Variation in the Early Trajectories of Autism Symptoms Is Related to the Development of Language, Cognition, and Behavior Problems

Objective The objectives of this study were to model more homogeneous subgroups within autism spectrum disorder (ASD) based on early trajectories of core symptoms; and to further characterize these subgroups in terms of trajectories of language, cognition, co-occurring (attention-deficit/hyperactivity disorder [ADHD]–related) traits and clinical outcome diagnosis. Method Children (N = 203) referred for possible ASD at ages 1 to 4 years were assessed at three time points at intervals ranging from 9 months to 3 years. Assessments included standardized measures for ASD (Autism Diagnostic Observation Schedule [ADOS]), language (ADOS-language item), nonverbal IQ (NV-IQ; different tests adequate to chronological/mental age), and parent-reported behavioral problems (Infant-Toddler Social and Emotional Assessment, Child Behavior Checklist). Results Latent-class growth curve analysis with ADOS total scores led to the identification of three main stable and two small improving groups: a severe–stable group (19.5% of sample)—the only group without considerable language improvement—showed persistent low NV-IQ and marked increase in attention problems over time; a moderate–stable group (21.7%) with below-average increasing NV-IQ; and a mild–stable group (48%) with stable–average NV-IQ and the highest scores on ADHD-related traits, whose ASD outcome diagnoses increased despite stable–low ASD scores. Two groups (each 5.4%) improved: one moved from severe to moderate ASD scores, and the other moved from moderate to mild/nonspectrum scores. Both of these groups improved on language, NV-IQ, and ADHD-related traits. Conclusion Results support the high stability of ASD symptoms into various severity levels, but also highlight the significant contribution of non-ASD domains in defining and explaining the different ASD trajectories.