ASD Symptoms in Unaffected Younger Siblings of Children with and without ASD: A Prospective Study

Background: Research following infants with an older sibling diagnosed with ASD has informed understanding of the early course of the disorder and has documented an infant sibling recurrence risk of 18.7% (Ozonoff et al., 2011). Prospective studies eliminate many limitations encountered in retrospective research, such as recall bias; prospective studies suggest that ASD symptoms emerge over the second half of the first year of life and beyond. There is a growing body of literature examining milder ASD symptoms present in relatives of individuals diagnosed with the disorder, frequently referred to as the broader autism phenotype (BAP). A recent study by Georgiades and colleagues (2013) suggested that 19% of unaffected siblings of children with ASD showed a BAP symptom profile at 12 months of age.

Objectives: This study investigates whether infant siblings of probands with ASD, who do not themselves meet diagnostic criteria for ASD at 24 or 36 months of age, are rated by parents as exhibiting more ASD symptoms at 12, 18, and 24 months than infants with typically developing siblings who also do not meet criteria for ASD.

Methods: Data are presented on 26 unaffected high risk (HR) younger siblings of probands (46.15% male), and 35 low risk (LR) control participants with typically developing siblings (45.71% male). Proband and infant sibling ASD diagnoses were made or ruled-out using gold-standard diagnostic measures and clinical judgment. All participants included in this report did not meet criteria for ASD or other developmental concerns (e.g., language delays). Parents of participants completed the Brief Infant-Toddler Social Emotional Assessment (BITSEA) at 12, 18, and 24 months of age. Analyses examine the ASD Problems subscale of the BITSEA, which consists of nine problem behaviors often present in young children diagnosed with ASD (e.g., Puts things in a special order over and over).

Results: A repeated measures ANOVA was used to assess mean differences on the BITSEA ASD Problems subscale between the HR and LR groups across 12, 18, and 24 months of age. Multivariate results revealed a significant reduction over time on BITSEA ASD Problems score. Wilks’ Lamba=.32, and this was statistically significant, (F(2,58)=62.10, p<.001). 68% of variance in BITSEA ASD Problems scores is accounted for by time (η_p²=.68). The interaction between time and risk group was non-significant, indicating that the difference between groups did not vary across time. Tests of between-subjects effects showed a significant main effect of group membership (F(1, 59)=4.94, p=.03), with HR participants having higher mean scores than LR participants. Final analyses will include imputed missing values and data from infant siblings who receive a diagnosis of ASD.

Conclusions: These preliminary results are consistent with recent findings regarding the early emergence of BAP traits in unaffected siblings (Georgiades et al., 2013); unaffected infant siblings of probands displayed more ASD-relevant problem behaviors than infants with typically developing siblings. Findings support the importance of monitoring the emergence of ASD symptoms in younger siblings of children diagnosed with ASD through routine developmental screening and surveillance as they may benefit from early intervention services.