hyperactivity disorder

hyperactivity disorder

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Attention defi cit hyperactivity disorder Anita Thapar, Miriam Cooper

Attention defi cit hyperactivity disorder (ADHD) is a childhood-onset neurodevelopmental disorder with a prevalence of 1·4–3·0%. It is more common in boys than girls. Comorbidity with childhood-onset neurodevelopmental disorders and psychiatric disorders is substantial. ADHD is highly heritable and multifactorial; multiple genes and non-inherited factors contribute to the disorder. Prenatal and perinatal factors have been implicated as risks, but defi nite causes remain unknown. Most guidelines recommend a stepwise approach to treatment, beginning with non-drug interventions and then moving to pharmacological treatment in those most severely aff ected. Randomised controlled trials show short-term benefi ts of stimulant medication and atomoxetine. Meta-analyses of blinded trials of non-drug treatments have not yet proven the effi cacy of such interventions. Longitudinal studies of ADHD show heightened risk of multiple mental health and social diffi culties as well as premature mortality in adult life.

Introduction Attention defi cit hyperactivity disorder (ADHD) is a childhood-onset neurodevelopmental disorder char ac- terised by developmentally inappropriate and impairing inattention, motor hyperactivity, and impulsivity, with diffi culties 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.

Defi nitions 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 defi ned in WHO’s International Classifi cation of Diseases (10th edition; ICD-10).3 This defi nition captures a more severely aff ected group of individuals, since reported prevalence of hyperkinetic disorder is lower than that of DSM-IV ADHD, even

within the same population.4 Key diagnostic criteria are listed in the panel. DSM-5 has longer symptom descriptors than those used in DSM-IV; these descriptors also capture how symptoms can manifest in older adolescents and adults. DSM-IV distinguished between inattentive, hyperactive–impulsive, and combined sub- types of ADHD; a diagnosis of the combined subtype required the presence of symptoms across the domains of inattention and hyperactivity–impulsivity. However, ADHD subtypes are not stable across time,5 and DSM-5 has de-emphasised their distinctions. ICD-10 does not distinguish subtypes; symptoms need to be present from the three separate domains of inattention, hyperactivity, and impulsivity for a diagnosis of hyperkinetic disorder.

The diagnosis of ADHD or hyperkinetic disorder also requires the presence of symptoms across more than one setting (eg, home and school) and requires that the symptoms needed for diagnosis result in impairment, for example in academic, social, or occupational functioning. Onset must be early, although DSM-5 has changed the age of onset from before age 7 years (ICD-10 and DSM-IV) to before age 12 years.

Like all complex medical and psychiatric disorders, ADHD shows marked heterogeneity at clinical, aetiological, and pathophysiological levels. Individuals with a diagnosis of ADHD diff er from each other in terms of their core symptom combinations, level of impairment and comorbidities, as well as on other background individual, family, and social factors.

For clinical purposes, defi ning ADHD categorically is useful given that clinical decisions tend to be categorical in nature—eg, whether to refer to specialist services or to treat. However, like many medical disorders (such as hypertension and diabetes), in terms of causes and outcomes, ADHD can be viewed as a continuously distributed risk dimension. In common with other continuously distributed phenotypes (eg, blood pressure), it could be argued that there is a lack of an objective cut-point that defi nes the diagnostic threshold. Indeed, individuals with subthreshold symptoms are at heightened risk of adverse outcomes6 (as is seen in hypertension). However, ultimately, categorical decisions on resource allocation and treatment have to be made,

Lancet 2016; 387: 1240–50

Published Online September 17, 2015

http://dx.doi.org/10.1016/ S0140-6736(15)00238-X

Child & Adolescent Psychiatry Section, Institute of

Psychological Medicine and Clinical Neurosciences, and

MRC Centre for Neuropsychiatric Genetics and

Genomics, Cardiff University School of Medicine, Cardiff , UK

(Prof A Thapar FRPsych, M Cooper MRCPsych)

