Introduction

Unintentional injuries in childhood According to WHO, injury can be defined as, ’The physical damage that results when a human body is suddenly subjected to energy in amounts that exceed the threshold of physiological tolerance or else the result of a lack of one or more vital elements, such as oxygen'.1 Therefore, unintentional injuries in children and young people (CYP) include traffic injuries, drowning, poisoning, falls or any other traumatic injury and burns. Childhood unintentional injury is a major cause of death and disability among children and adolescents: over 5 00 000 children die worldwide every year from unintentional injuries and many more are left with permanent disabilities.1 Injuries are especially relevant in childhood compared with adulthood. Developmental factors make CYP more prone to unintentional injuries compared with adults. Additionally, their anatomical fragility, smaller size and brain immaturity lead to more serious injuries and sequelae.2 Injury risk varies by sex, with a higher risk in males. It also varies with age. According to the WHO 2008 report on child injury prevention,1 in high-income countries children under 1 year and over 15 years have greater risks of death from unintentional injuries (28 and 23.9 death rates per 1 00 000, respectively). Socioeconomic deprivation is an additional factor associated with the probability of unintentional injury. Rates (per 100 000) of estimated mortality due to unintentional injuries in CYP in high-income countries were 12.2 as opposed to 41.7 in low-income and middle-income countries in this same report by WHO.1 Moreover, CYP from families from low socioeconomic areas have a higher incidence of unintentional injuries compared with those less deprived,1 for example, a study found that across England rates of serious injury in children as pedestrians were higher in the most deprived areas than in the least deprived (rate ratio (RR) 4.1; 95% CI 2.8 to 6.0 domestic product).3 As a result of the higher incidence of unintentional injuries in more economically deprived CYP, there is a contribution to ongoing inequalities between children within nations and comparing children from different nations. There is little evidence on the evidence of the economic costs of injuries as a proportion of gross domestic product globally. However, acute treatment costs of unintentional injuries sum €4000 million worldwide every year,4 whereas the National Health Service in the UK calculated that the extra cost of healthcare of injured children compared with non-injured children was €45 million.5 The injuries that occurred in the year 2000 in CYP under the age of 14 years from the USA will have an estimated lifetime cost from medical treatments of US$11 899 million and US$38 664 million from lost productivity.6 Importantly, an issue that makes unintentional injuries an even bigger healthcare priority is the fact that most of the times their consequences could be prevented or minimised with the proper educational, legal or environmental measures. In fact, injuries are the first preventable cause of death and disability.5

Attention-deficit hyperactivity disorder Attention-deficit hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder, with an estimated worldwide prevalence between 3% and 5% among children and adolescents, being three to four times more prevalent in males than females.7–9 The most recent version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), defines ADHD as a disorder characterised by a persistent pattern of hyperactivity/impulsivity and/or inattention, affecting both development and functioning. The symptoms need to be present in at least two settings and influence negatively academic, occupational or social activities from childhood to adult life.10 The economic burden of ADHD is very high. Healthcare spending in patients with ADHD has been estimated to be between US$600 and US$2000 greater than for an individual without ADHD.11 12 Importantly, a significant part of such increase in healthcare expenditure is a direct consequence of the higher likelihood of injuries in individuals with ADHD.13 The relationship between ADHD and the risk of unintentional injuries has been widely studied.14 15 However, available studies present with caveats and sample sizes and study methods have differed significantly across studies. Case-control studies are the most frequent in the literature, but, quite often, they assessed only one type of injuries such as dental,16 fractures of specific body bones17 or burns.18 An important limitation of this type of studies is that they have typically relied on a small sample size, which hinders the statistical control of confounding factors that could be leading to a spurious correlation between ADHD and unintentional injuries. Nevertheless, studies tend to show a higher incidence of injuries in ADHD CYP. Longitudinal cohort studies have also been conducted on the relationship between ADHD and physical injuries. Although few in number, they included large sample sizes, hence permitting an increased statistical rigour. Estimates of the differences have varied greatly between studies. In a large sample, Rowe et al found an OR of 1.6 (95% CI 1.6 to 2.3) for a statistically significant increased risk of fractures in ADHD compared with controls, while others have found ORs over 3.19–21 Furthermore, comorbidity with oppositional defiant disorder (ODD) and conduct disorder (CD) has been related to an increased risk of unintentional injuries in some studies, so that it could be argued that both disorders, highly comorbid with ADHD, could play a major role in the relationship between ADHD and unintentional injuries. However, while a recent European study with a total sample of 4517 individuals found no differences between children with ADHD and controls (OR 0.91, 95% CI 0.56 to 1.48) when comorbidity and other variables were controlled for,22 another study found a similar risk of injuries when ADHD with conduct problems were compared with controls than when ADHD without conduct problems were compared with controls (OR close to 1.5).23 Another factor that could contribute to the differences in the risk estimation between studies could be an interaction between diagnosis and variables that influence the risk of injuries in the general population, namely age and sex. Summarising, while there is evidence supporting a higher risk of injuries in ADHD CYP, the magnitude of the difference remains unclear. Additionally, to which extent variables such age and sex which could influence the possible association deserve further investigation.24 Finally, the suggestion that comorbidity with ODD and CD25 could lead to a higher risk of unintentional injuries needs to be more rigorously tested.