When Do the Developmental Trajectories of Siblings with ASD and BAP Diverge from Typically Developing Siblings?

Background: Younger siblings of children with ASD are at increased risk for developing ASD and Broader Autism Phenotype (BAP) (Ozonoff et al., 2011). BAP features include a wide range of subthreshold deficits and atypical behaviors in domains often affected in ASD. Extant evidence suggest that high-risk (HR) siblings with BAP already show developmental vulnerabilities and decreased social engagement by 12 months compared to typically developing (TD) infants without familial history of ASD (Ozonoff et al., 2014). More clinically informative would be to determine when the developmental trajectories of HR infants with typical and atypical development diverge. 

Objectives: To examine when HR-sibs begin to diverge in regard to social and communication adaptive functioning and severity of autism symptoms. 

Methods: All infants (n= 63, 66.7% males) underwent comprehensive evaluations at 12, 18, 24, and 36 months. Classification of ASD (n=15), BAP (n=15) and HR-TD (n=33) was based on a comprehensive assessment at 36 months. In order to examine divergence, the Vineland Adaptive Behavior Scales Communication and Socialization domains and the Autism Diagnostic Observation Schedule- Toddler (ADOS-T) were administered.  A higher standard score on the Vineland Communication and Socialization domains indicates higher functioning, while on the ADOS-T a lower calibrated severity score indicates higher functioning. 

Results: A repeated-measures age (3) x group (3) ANOVA indicated a significant main effect of group [F(2, 62) = 10.49, p < .01], a significant main effect of age [F(2, 62) = 19.581, p < .01], and a significant interaction of age by group [F(2,62) = 1.99, p < .025]. At 12 months, ASD displayed higher symptom severity on the ADOS than HR-TD (p < .01). At 18 and 24 months, ASD had higher ADOS-T severity (ps<.01) and lower Vineland Communication (ps < .01) and Socialization (ps < .01) scores. No significant group differences were found between BAP and HR-TD at 12 months (ps > .1). At 18 and 24 months, BAP demonstrated higher symptom severity on the ADOS (p < .01) and lower Vineland Communication scores (p < .05) than HR-TD. ASD had lower Vineland Socialization scores than BAP at 18 months (p < .01); all other comparisons between the two groups were non-significant. 

Conclusions: Already at 12 months infants who later develop ASD show more severe autism symptoms than TD siblings. At 18 months, those with ASD continued to show higher ADOS scores and also demonstrated lower abilities on adaptive communication and socialization domains, with group differences remaining stable at 24 months. BAP and HR-TD were differentiated less often: only at 18 and 24 months on Vineland Communication and ADOS. Finally, BAP and ASD only differed at 18 months on adaptive socialization. By and large, BAP presented as intermediate to ASD and HR-TD, but more closely aligned with those with ASD. The distinctive findings on the adaptive socialization scale highlight the potential contribution of “social ability” in differentiating BAP from ASD.  Future studies should further examine the role of social ability in the various atypical developmental trajectories of children at risk for developing ASD.