Correspondence to: Prof Anita Thapar, Institute of

Psychological Medicine and Clinical Neurosciences, Cardiff

University School of Medicine, Hadyn Ellis Building, Maindy

Road, Cathays, Cardiff CF24 4HQ, UK

thapar@cardiff .ac.uk

Search strategy and selection criteria

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”, “nonpharmacological interventions”, “diet”, and “outcomes”. Only articles published in English were included. Key recent reviews and book chapters were also examined. To reduce the number of papers cited, the most up-to-date review papers and meta-analyses were used where possible. We selected papers according to our judgment of the quality of the study or review paper, the relevance to controversial or commonly misunderstood issues, and whether fi ndings had clinical relevance. We included older papers that we judged to be important.

http://crossmark.crossref.org/dialog/?doi=10.1016/S0140-6736(15)00238-X&domain=pdf
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and ICD-defi ned or DSM-defi ned diagnosis provides a reliable way of balancing the risks and benefi ts of giving an individual a diagnostic label and providing treatments that are not free of adverse eff ects. A further challenge, which occurs in all psychiatric disorders and some neurological disorders (eg, migraine), comes from the diagnosis being based on reported symptoms alone; there are no biological tests. This diffi culty means that even with clear-cut diagnostic criteria, there is potential risk of overdiagnosis and underdiagnosis, which underscores the importance of careful and rigorous expert assessment.7 Concerns about underdiagnosis and overdiagnosis are not restricted to ADHD or psychiatric disorders.8

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 aff ected 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 a marked under-representation of studies on ADHD from low-income and middle-income countries.

One common assumption is that ADHD must be a modern occurrence. However, a case series of children presenting with the characteristic clinical features was published by the British paediatrician Sir George Still in The Lancet in 1902,12 and there are descriptions that pre-date this publication by several centuries. Time trends studies of non-referred population cohorts in the late 20th and early 21st centuries show no evidence of a rise in rates of ADHD symptoms or diagnosis across time.13,14 However, there has been a very marked rise in the number of prescriptions issued for ADHD pharma- cological treatment across high-income countries in the past decade.15–17 Rises in clinic incidence and treatment could simply indicate increased parent and teacher awareness of ADHD or changes in the impact of symptoms on children’s functioning, or both.18,19 Never- theless, European studies have repeatedly reported that despite the rise in ADHD treatment, the admin is trative prevalence is lower than the population fi gure, highlighting that in these countries there is still underdiagnosis.17,20,21 However, in the USA, similar types of studies show geographical variation in patterns of underdiagnosis and overdiagnosis or in ADHD medication prescribing.22,23 Such fi ndings highlight that there is the potential for misdiagnosis and inappropriate use of pharmacological interventions if safeguards are not in place. These safeguards include ensuring full, good-quality clinical assessments are undertaken, even though these require time, and adherence to national

and international treatment guidelines. However, there is no evidence of rising population rates of ADHD explained by social change, contrary to the opinion of some people.

An excess of aff ected male individuals is a strongly consistent epidemiological fi nding, although the male:female ratio of 3–4:1 recorded in epidemiological samples is increased in clinic populations to around 7–8:1, suggesting referral bias in relation to female patients with ADHD.24 The same male preponderance is seen for other neurodevelopmental disorders such as autism spectrum disorder, intellectual disability (intelligence quotient [IQ] <70), and communication disorders.25

The natural history of ADHD is best examined in prospective longitudinal studies. As is typical of neurodevelopmental disorders, the core defi ning features of ADHD tend to decline with age, although inattentive features are more likely to persist. However, in line with its heterogeneous clinical presentation, the develop mental trajectories of ADHD are highly variable. Although around 65% of patients continue to meet full criteria or have achieved only partial remission by adulthood, some patients do achieve full remission.26 Good-quality, large epidemiological studies of the prevalence of ADHD in adulthood are lacking, but one meta-analysis of adult ADHD yielded a pooled prevalence of 2·5% (95% CI 2·1–3·1).27 However, there are still uncertainties as to what constitutes the best way of defi ning ADHD (or indeed any

Panel: Key diagnostic symptoms of attention defi cit hyperactivity disorder2

Inattentive symptoms • Does not give close attention to details or makes careless

mistakes • Has diffi culty sustaining attention on tasks or play activities • Does not seem to listen when directly spoken to • Does not follow through on instructions and does not

fi nish schoolwork, chores, or duties in the workplace • Has trouble organising tasks or activities • Avoids, dislikes, or is reluctant to do tasks that need

sustained mental eff ort • Loses things needed for tasks or activities • Easily distracted • Forgetful in daily activities

