Longitudinal change in symptom severity in children with <scp>ASD</scp>: Results from the <scp>ELENA</scp> cohort

Autism spectrum disorder (ASD) is a lifelong neurodevelopmental condition and understanding the changes in autism symptoms over time is crucial for tailoring support and interventions. This study therefore aimed to investigate the changes in symptom severity in a large cohort of children with ASD over a three-year follow-up period and identify factors that influence these changes. The study included 575 children diagnosed with ASD, ranging in age from 2 to 12 years, who were assessed at baseline and again 3 years later using the Autism Diagnostic Observational Schedule-2 (ADOS-2). ASD severity changes were investigated using the ADOS calibrated severity score (CSS) scores for total, social affect (SA) and restricted and repetitive behaviors (RRB). Results highlight four distinct patterns: stable high, stable low, increased, and decreased severity. The ADOS CSS total score changed for half of the sample, reflecting an increase in ASD severity for 21.9% and a decrease for 29.1% of children. For the other half, the ADOS CSS score remained stable, either high (34.4%) or low (14.6%). While the majority of previous studies reported stability in ASD severity, our findings revealed significant variability with frequent improvements in SA symptoms whereas RRBs remained stable or worsened. Our findings also showed that an improvement in SA was associated with the youngest group and early diagnosis. However, no clinical or sociodemographic factors were linked to changes in RRB, emphasizing the necessity for RRB-specific therapies. The third six-year follow-up point of the ongoing ELENA cohort study will map the long-term trajectories of the severity of ASD symptoms and their potential risk factors.