Trajectories of Cognitive Development in Later-born Siblings of Children with ASD

Background: Autism is a highly-heritable neurodevelopmental disorder with sibling recurrence estimated at 4%-10%. The outcomes of later-born siblings of children with ASD (Sibs-ASD) are heterogeneous, and include autism or PDDNOS, language delays, characteristics of the broader autism phenotype (BAP), and typical development.

Objectives: To identify developmental trajectories in cognitive functioning associated with varying outcomes in Sibs-ASD.

Methods: Cognitive functioning of 35 Sibs-ASD and 30 younger siblings of typically developing children (Sibs-TD) was assessed using t-scores from the four Mullen Scales of Early Learning subscales at two time-points, approximately 18 months apart (T1 mean CA:15 mo; T2 mean CA:34 mo). At T2, Sibs-ASD were classified into 3 outcome groups based on ADOS scores and clinical judgment: ASD, BAP, and no ASD concerns (NC), and compared with Sibs-TD. A mixed-design MANOVA was used to examine changes over time within and between groups.

Results: Results revealed a significant main effect of Group for the Visual Reception (VR) (p=.041), Receptive Language (RL) (p=.007), and Expressive Language (EL) (p=.001) subscales, with the ASD group scoring lower than the BAP, NC, and Sibs-TD groups. The NC group did not differ from Sibs-TD on any subscale. A significant main effect of Time was found for the EL (p=.01) and Fine Motor (FM) (p=.001) subscales, with the former increasing over time and the latter decreasing. A significant Group by Time interaction was found for the FM subscale (p=.037), driven by a decline in FM skills from T1 to T2 in the ASD and BAP groups.

Conclusions: Subgroups of Sibs-ASD with different diagnostic outcomes showed different patterns of trajectories in cognitive development, compared to each other and to Sibs-TD. The decline in Fine Motor t-scores over time in Sibs-ASD subgroups warrants further investigation.