Hyperactivity or impulsivity symptoms • Fidgets with or taps hands or feet, or squirms in seat • Leaves seat in situations when staying seated is expected • Runs about or climbs when not appropriate (may present

as feelings of restlessness in adolescents or adults) • Unable to play or undertake leisure activities quietly • “On the go”, acting as if “driven by a motor” • Talks excessively • Blurts out answers before a question has been fi nished • Has diffi culty waiting his or her turn • Interrupts or intrudes on others

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neurodevelopmental disorder) in adulthood. DSM-5 explicitly allows for symptom decline and requires a reduced number of symptoms for diagnosis of adult ADHD.2 In clinical settings, diagnosis of ADHD in adults who did not present in childhood requires some caution in the absence of documented information because of the diffi culty for young adults and those who know them to date symptom onset retrospectively.28 Objective records (eg, school reports) could help in this regard. Despite these caveats, there is certainly suffi cient evidence to conclude that ADHD is not simply a problem that most children grow out of. However, transitioning from child to adult mental health clinics is diffi cult because of a scarcity of adult services.29

Early comorbidity ADHD shows high concurrent comorbidity with other neurodevelopmental disorders—namely, autism spec- trum disorder, communication and specifi c learning or motor disorders (eg, reading disability, developmental coordination disorder), intellectual disability, and tic disorders.30–32 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, oppositional defi ant and conduct disorders.31,34 Conduct disorder is a risk marker for greater neurocognitive impairment and worse prognosis in children with ADHD.35,36 This subgroup of children with hyperkinetic conduct disorder is distinguished in ICD-10 but not in DSM-5.

Risk factors Overview As for all complex disorders, no single risk factor is either necessary or suffi cient to explain ADHD—many genetic and non-genetic (or environmental) factors contribute to risk, and the pattern of inheritance is multifactorial for most aff ected individuals.

Genetics ADHD is a familial disorder. Its relative risk is about 5–9 in fi rst-degree relatives of probands with ADHD.37 Many twin studies of ADHD from diff erent countries have consistently yielded very high heritability estimates of about 76%, a magnitude similar to that reported for schizophrenia and autism.38

The genetic architecture of ADHD is similar to other neuropsychiatric disorders such as schizophrenia. Several diff erent classes of genomic variants have been identifi ed to be associated with ADHD risk.39 These variants include common (defi ned as >5% population frequency) DNA sequence variants called single nucleotide polymorphisms (SNPs), but associations have only been reported when thousands of SNPs are combined into a composite genetic risk score.40 Subtle chromosomal mutations, such as rare (defi ned as <1% frequency) deletions and

duplications called copy number variants (CNVs), are also associated with ADHD risk.41 These have larger eff ect sizes but are uncommon.

Before whole-genome investigations, specifi c single dopaminergic, serotonergic, and noradrenergic candidate genes were signifi cantly associated with ADHD status in meta-analyses.42,43 However, in the present era of whole-genome investigation, psychiatric candidate gene studies of DNA variants in single genes are viewed with caution because of the potential for false positives.44

ADHD-associated genomic variants are non-specifi c; composite genetic risk scores show signifi cant overlap with those contributing to schizophrenia and mood disorders.45,46 ADHD-associated CNVs also show overlap with ones associated with schizophrenia, autism, and intellectual disability.41,47 Although testing for rare CNVs is now recommended for individuals with intellectual disability, this is not the case for ADHD. Ascertaining causality requires further and diff erent types of investigation, as reviewed elsewhere.39

Although most cases of ADHD are multifactorial in origin, there are several known, rare genetic syndromes (such as fragile X syndrome, tuberous sclerosis, 22q11 microdeletion, and Williams syndrome) characterised by high rates of ADHD and ADHD-like features. These syndromes are also associated with high risk of other disorders, such as autism (especially in fragile X syndrome and tuberous sclerosis) and schizophrenia (22q11 microdeletion syndrome). In typical clinic populations with ADHD, there is no evidence that routine screening for these genetic syndromes is warranted in the absence of intellectual disability.48

