Adhd
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"attention deficit hyperactivity disorder"[MeSH Terms] AND epidemiology
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Attention-Deficit/Hyperactivity Disorder (ADHD) - Overview
Definition
ADHD is a neurodevelopmental disorder characterized by persistent, developmentally inappropriate levels of inattention, hyperactivity, and/or impulsivity that impair functioning across multiple settings. It is one of the most well-researched childhood psychiatric disorders, with several evidence-based treatments available.
The historical terminology has evolved considerably - from "hyperactive syndrome" in the early 1900s, to "minimal brain damage" in the 1960s, to the current DSM-5 classification. Early theories proposed abnormal arousal and reduced ability to modulate emotions, initially supported by the observation that stimulant medications improved sustained attention and focus.
Epidemiology
| Population | Prevalence |
|---|---|
| School-aged children (US, Danielson et al. 2018) | 9.4% |
| Children worldwide (pooled estimate, Thomas et al. 2015) | 7.2% |
| Adults (DSM-5 estimate) | ~2.5% |
| Children (DSM-5 conservative estimate) | ~5% |
| Preterm infants (vs. term) | 2-4x higher rate |
Sex difference: ADHD occurs approximately twice as often in males. When females are affected, they are more likely to present predominantly with inattentive features (rather than hyperactive/impulsive).
Course: Contrary to older thinking that recovery was the rule, ADHD is now recognized as a chronic condition. It persists into adulthood in 60-70% of people diagnosed in childhood (Kaplan & Sadock's data; Kessler et al., 2005). The Kaplan & Sadock's Comprehensive Textbook also notes persistence into adulthood in "roughly 50%" of patients.
DSM-5 Subtypes / Presentations
DSM-5 replaced the old "subtypes" with three specifiers (presentations):
- Combined presentation - meets criteria for both inattention AND hyperactivity-impulsivity
- Predominantly inattentive presentation - meets criteria for inattention only
- Predominantly hyperactive-impulsive presentation - meets criteria for hyperactivity/impulsivity only
These largely map onto the older subtype categories but use "presentation" to reflect that the clinical picture can shift over time.
Symptom Thresholds (DSM-5):
- Children under 17 years: 6 or more symptoms in either (or both) domains
- Adolescents 17+ and adults: only 5 or more symptoms required (reflecting that hyperactivity often decreases with age)
Symptoms must:
- Be present before age 12 years (changed from age 7 in earlier DSM editions)
- Persist for at least 6 months
- Occur in at least two settings (e.g., home and school/work)
- Cause impairment in academic, social, or occupational functioning
- Be excessive for the developmental level
Core Symptom Domains
Inattention symptoms include:
- Short attention span, easy distractibility
- Failure to finish tasks, perseveration
- Difficulty following instructions
- Poor concentration, forgetfulness
Hyperactivity-Impulsivity symptoms include:
- Inability to remain seated
- Excessive talking, always "on the go"
- Acting before thinking, abrupt shifts in activity
- Inability to delay gratification
- Jumping up in class, blurting out answers
The most cited characteristics in order of frequency are: hyperactivity, attention deficits (inattention), impulsivity, memory/thinking deficits, specific learning disabilities, and speech/hearing deficits.
Severity Specifiers (DSM-5)
| Severity | Description |
|---|---|
| Mild | Few or no symptoms beyond diagnostic threshold; minor functional impairment |
| Moderate | Between mild and severe |
| Severe | Many excess symptoms or marked impairment in social/occupational functioning |
Onset and Clinical Course
- Onset can occur in infancy, though it is rarely recognized before toddler age
- School-age presentation is the most common point of recognition
- Children with combined or hyperactive-impulsive presentations are referred for treatment earlier and are more likely to have comorbid conduct disorder
- Children with the inattentive subtype may go undetected longer
- Comorbidities are common: specific learning disorders (reading, arithmetic, writing), communication disorders, mood disorders, anxiety, and developmental coordination disorder
Neurobiology (Brief)
Theories center on dopaminergic and noradrenergic dysregulation in prefrontal circuits governing executive function and inhibitory control. This is supported by the mechanism of action of effective treatments:
- Stimulants (methylphenidate, amphetamines) block dopamine/norepinephrine reuptake
- Non-stimulants (atomoxetine, viloxazine) are selective norepinephrine reuptake inhibitors
- Alpha-2 agonists (clonidine, guanfacine) modulate noradrenergic signaling
Genetics play a significant role - heritability estimates are high, and ADHD rates are 2-4x greater in preterm children, especially the most premature.
Sources: Bradley and Daroff's Neurology in Clinical Practice; Kaplan & Sadock's Comprehensive Textbook of Psychiatry; Kaplan and Sadock's Synopsis of Psychiatry; Goodman & Gilman's Pharmacological Basis of Therapeutics
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