About 20 percent of younger siblings of children with Autism Spectrum Disorder (ASD) will develop the condition by age 3, researchers said.
The study by Yale School of Medicine researchers has found that 57 per cent of these younger siblings who later develop the condition already showed symptoms at age 18 months.
This is the first large-scale, multi-site study aimed at identifying specific social-communicative behaviours that distinguish infants with ASD from their typically and atypically developing high-risk peers as early as 18 months of age.
"While the majority of siblings of children with ASD will not develop the condition themselves, for those who do, one of the key priorities is finding more effective ways of identifying and treating them as early as possible," said lead author Katarzyna Chawarska, associate professor in the Yale Child Study Centre and the Department of Pediatrics at Yale School of Medicine.
"Our study reinforces the need for repeated diagnostic screening in the first three years of life to identify individual cases of ASD as soon as behavioural symptoms are apparent," said Chawarska.
Chawarska and her co-authors closely examined social, communicative, and repetitive behaviours in 719 infants when they were 18 months old.
The team looked for patterns that might predict a later diagnosis of ASD. It then followed up when the participants were age 3.
"Our research suggests that approximately half of the siblings who are later diagnosed with ASD display signs suggestive of ASD at 18 months, and in those who appeared asymptomatic at 18 months, symptoms appeared between 18 and 36 months," said Chawarska.
Chawarska said what was most interesting to the research team was that different patterns of behaviours at 18 months may be predictive of ASD later on.
In about 50 per cent of siblings, a combination of poor eye contact and lack of communicative gestures or imaginative play is most strongly associated with later ASD diagnosis.
In a small percentage of those later diagnosed with ASD, eye contact may be relatively normal, but they begin to display early signs of repetitive behaviours and have limited non-verbal communication skills.
"So not only do the behavioural symptoms appear at different ages, but different combinations of early symptoms may predict the diagnostic outcome," Chawarska added.
The study was published in the Journal of the American Academy of Child and Adolescent Psychiatry.

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