Meta-analytical studies of community samples of children and adolescents with ADHD estimate that the prevalence of ADHD is 5.29% (95% confidence interval [CI] 5.01‒5.56) (Polanczyk G, et al. 2007). In adults aged 19‒45 years, meta-analyses suggest that the prevalence of ADHD in adults is 2.5% (95% CI 2.1‒3.1) (Simon V, et al. 2009). At least four developmental trajectories have been suggested for ADHD as, although it is a chronic disorder, its presentation can vary between individuals and change over time. These include: early (preschool [age 3‒5 years]) onset, middle childhood (age 6‒14 years) onset with a persistent course, middle childhood onset with adolescent offset, and adolescent or adult (age ≥16 years) onset. It is estimated that ADHD will persist into adulthood in 15% of individuals, and that 40‒60% will become partial remitters (Faraone SV, et al. 2006). The prevalence of ADHD in adults is higher than would be expected based on the persistence rates in children, indicating that new cases of ADHD may occur during development.
Evaluation of functional impairment as well as current and previous symptoms should form the basis of a clinical diagnosis of ADHD. Clinicians should also take a full family, gestational and developmental history when making a diagnosis. The Diagnostic and Statistical Manual of Mental Disorders –5th Edition (DSM-5TM) defines ADHD as the presence of ≥6 symptoms in either the inattentive or hyperactive and impulsive domains, or both. For adults, only the presence of ≥5 symptoms is required for a diagnosis (American Psychiatric Association, 2013). To allow for greater flexibility when diagnosing adults, symptom onset before age 12 years is required versus before age 7 years in the Diagnostic and Statistical Manual of Mental Disorders – 4th Edition (DSM-IV) (American Psychiatric Association, 2004). Previously, the DSM-IV had divided ADHD into three subtypes based on the most dominant symptoms (inattentive, hyperactive and impulsive, or combined), whereas the DSM-5TM has replaced ‘subtype’ with ‘presentation’ to highlight that clusters of symptoms may develop and change over time (American Psychiatric Association, 2004; American Psychiatric Association, 2013).
The International Classification of Diseases 11th Edition (ICD-11) was updated in 2018 having moved ADHD from the ‘disruptive’ to the ‘neurodevelopmental disorder’ domain and replaced ‘hyperkinetic disorder’ with ‘ADHD’ in line with the DSM-5TM (World Health Organization, 2018). Inattentive and hyperactive-impulsive presentations of symptoms have also been included in the ICD-11, which, unlike the International Classification of Diseases 10th Edition, includes a description of the essential features of the disorder without providing an age of onset (World Health Organization, 1993; World Health Organization, 2018).
Aetiology and pathophysiology
The heritability of ADHD is high and genome-wide significant risk loci have been identified for ADHD but these associations do not account for all of the disorder’s heritability. A number of environmental exposures are associated with ADHD but many of these are yet to be shown as causal. Prematurity and low birthweight have been consistently linked to ADHD, as well as intrauterine tobacco exposure and maternal stress and obesity during pregnancy, but these can also be explained by confounding genetic factors. Postnatally, neglect and deprivation soon after birth have been linked to ADHD, independent of pre-existing genetic or intrauterine risk factors.
The cognitive domains affected by ADHD include arousal, executive function, behavioural inhibition, motivation, set shifting and working memory. ADHD-related impairments may vary from setting to setting and the disorder is not a result of a fixed deficit. For example, symptoms of ADHD and cognitive problems may be more common when tasks are long and repetitive compared with highly stimulating and engaging tasks.
Clinical advances in ADHD based on scientific findings
The authors provided their thoughts on how scientific advances are challenging the way ADHD is viewed, and how this may impact the diagnosis and treatment of ADHD:
- Based on the DSM-5TM and ICD-11, ADHD is defined by symptom thresholds that suggest there are discrete boundaries between those with and without ADHD. However, these clinical boundaries are somewhat arbitrary as diagnosis of ADHD is often made based on clinical experience regarding the number and severity of symptoms and whether the associated impairment requires intervention. The authors questioned whether there are appropriate alternatives to the current models that categorise ADHD and instead accurately reflect the reality of the disorder.
- ADHD is a heterogeneous disorder with variation in aetiology and pathophysiology. Studies designed to explore this heterogeneity may be challenging to conduct as they require large sample sizes, multiple measures that capture all deficits and the use of multivariate analytical approaches. However, by doing so, this may lead to precision medicine for ADHD where the individual’s underlying cognitive and brain processes are targeted.
- ADHD is now recognised as a lifespan disorder with at least four developmental trajectories emerging, but how this affects prognosis and treatment responses is not yet understood. Having a developmental approach means that early intervention and preventative strategies could be implicated; however, there is a lack of effective interventions that can be used in the first few years of life, and a lack of understanding of the early cognitive and behavioural precursors for ADHD that would allow for cost-effective early intervention.
- In terms of aetiology and pathophysiology, there is overlap between ADHD and other psychiatric conditions. The authors suggested that using a transdiagnostic framework may enable the shared symptoms of different conditions to be successfully targeted. This may also provide some explanation as to why causal factors can lead to ADHD and other psychiatric conditions.
The authors concluded that ADHD is a common and highly heritable condition associated with impairments. Although there are effective treatments for ADHD, scientific progress in understanding the nature and causes of the disorder challenges the currently accepted models of ADHD and highlights the need for clinical improvement.
Read more about a review of ADHD in light of scientific advances here
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