Environment and gene–environment interplay Environmental factors are also known to be important in ADHD. Because evidence for modifi able causes can aff ect clinical decision making, public health priorities, and clinician and patient behaviour,49 we will discuss whether fi ndings on individual environmental risks meet accepted standards for inferring causation.50

Observational case-control and epidemiological studies show that exposures to a range of prenatal and perinatal factors, environmental toxins, dietary factors, and psycho- social factors are all associated with ADHD.38 If these associations are causal, it means manipulation of the risk factor can alter the outcome. However, association does not mean causation, because exposures to risks are not randomly allocated and can be aff ected by unmeasured confounders, selection factors, and reverse causation, whereby the phenotype infl uences the environmental exposure. Evidence for environmental causation must be interpreted with these caveats in mind.

Prenatal and perinatal factors reported to be associated with ADHD are low birthweight and prematurity,51 and in-utero exposure to maternal stress, cigarette smoking, alcohol, prescribed drugs (eg, paracetamol), and illicit substances.38,52,53 In relation to prenatal smoking and

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stress, quasi-experimental designs suggest that most or all of the association with off spring ADHD, unlike with off spring birthweight, is explained by unmeasured confounding factors.54–58

Environmental toxins, specifi cally in-utero or early childhood exposure to lead, organophosphate pesticides, and polychlorinated biphenyls, are risk factors for ADHD, as reviewed elsewhere.38 Nutritional defi ciencies (eg, zinc, magnesium, and polyunsaturated fatty acids), nutritional surpluses (eg, sugar and artifi cial food colourings), and low or high IgG food have not been shown convincingly to precede ADHD and at present should be regarded as correlates. Eff ective treatments for any disorder, unlike prevention, do not necessarily have to deal with its causes or origins.

Psychosocial risks, such as low income, family adversity, and harsh or hostile parenting, although robustly causal for some psychiatric disorders, are also correlates rather than proven causes of ADHD. Longitudinal studies,59 treatment trials,60 and a study of children adopted away at birth61 suggest that observed negative mother–child relationships (even in unrelated mothers) arise as a consequence of early child ADHD symptoms (reverse causation) and improve with treatment. However, exposure to very severe, early social deprivation seems to be diff erent and causal. After being adopted away in the UK, Romanian orphans raised in institutions and exposed to extreme early privation in the fi rst year of life showed increased rates of ADHD-like features, cognitive diffi culties, and quasi-autistic features that persisted into adolescence.62,63 Psychosocial context may well shape ADHD presen ta- tions and alter developmental trajectories, outcomes, and impairments, but surprisingly this has not been investigated widely (fi gure 1). Irrespective of the cause of ADHD, treatment is based on clinical features and not assumed aetiology.

As a fi nal word on risk factors, many individuals mistakenly assume that the actions of genes (or biology) and environment are distinct. Potentially important environmental risks for ADHD and its outcomes may be brought about as a consequence of genetic propensities (gene–environmental correlation61). The eff ect of environmental factors on clinical phenotype may also depend on genetic liability. For example, animal studies have robustly shown that environment can alter behaviour in diff erent ways depending on the variant of gene carried (gene–environment interaction39). Eff ects of gene–environment interplay are subsumed in twin heritability estimates. Finally, there is very good evidence that environmental exposures result in biological changes,49 including ones involving brain structure, function, and altered DNA methylation (epigenetics). These fi ndings highlight that genes, environment, and biology work together. However, in man, these issues are complex; they will not be discussed in detail here, but have been reviewed elsewhere.38,39,49

Pathophysiology Biology The biological mechanisms through which genetic and environmental factors act and interact to alter neuro- development 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, fi ndings in animal models have suggested involvement of dopaminergic and noradrenergic neurotransmission (in line with the neurochemical eff ects of ADHD medications) as well as involvement of serotonergic neurotransmission.65

