Donders Institute for Brain, Cognition and Behaviour
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Thesis defense Janne Visser (Donders series 286)

4 July 2017

Promotor: prof. dr. J. Buitelaar


Presentation, risk factors and trajectories

The high heterogeneity of autism spectrum disorders (ASD) in terms of presentation, course and (genetic) etiology, and their overlap with other neurodevelopmental disorders is a big challenge for clinical and basic scientific work. Focusing on early development, this thesis aims to clarify (1) to which extent ASD can be differentiated from other (neurodevelopmental) disorders, in particular attention deficit-hyperactivity disorder (ADHD) and (2) if ASD can be differentiated into more homogeneous subtypes, based on prenatal risks and on developmental  trajectories. Starting with an in-depth review of the literature on the shared and unique characteristics of ASD and ADHD in early childhood, the studies in this thesis are based on (longitudinal) data on symptoms, cognitive and behavioral characteristics, and on prenatal risks in children aged between 1 and 7 years.

Previous research suggests that ASD and ADHD show more overlap at the level of symptoms and traits – particularly attentional problems - than at the level of underlying behavioral, cognitive and motivational components. Results show that during early childhood, ASD may be differentiated from typical development and from other disorders groups, by a more pronounced lack of competencies and relative absence of disruptive behavioral problems. As regards the differentiation within ASD, the severe and mild ASD phenotypes show differences in smoking during pregnancy which suggest etiological differences. ASD can further be parsed into meaningful subtypes based on early developmental trajectories of ASD symptoms. These ASD subtypes - two of which (10%) show marked improvements - differ in trajectories of language, non-verbal cognition and ADHD-related traits.

In conclusion, results show that a better differentiation of ASD can be achieved by broadening the focus to trans-diagnostic characteristics, by examining the components and mechanisms underlying the overt symptoms, and by following their development starting in early life.