Developmental Course of Symptom Severity in Preschool Children with ASD

Background: Although it is generally well appreciated that the severity of ASD symptoms reduces over time, the timing and rate of that change is not well understood. Moreover, previous studies have generally considered ASD to be a homogenous disorder and thus have neglected the potential impact of clinical characteristics that could lead to the indentification of more homogeneous sub-groups.

Objectives: (a) To identify more homogeneous sub-groups of preschool children with ASD who show differential rates of change on a measure of autism severity; and (b) to identify predictors of sub-group membership as well as outcomes of those sub-groups.  

Methods: 420 children with ASD 2-4 years of age were assessed soon after receiving an initial diagnosis of ASD, and then 6 and 12 months later and then at 6 years of age. ASD symptom trajectories based on the severity metric of the ADOS were used to identify more homogeneous sub-groups. Possible predictors of group membership included language skills (PLS-4), developmental level (MP-R), age at diagnosis, and sex. Outcome measures included adaptive behaviour functioning (VABS II), internalizing/externalizing problems (CBCL), and parent report ASD symptoms (ADI-R). Semi-parametric group based modeling (PROC TRAJ) of longitudinal data was used to identify distinct trajectories. Analysis of variance (ANOVA) and cross tabulations using chi-squared tests were used to examine trajectory group differences on predictor and outcome variables.

Results: Two distinct developmental trajectories of ADOS severity were identified in preschool children with ASD. Group 1 (11% of the sample) showed a significant reduction (declining trajectory; p<0.001) in symptom severity as indexed by the ADOS; the other group showed no significant change. Children in Group 1 had higher language skills (p=0.029) at baseline but there was no difference in IQ, age of diagnosis, or sex. Children in Group 1 had higher adaptive functioning skills (p=0.001) and fewer internalizing/externalizing problems (p=0.013) at age 6. On the ADI-R, children in Group 1 had lower social and communication domain scores (p=0.001 for both) at age 6. In terms of ADOS classification, in Group 1 there was a significant shift from Autism/ASD to non-ASD during T1 to T4 (p<0.001). There was little or no change in ADOS classification in Group 2.

Conclusions: These results suggest that there are at least two sub-groups of children with ASD  that show different rates of change on the ADOS severity metric from the point of initial diagnosis at 2-4 years of age until 6 years. The existence of those two groups is also validated through differential patterns of predictor and outcome variables. At least 10% of preschool ASD children show notable improvement in autism symptoms over the first few years. This finding emphasizes the heterogeneity of the prognostic course in ASD and might be used to generate new hypotheses with respect to the clinical variables that influence the developmental trajectories of children with ASD.