according to a study published in Pediatrics.
When a baby had more than one older sibling with autism, the family recurrence rate rose to 36.9%, the study found.
The researchers, led by Sally Ozonoff, PhD, Department of Psychiatry and Behavioral Sciences at University of California Davis Health in Sacramento, analyzed data from 1,605 infants who had an older sibling with ASD using data from the global Baby Siblings Research Consortium.
They calculated that the rate of autism recurrence is seven times higher in families who already have one autistic child than in the general population, which points to the importance of close developmental observance in infants born in families with autistic children, particularly male infants in those families. This study replicated a 2011 study, also led by Dr. Ozonoff, which found a similar rate of familial recurrence.
Differences by Sex and Race
Dr. Ozonoff’s team found that sex and race played a part in likelihood of recurrence. Younger siblings of females with ASD were much more likely to develop the disorder (34.7%) than siblings of boys (22.5%). And male younger siblings were more likely to have ASD than girls (25.3% vs. 13.1%).
Additionally, ASD recurrence in White families was 17.8% while across other races collectively the recurrence rate was 25%.
Links with Maternal Education
Differences by maternal education were also striking. Recurrence was 32.6% when mothers had a high school or less education; 25.5% with some college; 19.7 with a college degree; and 16.9% with a graduate degree. The parental education revealed a significant effect only for mothers (P < .01); paternal education was not significant (P = .09).
Suzanne Rybczynski, MD, chief medical officer at East Tennessee Children’s Hospital in Knoxville, who was not part of the study, praised the study for following babies over time, “doing serial evaluation using two very standard tools in diagnosing autism and developmental delay.”
The babies were evaluated as early as 6 months of age, for up to seven visits. A final assessment was made at 36 months.
Dr. Rybczynski said it was interesting to see that, although ASD prevalence has increased substantially from the 2011 study (0.9%-2.8%), the findings regarding the sibling link have been consistent (18.7% in the 2011 study to 20.2% now).
Eliminating Biases
Dr. Rybczynski noted the current study also used diagnoses only from autism experts, which strengthened the findings, noting the potential for overdiagnosis when interviews are with the parents. “This really eliminates those biases.”
The authors explained the factors driving the need to update recurrence rate studies, including the growth in the prevalence of ASD in the last decade to 1 in 36. That may be caused partly by “greater awareness and identification of autistic females and cognitively able, verbal children.”
Also, new diagnostic criteria have been published, with different diagnostic thresholds since the last study. This study, they noted, had a sample size twice as large and more diverse than the 2011 sample.
The size and the diversity are particularly important, Dr. Rybczynski said, as it helps support more recent findings that ASD is not as heavily centered in White males as previously thought.
“We need to make sure we’re monitoring all children, especially from groups where there’s at least one older sibling or multiple siblings with autism or a sister with autism,” she said. The findings of this study are important not just for pediatricians but for families and all who have professional interactions with children.
Dr. Ozonoff reports travel reimbursements and honoraria from Autism Speaks and the Autism Science Foundation and book royalties from Guilford Press. One coauthor has served as a paid consultant to F. Hoffmann–La Roche and Servier and has received royalties from Sage Publications and Guilford Publications. Another is supported by the Stollery Children’s Hospital Foundation Chair in Autism. One coauthor reported a consulting agreement with EarliTec Diagnostics and book royalties from Wiley. A fourth coauthor has received funding from the Simons Foundation and consults for the Beasley Law Firm and Linus Technology. Dr. Rybczynski reported no relevant financial relationships.