This study examined variability in autism symptom trajectories in toddlers referred for possible autism spectrum disorder (ASD) who had frequent observations from 14 to 36 months of age.

In total, 912 observations of the Autism Diagnostic Observation Schedule (ADOS) were obtained from 149 children (103 with ASD) followed from 14 to 36 months of age. As a follow-up to a previous analysis of ADOS algorithm scores, a different analytic approach (Proc Traj) was implemented to identify several courses of symptom trajectories using ADOS Calibrated Severity Scores in a larger sample. Proc Traj is a statistical method that clusters individuals into separate groups based on different growth trajectories. Changes in symptom severity based on individual ADOS items also were examined.

Trajectory analysis of overall symptom severity identified 4 clusters (non-spectrum ∼25%; worsening ∼27%; moderately-improving ∼25%; severe-persistent ∼23%). Trajectory clusters varied significantly in the proportions of confirmatory ASD diagnosis, level of baseline and final verbal and nonverbal abilities, and symptom severity. For the moderately-improving group, social communication improved, whereas restricted and repetitive behaviors were stable over time. Language and verbal and nonverbal communication improved for many children, but several social affect and restricted and repetitive behavior symptoms remained stable or worsened.

Significant variability in symptom trajectories was observed among toddlers referred for possible ASD. Changes in social and restricted and repetitive behavior domain scores did not always co-occur. Similarly, item-level trajectories did not always align with trajectories of overall severity scores. These findings highlight the importance of monitoring individual symptoms within broader symptom domains when conducting repeated assessments for young children with suspected ASD.