Abstract
OBJECTIVES: To plot longitudinal trajectories of autism spectrum disorder (ASD) severity from early childhood to early adolescence. In line with reported trajectories in toddlers, we hypothesize that a substantial minority of children will show marked changes in ASD severity over time, with "Improvers" demonstrating the highest mean baseline and rate of growth in verbal IQ (VIQ). METHODS: Patients included 345 clinic referrals and research participants with best-estimate clinical diagnoses of ASD at 1 or more time points, and repeated Autism Diagnostic Observation Schedule (ADOS), VIQ, and nonverbal IQ scores. Standardized ADOS severity scores were applied to 1026 assessments collected longitudinally between the ages of 2 and 15 (VIQ at most recent assessment: mean = 58, SD = 35). Scores were fitted for latent severity trajectory classes with and without covariates. Adaptive behavior and VIQ trajectories over time were modeled within each of the best-fit latent classes. RESULTS: A 4-class model best represented the observed data. Over 80% of participants were assigned to persistent (stable) high or moderately severe classes; 2 small classes respectively increased or decreased in severity over time. Age, gender, race, and nonverbal IQ did not predict class membership; VIQ was a significant predictor. Baseline VIQ was highest in the improving and worsening classes; it increased at the greatest rate in the improving class. Adaptive behavior declined in all but the improving class, with consistent impairment in all classes. CONCLUSIONS: If replicated, identified trajectory classes of ADOS severity may contribute to clinical prognosis and to subtyping samples for neurobiological and genetic research.