Children who go on to develop ADHD may exhibit more risk taking behaviours as young children and are therefore more likely to experience early injuries, suggest the authors.
There is considerable debate over the cause of ADHD. The National Institute for Health and Clinical Excellence guidelines state that risk factors for ADHD may interact rather than act in isolation. For example, they say that genetics is important, but environmental factors such as injury or maternal smoking may also contribute.
The exact association between head injury and ADHD is not clear. Previous studies have shown that injury is more common in children with ADHD, and that moderate to severe traumatic brain injury in school age children results in the development of ADHD.
So Professor Heather Keenan and colleagues investigated whether medically attended head injury, before 2 years of age, increases the incidence of ADHD occurring.
The study involved over 62 088 children from 308 general practices in the UK between 1988 and 2003, using data from the health improvement network database.
They compared three groups of children—children with medically attended head injury before age 2 (2782), children who had a medically attended burn or scald without a head injury before age 2 (1116), and non-injured children before age 2 (58 190).
They hypothesised that if head injury was a causal factor in the development of ADHD, the risk of ADHD would be higher in the head injury group than in the other two groups.
The authors found both injury groups had similar and significantly higher rates of ADHD compared with non-injured controls—children with a head injury before age 2 were twice as likely to be diagnosed with ADHD than non-injured children, but not more likely than the other burn injury group.
Thus, head injury does not seem to be a cause of ADHD, say the authors, but some other factor appears to be associated with early injury and the development of ADHD.
They conclude that medically attended injury may be an early marker for behavioural traits that lead to a diagnosis of ADHD.
The study also found that children who had a head injury after age 2 had a greater likelihood of being diagnosed with ADHD before their 10th
birthday among all three groups. This suggests that the main symptoms of ADHD—excessive attention, hyperactivity, and impulsivity—might predispose these children to injury, says Morris Zwi, consultant child and adolescent psychiatrist from South West London and St George's NHS Mental Health Trust in London, in an accompanying editorial.
The study highlights the need for primary care clinicians to assess children for symptoms of ADHD and monitor them over time, he concludes.