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Having an older sibling with autism spectrum disorder (ASD) or attention-deficit/hyperactivity disorder (ADHD) raised the risk of being diagnosed with either condition, according to a study of population-based medical records.

Children whose older brother or sister had autism were 30 times more likely to receive an autism diagnosis and 3.7 times more likely to be diagnosed with ADHD than children whose older sibling did not have either disorder, reported Meghan Miller, PhD, of the University of California Davis MIND Institute, and colleagues.

Likewise, children with an older sibling who had ADHD were 13 times more likely to receive an ADHD diagnosis and 4.4 times more likely to be diagnosed with autism than control children, they wrote in

"We've known for quite some time that risk for autism is elevated among younger siblings of children with autism, that ADHD is highly heritable, and that autism and ADHD frequently co-occur," Miller told 番茄社区app. "But we didn't have good data on recurrence risk for ADHD specifically among younger siblings of children with ADHD, nor did we know much about how these conditions aggregate across families."

While ADHD and autism may have shared familial mechanisms, Miller offered a word of reassurance for parents of a child with autism or ADHD: "Although risk is increased among these younger siblings, most do not develop autism or ADHD." Indeed, the risk was about one in eight.

In this study, Miller and her team extracted data from medical records of children born or seen between 1995 and 2013 in two large health care systems: one in the Upper Midwest region of the U.S., the other in Oregon and Washington. Both systems identified siblings by their mother.

The data included 730 younger siblings of children with ADHD, 158 younger siblings of children with autism, and 14,287 younger siblings of children with no known diagnosis of either disorder. If a child was identified as having both autism and ADHD, the autism diagnosis was considered primary. Only families who had at least one younger child after a diagnosed child were included in the study, and children under age 5 were excluded.

Across all children in the sample -- later-born children and their older siblings -- the incidence of autism was 0.8% and of ADHD was 3.3%, considerably less than the or in the United States. These lower levels may be due to conservative definitions of ADHD and autism, the researchers noted: cases were counted as positive only if identified at two or more points.

The analyses showed the following absolute percentages and ORs, compared with younger siblings of children who had neither autism nor ADHD:

• 12.03% of younger siblings of children with autism had a diagnosis of autism; OR 30.38, 95% CI 17.73-52.06
• 3.8% of younger siblings of children with autism had an ADHD diagnosis; OR 3.70, 95% CI 1.67-8.21
• 12.47% of younger siblings of children with ADHD had a diagnosis of ADHD; OR 13.05, 95% CI, 9.86-17.27
• 1.92% of younger siblings of children with ADHD had an autism diagnosis; OR 4.35, 95% CI 2.43-7.79

A novel finding may be the cross-aggregation rates of later-born siblings, the researchers noted: "This finding is consistent with numerous studies using genetically informed methods that have suggested on both ASD and ADHD traits and diagnoses, and several that have highlighted shared copy number variants, although other studies have ," they wrote.

"These within- and cross-condition recurrence figures are of important clinical usefulness in terms of informing discussions with parents about the need for enhanced developmental surveillance for neurodevelopmental conditions, such as ASD and ADHD, in their younger children," observed Tony Charman, PhD, of King's College London, England, and Emily J.H. Jones, PhD, of the University of London, in an .

But the data should be interpreted cautiously: the sample sizes were modest and the confidence intervals of the elevated ORs were wide, "and this imprecision should add caution to how this information is conveyed to parents, although it is clear that the later-born siblings are at considerably elevated likelihood of developing ASD and ADHD," Charman and Jones wrote.

Miller and co-authors noted several limitations to their study. It was based on a population sample that was not ascertained epidemiologically. The researchers had no information about full- versus half-sibling status, nor data about parental psychopathology, comorbidities, or other potential confounders. Data came from general medical records and diagnostic procedures were unknown, but the relatively low incidence of autism and ADHD in this study (because two or more instances of a given diagnosis were required) suggests there was not much over-identification, they noted.