1 9 General introduction General introduction Over the course of the last few years, there has been a strong increase in scientific interest in the potential of early interventions to alter the developmental trajectories of young children with autism spectrum disorder (ASD) or at elevated likelihood of ASD (French & Kennedy, 2017; Fuller & Kaiser, 2020). Multiple studies show that early intervention of ASD can have a positive impact on the child’s development, but also on family and society functioning, and that early interventions are cost-effective (e.g. Dawson et al., 2012; Fuller & Kaiser, 2020; Horlin et al., 2014; Tinelli et al., 2023). In line with this, the aim of this thesis is to enhance early detection and, most importantly, ensure timely access to suitable interventions for all infants and toddlers experiencing difficulties in their social-communicative development, play, and flexibility, as well as for their families. This introductory chapter starts with a general description of ASD and its early clinical manifestations, followed by the importance and challenges of early detection and intervention. Next, we focus specifically on multiple strategies to improve early detection and intervention of ASD in the Netherlands, including a screening guideline, the founding of an interdisciplinary expertise network and the development of an innovative early intervention program called BEAR (Blended E-health for Children at eArly Risk). This chapter concludes with the specific aims and outline of this thesis. Autism spectrum disorder – it’s definition and prevalence Autism Spectrum Disorder (ASD) – often simply referred to as ‘autism’ in daily practice – is an early onset neurodevelopmental disorder characterized by A) deficits in social communication and interaction and B) restricted repetitive patterns of behaviors and interests (American Psychiatric Association, 2013). In Box 1.1 the current diagnostic criteria for ASD as defined in in the DSM-5 are presented. ASD is a heterogeneous disorder at multiple levels of analysis: phenotypically in terms of the expression of symptoms and clinical severity, indicating a wide variety in how it manifests; etiologically in that different sets of common and rare genes together with environmental factors play a role in causing the disorder; and biologically in that different parts of the brain have been implicated in ASD (Lombardo, Lai & Baron-Cohen, 2019). For example, verbal children with milder symptoms of ASD and (above) average intelligence often show a different presentation of ASD than non-verbal children with more severe symptoms and cognitive impairments (Oosterling et al., 2010). ASD affects about 1 in 100 children (or 100 in 10,000) worldwide (Zeidan et al., 2022). This is a slight increase compared to the previous global prevalence report conducted in 2012 where prevalence was estimated to be 62 in 10,000 children (Elsabbagh et al., 2012). It is important to note that there is a wide variation in the prevalence of ASD in populations worldwide. The
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