Cognition Although there is no cognitive profi le that defi nes ADHD, defi cits in various neuropsychological domains have been reliably identifi ed. In terms of executive functioning, the most consistent and strong associations are seen for response inhibition, vigilance, working memory, and planning.66 In terms of non-executive defi cits, associations are seen with timing,67 storage aspects of memory,68 reaction time variability,69 and decision making.70 However, there is substantial heterogeneity in cognitive functioning even within single samples,71 and there is not a straightforward association between cognitive performance and the trajectory of clinical symptoms.72,73 There is evidence, though, that some cognitive defi cits are improved by methylphenidate, with a meta-analysis showing improvements in executive and non-executive memory, reaction time, reaction time variability, and response inhibition.74

Figure 1: Origins and trajectories of attention defi cit hyperactivity disorder (ADHD)

Multiple early risk factors contributing to the development of ADHD

Genetics Early environment (prenatal and postnatal)

ADHD Later risk and protective factors modifying the course

Family and social environment

Genetics

Heterogeneous outcomes

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Imaging Functional MRI (fMRI) studies in patients with ADHD have found abnormalities in the function of many neural networks in response to cognitive tasks. A meta-analysis of task-based fMRI studies identifi ed alterations in several networks, including those related to attention and executive function.75 In terms of brain structure, a meta-analysis of structural MRI studies highlighted alterations in the basal ganglia and limbic areas.76 In a meta-analysis of diff usion MRI studies investigating white matter microstructure, alterations were reported to be widespread, but most reliably seen in the right anterior corona radiata, right forceps minor, bilateral internal capsule, and left cerebellum.77 Reduced total grey matter and altered basal ganglia volumes seem to index familial risk for ADHD.78

The scientifi c literature is increasingly suggesting that the pathophysiology of ADHD involves abnormal interactions between large-scale brain networks; however, current imaging studies do not yet have relevance to clinical practice.79 Interpretation is complex because of many factors, including the cross-sectional nature of most studies. Longitudinal data regarding the trajectory of cortical development suggest that the brain may show maturational delay, with persistence of ADHD indexed by progressive divergence from the normal trajectory,80 but it is not known whether this phenomenon can be

extrapolated to other metrics of structure, microstructure, and function. The eff ect of pharmacological treatment is also a consideration because there is some evidence to suggest that it normalises macrostructure and function.81 Nonetheless, there is some evidence from longitudinal studies of adults with childhood ADHD that grey and white matter abnormalities persist well into adulthood.82,83

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 medical history and an assessment of family processes and social circumstances (strengths as well as weaknesses) are also required. Figure 2 summarises the key steps in the assessment of children.

It is important to consider whether endorsed symptoms are better explained by other diffi culties that are amenable to intervention—eg, hearing diffi culties presenting as inattention. However, diagnosis is based on clinical phenotype and not generally excluded by presumed cause. Information should be obtained from more than one informant, including those who know the aff ected individual best at home and at school (or college or work). To decide which individuals need referral to a specialist assessment service or to monitor treatment response, use of standardised ADHD questionnaires (eg, Strength and Diffi culties Questionnaire,84 Conners’ Parent and Teacher Rating Scales85) are helpful, but these are not a substitute for detailed history taking before diagnosis. Structured interviews are more likely to be encountered in a research setting, but might be valuable in a clinical context, especially interviews that do not require extensive, expensive training (eg, the Development and Wellbeing Assessment).86 The use of structured interviews in a clinical setting requires further investigation. ADHD symptoms are commonly associated with a range of neurobehavioural diffi culties, which could be comorbid features of the disorder but should also be considered as diff erential diagnoses because their treatments are very diff erent.

Mental health symptoms, which should also be screened for, include those of oppositional defi ant disorder, conduct disorder, anxiety, and mood disturbance. Developmental and learning problems such as reading disorders, developmental coordination disorder, and tic disorders are also common.87–89 Because ADHD and autism spectrum disorder co-occur so frequently,90 autistic symptomatology should be considered. ADHD is also associated with lower IQ or intellectual disability91 and emotion dysregulation

Figure 2: Summary of the clinical assessment process for children ADHD=attention defi cit hyperactivity disorder.

Obtain detailed clinical history from parents or carers and young person

Carry out core ADHD symptom enquiry: are symptoms out of keeping with child’s age and developmental stage?

