Diagnostic stability and developmental progression from the second to third year of life in toddlers with ASD vary depending on their initial early profiles of relative strengths and deficits. Although a small minority of toddlers with more complex clinical presentations may not retain their diagnoses by the age of three, most children continue to exhibit symptoms of autism. Despite limited improvements in symptom severity, many children show significant gains in verbal functioning. Only a small proportion of children (17%) exhibit very limited gains despite intensive intervention. These findings support continued efforts to examine determinants of developmental trajectories including factors mediating and moderating response to treatment.

Background Although temperament has been recognized as an important contributor to childhood psychopathology, its role in emergent autism spectrum phenotypes is not well-understood. This study examined whether toddlers with Autism Spectrum Disorder (ASD) display temperamental vulnerabilities compared to toddlers with other developmental challenges, whether these characteristics are distinct from core autism symptoms, if they are stable over time, and if they contribute to social outcomes in preschool. Methods Parents of 165 toddlers with ASD, 58 nonverbal ability- and chronological age- (CA) matched developmentally delayed (DD) toddlers and 92 CA-matched typically developing (TD) toddlers completed the Toddler Behavior Assessment Questionnaire-Supplemental (TBAQ-S) at 26 months (SD=6; Time 1). TBAQ-S data were also available for a subset of toddlers with ASD (n=126) at 43 months (SD=9; Time 2). Results Compared to the DD and TD groups, toddlers with ASD exhibited vulnerabilities within the Effortful Control domain as well as the Surgency domain. They also displayed greater Negative Emotionality compared to TD peers. In the ASD group, temperamental characteristics were not concurrently related to autism severity or developmental level and individual differences were highly stable over time. Changes in Perceptual Sensitivity, Inhibitory Control, and Low-Intensity Pleasure from age 2 to 3 ½ uniquely predicted autism symptom severity and adaptive social skill level at Time 2. Conclusions Temperamental vulnerabilities in toddlers with ASD are stable over time and involve attentional and behavioral control as well as affective reactivity. They contribute uniquely to social outcomes in preschool and are likely to signal risk for developing later maladaptive attentional, affective, and behavioral symptoms. Considering biologically-based dimensions may shed light on non-core facets of the early ASD phenotype that are potentially relevant to the emergence of comorbid conditions later in childhood.

It is widely believed that early diagnosis and treatment of autism spectrum disorder is essential for better outcome. This is demonstrated by the American Academy of Pediatrics recommendation to screen all 1.5–2.5-year-old toddlers for autism spectrum disorder. However, multiple longitudinal studies of children diagnosed with autism spectrum disorder at 1.5–6 years of age and treated in community settings have not reported any associations between earlier diagnosis and improved outcome in core symptoms. Here, we quantified Longitudinal changes in core autism spectrum disorder symptoms of 131 children diagnosed at 1.2–5 years of age using the Autism Diagnostic Observation Schedule–Second Edition Calibrated Severity Scores over a 1-2 year period. We examined the prevalence and magnitude of Calibrated Severity Scores changes across children who were diagnosed at different ages. The results revealed that age of diagnosis was significantly correlated with poorer outcome ( r(129) = 0.41, p < 0.001). Approximately 65% of the children diagnosed before 2.5 years of age exhibited improvements in Autism Diagnostic Observation Schedule–Second Edition Calibrated Severity Scores (⩾2 points) in contrast to only 23% of the children diagnosed after this age. Changes in younger children were driven by improvements in social symptoms despite deterioration in restricted and repetitive behaviors. These findings reveal that autism spectrum disorder diagnosis before the age of 2.5 is associated with considerable improvement in social symptoms. We suggest that greater brain plasticity and behavioral flexibility enable younger children to benefit more from autism spectrum disorder interventions even in community settings with heterogeneous services. This motivates further prioritization of early autism spectrum disorder screening as recommended by American Academy of Pediatrics guidelines. Lay abstract It is widely believed that early diagnosis and treatment of autism spectrum disorder is essential for better outcome. This is demonstrated by the American Academy of Pediatrics recommendation to screen all 1.5–2.5-year-old toddlers for autism spectrum disorder. However, multiple longitudinal studies of children diagnosed with autism spectrum disorder at 1.5–6 years of age and treated in community settings have not reported any associations between earlier diagnosis and improved outcome in core autism spectrum disorder symptoms. In this study, we measured changes in core autism spectrum disorder symptoms over a 1–2-year period in 131 children diagnosed with autism spectrum disorder at 1.2–5 years of age, and treated in the community. The results revealed that children who were diagnosed before 2.5 years of age were three times more likely to exhibit considerable improvements in social autism spectrum disorder symptoms in comparison to children diagnosed at later ages. These results highlight the importance of early diagnosis and treatment of autism spectrum disorder even in community settings with heterogeneous services. In addition, these results motivate further prioritization of universal screening for autism spectrum disorder before 2.5 years of age.

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