To identify studies for this Seminar, we searched PubMed for papers published between Jan 1, 2010, and March 31, 2015, using the search terms “ADHD”, “aetiology”, “epidemiology”, “prevalence”, “gender”, “time trends”, “prescribing”, “genetic”, “prenatal”, “psychosocial”, “toxins”, “institutional rearing”, “longitudinal”, “prognosis”, “animal model”, “biological pathway”, “cognition”, “neuroimaging”, “comorbidity”, “neuropsychological”, “medication”, “stimulants”, “behavioural interventions”,
SeminarAttention deficit hyperactivity disorder
Introduction
Attention deficit hyperactivity disorder (ADHD) is a childhood-onset neurodevelopmental disorder characterised by developmentally inappropriate and impairing inattention, motor hyperactivity, and impulsivity, with difficulties often continuing into adulthood. In this Seminar, we aim to update and inform early career clinicians on issues relevant to clinical practice and discuss some controversies and misunderstandings.
Section snippets
Definitions of ADHD
ADHD is a diagnostic category in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders 4th edition (DSM-IV)1 and the more recent DSM-5.2 The broadly equivalent diagnosis used predominantly in Europe is hyperkinetic disorder, which is defined in WHO's International Classification of Diseases (10th edition; ICD-10).3 This definition captures a more severely affected group of individuals, since reported prevalence of hyperkinetic disorder is lower than that
Epidemiology
In the general population, the estimated prevalence of ADHD in children is 3·4% (95% CI 2·6–4·5) according to the most recent meta-analysis,9 with lower rates of around 1·4% reported for hyperkinetic disorder from European studies.10 International comparisons show that prevalence does not vary by geographical location but is affected by heterogeneity in assessment methods (eg, use of an additional informant to the parent or carer) and diagnostic conventions (eg, ICD vs DSM).11 Notably, there is
Early comorbidity
ADHD shows high concurrent comorbidity with other neurodevelopmental disorders—namely, autism spectrum disorder, communication and specific learning or motor disorders (eg, reading disability, developmental coordination disorder), intellectual disability, and tic disorders.30, 31, 32 Unsurprisingly, rates of comorbidity are higher in individuals who are clinically referred than in those who are not referred.33 ADHD also shows high concurrent comorbidity with behavioural problems—namely,
Overview
As for all complex disorders, no single risk factor is either necessary or sufficient to explain ADHD—many genetic and non-genetic (or environmental) factors contribute to risk, and the pattern of inheritance is multifactorial for most affected individuals.
Genetics
ADHD is a familial disorder. Its relative risk is about 5–9 in first-degree relatives of probands with ADHD.37 Many twin studies of ADHD from different countries have consistently yielded very high heritability estimates of about 76%, a
Biology
The biological mechanisms through which genetic and environmental factors act and interact to alter neurodevelopment in ADHD are not yet understood and there remains no diagnostic neurobiological marker. The validity of animal models of ADHD are limited by our incomplete understanding of its pathophysiology in man and the extent to how well inattention, motor overactivity, and impulsive responses on behavioural tasks in non-human species represent ADHD.64 However, findings in animal models have
Clinical assessment
The assessment process for ADHD requires careful clinical history taking that goes beyond asking yes or no questions in relation to core ADHD symptoms. A missed diagnosis has potential to jeopardise an individual's learning or occupational and social relationships, whereas a misdiagnosis could lead to the use of pharmacological treatment that is not needed. History taking should not be reductionist—ie, exclusively focused on asking about diagnostic items. A detailed developmental as well as
Treatment
There are specific guidelines for the stepwise management of ADHD, such as those developed by the National Institute for Health and Care Excellence (NICE)7 and the Scottish Intercollegiate Guidelines Network (SIGN)93 in the UK, by the Eunethydis European ADHD Guidelines Group (EAGG)94 in Europe, and by the American Academy of Pediatrics (AAP)95 and the American Academy of Child and Adolescent Psychiatry (AACAP)96 in the USA. The main difference between these guidelines is that US guidance does
Prognosis
Not only do core ADHD symptoms themselves persist, but individuals with childhood ADHD are also at substantial risk of adverse outcomes in adolescence and adulthood. In this regard, ADHD behaves dimensionally: there is no distinct threshold at which adverse outcomes appear. A diagnosis of ADHD is associated with low academic attainment and premature cessation of education, and poor educational outcomes also extend to individuals with subthreshold symptoms.111 ADHD also predicts serious
Future research and clinical directions
The early age of onset, male preponderance, and strong comorbidity with other childhood-onset neurodevelopmental disorders support the inclusion of ADHD in the DSM-5 grouping of neurodevelopmental disorders. The previous practice of not diagnosing ADHD in the presence of autism spectrum disorder or intellectual disability has been a crucial barrier to research on aetiological and clinical overlaps and distinctions as well as to clinical and educational practice. Unfortunately, referral and
Conclusions
ADHD is a very important condition because of its high prevalence, persistence into adult life, and adverse outcomes that extend beyond the affected individual. Although ADHD is still viewed with scepticism by some and often remains stigmatised by the media, the evidence for it being a clinically and biologically meaningful entity is robust and consistent across design type and sample. There are established assessment methods and good treatment evidence. However, as is true for any chronic
Search strategy and selection criteria
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