Obtain information across settings; consider questionnaires as an adjunct

Screen for associated difficulties (eg, mental health symptoms, other neurodevelopmental or learning problems)

• Developmental history (eg, motor delay) • Medical history (eg, epilepsy) • Family history (eg, mental health, educational history, physical health

problems) • Medical histories especially important in relation to cardiac or other risk

factors if pharmacological treatment is being considered

Consider severity of symptoms, effects on functioning, comorbid symptoms, medical history, and the family and child’s strengths, resources, demands, and psychosocial context when deciding on treatment options

Physical assessment: • Signs of other disorders (eg, dysmorphic features, skin lesions) and motor

coordination (eg, handwriting, balance); to be undertaken more completely if considering pharmacological treatment

• Baseline height, weight, blood pressure, pulse

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symptoms (eg, irritability),92 both of which can further complicate the presentation and interpretation of symptoms. In practice, it will be rare to fi nd an individual who presents with so-called uncomplicated ADHD, even if full diagnostic criteria for other comorbid disorders are not met. This situation makes formalising diff erential diagnoses conceptually diffi cult, because in reality an individual with neurodevelopmental problems is unlikely to have a pure presentation of any one condition as a unifying explanation for their diffi culties. A formulation should capture the full range of developmental, behavioural, and psychiatric diffi culties, even if some of these need to be described in terms of subthreshold problems.

Neuropsychological testing does not have a role in diagnosis of ADHD because cognitive processes are not a defi ning characteristic.66 However, cognitive comorbidities such as learning disability and dyslexia should be considered, which may require specialist assessment from education services.

Treatment There are specifi c 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 diff erence between these guidelines is that US guidance does not preclude the use of pharmacological treatment for preschool children or for those with mild ADHD; practice that is not recommended in Europe where a stepwise approach is recommended. If pharmacological treatment is prescribed, it should be in conjunction with behavioural interventions—namely, optimised classroom manage- ment strategies, parental psycho education, and behav- ioural management tech niques. However, there is no one-size-fi ts-all solution to management. Individual circumstances such as current academic or employment demands and medical history should be taken into account, and appropriate evidence-based treatments for comorbidities should also be initiated.

Non-pharmacological interventions have been investigated extensively over the years. The only non- pharma cological interventions that currently form a core part of treatment guidelines are behavioural inter- ventions. Initial results from the largest trial of ADHD interventions so far, the multimodal treatment study of children with ADHD (MTA),97 suggested that the combination of intensive behavioural treatment plus pharmacological treatment did not off er additional benefi t over pharmacological treatment alone for core ADHD symptoms, but that the combination might have provided some benefi t in terms of associated symptoms

and levels of functioning as well as the need for a lower drug dose. In a more recent series of meta-analyses investigating randomised controlled trials of non- pharmacological interventions, the investigators concluded that, along with neurofeedback, cognitive training, and restricted elimination diets, behavioural interventions cannot be recommended as interventions for core ADHD symptoms until better evidence of their eff ectiveness is reported by blinded assessments.98 Elimination of artifi cial food colouring98 might be benefi cial, but to what extent and for which population of patients is unclear.99 A meta-analysis has shown that children with ADHD have lower concen trations of omega-3 fatty acids than controls and that supple- mentation improves ADHD symptoms to a modest degree (an eff ect size about a quarter as large of that seen for pharmacological treatment); but whether subnormal blood concentrations should be the indication for treatment is not understood.100 However, there is evidence from blinded randomised controlled trials of a benefi cial eff ect of behavioural interventions on parenting and child conduct problems,101 and there is evidence that cognitive behaviour therapy may be useful for adults with ADHD when used in conjunction with pharmacological treatment.102

Stimulants such as methylphenidate and dexamfetamine are the fi rst-line pharmacological treatments for ADHD, and the noradrenaline reuptake inhibitor atomoxetine is the second-line treatment. Each of these treatments increases catecholamine availability. Meta-analyses have provided evidence for the effi cacy of stimulants for ADHD in children,103 in children with co-occurring autism spectrum disorder,104 and in adults.105 Although it is recommended that ADHD is treated in individuals with autism spectrum disorder or intellectual disability, or both, side-eff ects of pharmacological treatment in these individuals are more common than in those with ADHD alone.106,107 Meta-analyses have shown benefi cial eff ects of atomoxetine in children108 and in adults.109 Extended- release guanfacine and extended-release clonidine are licensed for use in the USA. Atypical antipsychotics are not indicated for treatment of core ADHD symptoms.

