Messinger D, Yirmiya N, Zwaigenbaum L, Bryson S, Carver LJ, Constantino JN, Dobkins K, Hutman T, Iverson JM, Landa R, Rogers SJ, Sigman M, & Stone WL (2011). Recurrence Risk for Autism Spectrum Disorders: A Baby Siblings Research Consortium Study. Pediatrics PMID: 21844053

Tager-Flusberg H (2010). The origins of social impairments in autism spectrum disorder: studies of infants at risk. Neural networks : the official journal of the International Neural Network Society, 23 (8-9), 1072-6 PMID: 20800990

Yoder P, Stone WL, Walden T, & Malesa E (2009). Predicting social impairment and ASD diagnosis in younger siblings of children with autism spectrum disorder. Journal of autism and developmental disorders, 39 (10), 1381-91 PMID: 19449096

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