ADHD

Definition

Attention-Deficit/Hyperactivity Disorder is diagnosed from two symptom lists — inattention and hyperactivity-impulsivity — requiring six symptoms in either or both for children, and five for those aged 17 and over.

Three presentations are specified: predominantly inattentive, predominantly hyperactive-impulsive, and combined. The presentation is not stable over a lifetime; hyperactivity typically becomes less visible while inattention and executive difficulties persist.

The name is unhelpful. The difficulty is less an absence of attention than dysregulated attention: attention that will not go where it is directed and will not leave where it has landed.

History

Descriptions of the pattern go back to at least the early twentieth century. The category has been renamed repeatedly — minimal brain dysfunction, hyperkinetic reaction, attention deficit disorder — and each renaming shifted what was emphasised.

DSM-5 raised the age-of-onset threshold from 7 to 12 and added adult-adjusted criteria, both of which increased recognition in adults.

Theory

Executive function accounts, particularly Barkley's, place behavioural inhibition as the primary deficit, from which working memory, self-regulation of affect and internalised speech difficulties follow.

Delay aversion and reward-sensitivity accounts propose the core difference is in how future rewards are discounted, which explains motivational patterns that a pure inhibition account does not.

Dual pathway models combine both, and fit the heterogeneity better than either alone.

No model explains everyone with the diagnosis, and the group is neurobiologically heterogeneous.

Research

Prevalence around 5 percent in children and around 2.5 percent in adults, though adult figures are rising with recognition rather than incidence.

Heritability is high, commonly estimated around 70 to 80 percent.

Stimulant medication has large short-term effects on core symptoms, among the largest in psychiatry. Long-term functional outcomes are much less clear, and the MTA follow-up found the medication advantage over behavioural treatment diminished over years rather than persisting.

The relative age effect is well replicated: children born just before the school-year cutoff, and therefore youngest in their class, are substantially more likely to be diagnosed. Some of what is diagnosed is immaturity relative to classmates.

Applications

Diagnosis requires evidence of impairment in more than one setting and a developmental history; a rating scale alone is not sufficient.

Treatment is usually medication plus environmental and behavioural adjustment. For adults, accommodation and skills often matter more than symptom reduction.

Under-recognition in girls and women is well documented, and inattentive presentations without disruptive behaviour are missed most.

Criticism

• The relative age effect suggests a share of diagnoses reflect classroom norms rather than a condition.
• Long-term benefit is not established. Strong short-term symptom effects are not the same as better outcomes at ten years, and the evidence for the latter is weak.
• Diagnostic thresholds are conventions on a continuous trait, and where the line sits determines prevalence.
• Under-diagnosis and over-diagnosis coexist in different groups, which is why arguments about which is happening usually talk past each other.
• Adult diagnosis has become commercially entangled, with private services whose business model depends on diagnosing.

References

• Barkley, R. A. (1997). Behavioral inhibition, sustained attention, and executive functions. Psychological Bulletin, 121(1), 65-94.
• Sonuga-Barke, E. J. S. (2003). The dual pathway model of AD/HD. Neuroscience & Biobehavioral Reviews, 27(7), 593-604.
• Molina, B. S. G., et al. (2009). The MTA at 8 years. Journal of the American Academy of Child & Adolescent Psychiatry, 48(5), 484-500.
• Layton, T. J., et al. (2018). Attention deficit-hyperactivity disorder and month of school enrollment. New England Journal of Medicine, 379(22), 2122-2130.