Pretreatment checks, including in relation to medical and family medical history (in particular cardiac disorders), are especially important if medication is to be initiated (fi gure 2). Height, weight, blood pressure, and pulse should be checked at baseline before starting treatment, and compared with normative data. It is reasonable but not mandatory to consider the routine performance of an ECG before starting pharmacological treatment, and the need to do so should be at the treating clinician’s discretion, taking into account factors such as medical history, family medical history, and physical examination fi ndings.7,110

It is best practice to start with a low dose, titrate up according to response, and monitor side-eff ects carefully.7 The most common side-eff ects of medications for ADHD are shown in the table. There is no evidence that

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pharmacological treatment for ADHD is associated with changes in QT interval, sudden cardiac death, acute myocardial infarction, or stroke.110 A comprehensive review of best practice in managing adverse events associated with pharmacological treatment for ADHD has been published elsewhere.110 Once an optimum response is achieved, height, weight, and growth will need regular monitoring. NICE guidance recommends that height is measured every 6 months in children and young people; weight is measured 3 months and 6 months after initiation of treatment and every 6 months thereafter in children, young people, and adults; and height and weight in children and young people should be plotted on a centile chart.7 Blood pressure and pulse should also be plotted on a centile chart before and after each change in dose and routinely every 3 months.7 Adverse side-eff ects of stimulant medication for ADHD include appetite suppression and growth retardation, which can be off set to a degree by so-called stimulant holidays on days when symptom control is deemed less crucial, such as weekends and holidays, and by adjusting the timing of doses. Other side-eff ects of stimulants and atomoxetine include gastrointestinal symptoms, cardiac problems, insomnia, and tics (although tics are less common with atomoxetine). Stimulants are controlled drugs with potential for diversion for misuse, and if there is a concern in this regard then an alternative drug may be preferable.

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 antisocial behaviour, involvement with the police, and substance misuse in adolescence.36

Until recently, few data for broad outcomes beyond the third decade of life were available. However, one long-term follow-up study has shown that childhood ADHD (participants aged 6–12 years) is also associated with adverse occupational, economic, and social outcomes, antisocial personality disorder, and risk of substance use disorders, psychiatric hospital admissions, incarcerations, and mortality.112 A Danish registry-based investigation showed substantially increased mortality in adult life in individuals with ADHD compared with individuals without the disorder. This increase in mortality was mainly a result of accidents, and was especially increased in those with comorbid oppositional defi ant disorder, conduct disorder, and substance misuse.113

A meta-analysis of ADHD in prison inmates showed an estimated prevalence of 30·1% in youth prison populations and 26·2% in adult populations, with the risk for female prisoners being nearly as high as that for male prisoners.114 Psychiatric comorbidity is high in prisoners with ADHD, especially in adults.115 Although randomised controlled trials of ADHD treatment have reported immediate but not as yet longer term benefi ts, there is epidemiological evidence that pharmacological treatment might reduce criminal behaviour116 and trauma-related visits to emergency departments.117

Most people with ADHD do not develop psychosis or a mood disorder. The largest studies only fi nd a small subgroup of individuals who additionally develop schizophrenia or bipolar disorder.118,119 Evidence about associations between ADHD and later unipolar depression is inconsistent;112,120 this might be because depression is more common in female patients who are under-represented in ADHD samples.

Future research and clinical directions The early age of onset, male preponderance, and strong comorbidity with other childhood-onset neuro- developmental 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 treatment pathways and service provision in health and education tend to be diagnostically focused (ie, autism only or intellectual disability only), although some clinics and services are focusing more broadly on childhood neurodevelop- mental disorders, a change which is welcomed and supported by research.

Methylphenidate (MPH)

Atomoxetine (ATX)

Loss of appetite + +

Growth restriction ++ +

Other gastrointestinal symptoms: abdominal pain, nausea, vomiting, diarrhoea (MPH), constipation (ATX), dyspepsia, dry mouth

+ +

Increase in blood pressure and heart rate + +

Cough, nasopharyngitis + ··

Sleep disturbances ++ +

Tics + ··

Irritability, mood changes + +

Drowsiness + ++

Dizziness + +

Headache ++ +

+=common side-eff ect. ++=if the side-eff ect is common for both drugs, the eff ect is more pronounced for this drug compared with the other. ··=side-eff ect not common.

Table: Some of the more common side-eff ects associated with pharmacological treatment

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We accept that for clinical practice, there is a need for strict categories, otherwise diagnostic spread would become at best unhelpful and at worst risky and unethical (eg, use of pharmacological treatment where not indicated) and application of evidence-based treatments would become impossible (eg, interpreting the severity of diffi culties of individuals included in a trial). However, for aetiological and outcome research purposes, there is strong evidence in favour of viewing ADHD dimensionally. At present, we do not know what sorts of dimensions best capture ADHD and at what level they should be measured—eg, reported symptoms, cognitive tests, brain imaging markers, or other biological signatures.

Genetic research is progressing via large-scale collaboration, but there is a need to understand the clinical as well as biological meaning of fi ndings, if they are to aff ect our understanding and treatment of ADHD. Currently, there is no rationale for routine genetic testing in ADHD because of limited predictive power. However, because the disorder is heritable, rates of ADHD in parents of those with ADHD are increased. A pertinent future research question is how might treatment of parent ADHD aff ect child ADHD features and comorbidity? There is, for example, evidence that treating parent depression seems to improve off spring mental health.121,122 Another issue for future consideration is that genetic and environmental risk factors that cause ADHD are not necessarily the same as those that alter the later course of the disorder or contribute to adverse outcomes. What is greatly needed is research that tests which environmental risks (eg, social and other potentially modifi able risk factors) contribute to and modify the longitudinal course of ADHD across time, including better prognosis, with designs that can control for unmeasured confounders and genetic contributions from the aff ected person (eg, twin studies) and related parents (eg, adoption studies). This could inform interventions aimed at optimising outcomes.

So far, pharmacological and behavioural treatments for ADHD have focused on symptomatic relief of the core symptoms of inattention, overactivity, and impulsivity. However, according to trial-based data, benefi ts seem to be short-lived. Another issue is that treatment typically begins after a child has already begun to fail across multiple domains. ADHD in many respects behaves like a chronic medical disorder. Many features remain problematic long term, although the most prominent or presenting features can change with age and development. ADHD creates risks of its own and secondary mental health problems commonly arise in mid-childhood and after puberty. Almost certainly, for many individuals, multimodal interventions that are carefully adjusted over time to prevent complications will be needed, perhaps in the way that is undertaken for optimising diabetes control. How ADHD is best managed across the lifespan and across key transition periods (eg, school entry,

comprehensive or high-school entry, transition to adult services, and transition to parenthood) needs much more investigation. Until now, guidelines have been based on evidence, but unless research keeps pace, guidance will have to be based on professional consensus, which is not very satisfactory for a prevalent, impairing disorder.

Conclusions ADHD is a very important condition because of its high prevalence, persistence into adult life, and adverse outcomes that extend beyond the aff ected 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 disorder, repeated assessment is likely to be needed and treatment will typically need many adjustments over time. Impairments beyond core diagnostic criteria, developmental change, and an individual’s psychosocial strengths, weaknesses, and resources are all important aspects for consideration. Contributors AT drafted the initial outline and structure and wrote the fi rst draft of the summary and sections on introduction, defi nitions of ADHD, epidemiology, early comorbidity, genetics, environment and gene–environment interplay, and future research and clinical directions. MC wrote the fi rst draft of the sections on pathophysiology, clinical assessment, treatment, and prognosis, and prepared the table, panel, and fi gures. Both authors undertook scientifi c literature searches and edited the manuscript.

Declaration of interests We declare no competing interests.

Acknowledgments We have been funded by the MRC, ESRC, and the Wellcome Trust.

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Attention deficit hyperactivity disorder
Introduction
Definitions of ADHD
Epidemiology
Early comorbidity
Risk factors
Overview
Genetics
Environment and gene–environment interplay
Pathophysiology
Biology
Cognition
Imaging
Clinical assessment
Treatment