an Economist Intelligence Unit business healthcare
Department of Health
Attention deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder characterised by pervasive and impairing inattention, impulsivity and hyperactivity, which arise early in life. It results in a broad range of psychological, social and academic burden for sufferers, their families and society in general. There is an increasing awareness of the condition and the numbers diagnosed and treated for ADHD are increasing, and as such these increasing numbers have policy implications for the Department of Health. Critically, the causes of ADHD remain unknown, though there is no shortage of theories.
The Policy Research Programme of the Department of Health wished to commission a systematic review of research literature on the causes of Attention Deficit Hyperactivity Disorder (ADHD). It was anticipated that the review would lead to a better understanding of the underlying causal factors and the complex interaction between genes/environment, with the ultimate aim of improving strategies for preventing and managing the condition. The review was put out to tender and Bazian, in collaboration with the University of Dundee, won the contract to carry out the review.
This was a full scale systematic review. To identify relevant studies we searched electronic databases, hand searched the reference lists of relevant studies, and obtained references from the expert reference group. The search aimed to identify research regarding any potential causes of ADHD, both genetic and environmental. We included only studies which established that exposure to the potential causal factor occurred before the diagnosis of ADHD; such a study design is best able to support the possibility of a causal link, and avoid the possibility of reverse causality. Potential causal agents include neurotransmitter receptor and transporter genes, perinatal factors (such as smoking in pregnancy), environmental and psychosocial factors (such as early childhood deprivation and watching television), environmental toxins (such as lead and mercury), and diet (including food additives and colourings). We identified 6061 publications, of which 496 were selected for screening in full text and 168 met our pre-established quality thresholds and were therefore included in the review.
In summary, the evidence clearly supported a causal role for particular genetic and environmental factors in ADHD, and pointed to the likelihood that these act together to increase the risk of the disorder. Whilst the available evidence did not yet meet the standards required to be certain of an unequivocal causal relationship between any specific environmental or genetic factors and a categorical diagnosis of ADHD, we were able to identify those factors with the most evidence supporting their claim to causality. We were also able to identify those factors for which there is currently no longitudinal evidence of a link with ADHD diagnosis. A systematic review of all possible causes of a condition this complex is a massive undertaking. In collaboration with our expert reference group, we were able to complete this systematic review in under a year, within a competitive budget. To our knowledge, this is the most comprehensive systematic review of the potential causes of ADHD. This type of review can have a number of practical policy implications. More specifically, they can be used to: