Early Cognitive Trajectories Associated with Autism Spectrum Disorders In A High-Risk Longitudinal Cohort

Background:  As our understanding of autism spectrum disorders (ASD) evolves, so too do diagnostic definitions and conceptualizations of the core and co-occurring conditions. Although not a cardinal diagnostic feature of ASD, cognitive ability can significantly influence daily functioning, response to treatment, school placement, and overall prognosis in ASD (e.g., better outcomes for children with higher IQ). As with many features of ASD, cognitive functioning varies along a continuum from severely impaired to well above average, but little is known about the early developmental trajectories of cognitive functioning in ASD. Identification of trajectories predictive of later diagnosis can enhance our understanding of the mechanisms underlying the emergence of developmental challenges in children with ASD, and may in turn guide the development of screening methods, program planning and focused early interventions.

Objectives: To examine and compare cognitive trajectories across developmental domains from 6 to 36 months in high-risk infants/toddlers later diagnosed with ASD, relative to high-risk infants with no ASD, and low-risk controls.

Methods:  239 younger siblings of children with ASD (hereafter, ‘high-risk sibs’) and 90 low-risk controls received cognitive assessment at 6, 12, 24 and 36 months of age, using the Mullen Scales of Early Learning (MSEL). MSEL Early Learning Composite (ELC) standard scores are presented here.

Results:  Semi-parametric group-based modeling identified 3 distinct developmental trajectories in MSEL-ELC: (1) average-to-high average performance throughout the assessment interval, with a slight increase over time; (2) solidly average performance across time; and (3) marked decline in standard scores from the average to the intellectually disabled range by age 2, and persisting to age 3. Trajectory group membership was a significant predictor of 36-month diagnostic status (χ² = 117.84; p < .001); ASD sibs were most highly represented in MSEL Trajectory groups 3 (44%) and 2 (40%). Non-ASD sibs were most likely to be members of Trajectory groups 1 (65%) or 2 (31%), and the vast majority of controls were in group 1 (83%); none were in group 3. Language and nonverbal cognitive skills appear to play the greatest role in defining these trajectories. Specifically, the declining trajectory (3) appears to be attributable mostly to declining performance in Visual Reception and Receptive and Expressive Language scores across age. Gross Motor development remained relatively consistent across time for all Trajectory groups, and Fine Motor performance followed a u-shaped curve, with slight improvements for all groups at 12 months and a slight drop for all at 24 months.  

Conclusions: A pattern of decreasing cognitive abilities between 12 and 24 months is highly predictive of a diagnosis of ASD at age 3. However, it is important to underscore that 40% of high-risk infants later diagnosed with ASD, and 96% of high-risk siblings with no diagnosis of ASD had average-to-high-average cognitive abilities, as measured by the MSEL-ELC, that were stable over this period.