A 7-year-old boy is brought to the child psychiatry clinic by his mother with complaints of inattention, difficulty completing schoolwork, and frequent fidgeting in class for the past 2 years. The teacher reports that he is unable to sit still during lessons and often interrupts other children. On evaluation, he meets DSM-5 criteria for Attention-Deficit/Hyperactivity Disorder. Which is the most common neurobiological abnormality found in children with ADHD?

Explanation

## Neurobiological Basis of ADHD ### Catecholamine Hypothesis **Key Point:** The most widely accepted neurobiological model of ADHD involves dysfunction of the dopaminergic and noradrenergic systems in the prefrontal cortex and anterior cingulate cortex. **High-Yield:** Reduced dopamine (DA) and noradrenaline (NA) availability in the prefrontal cortex leads to: - Impaired executive function (planning, working memory, impulse control) - Reduced sustained attention - Difficulty with response inhibition ### Prefrontal Cortex Dysfunction The prefrontal cortex is critical for: 1. Executive functions 2. Impulse control 3. Working memory 4. Sustained attention DA and NA dysregulation in this region directly explains the core symptoms of ADHD (inattention, hyperactivity, impulsivity). ### Supporting Evidence **Clinical Pearl:** Stimulant medications (methylphenidate, amphetamines) work by increasing DA and NA availability in the prefrontal cortex, which is why they are the first-line pharmacological treatment for ADHD. Non-stimulants like atomoxetine (NA reuptake inhibitor) and guanfacine (α-2 agonist) also target catecholamine systems. ### Table: Neurotransmitter Systems in ADHD | Neurotransmitter | Location | Role in ADHD | Evidence | | --- | --- | --- | --- | | Dopamine | Prefrontal cortex, striatum | Executive function, reward, motivation | Stimulant response | | Noradrenaline | Prefrontal cortex, locus coeruleus | Attention, arousal, impulse control | Non-stimulant efficacy | | Serotonin | Limbic system | Mood, impulse control | Secondary role; not primary | | GABA | Distributed | Inhibitory neurotransmission | Not primary in ADHD pathophysiology | | Acetylcholine | Basal ganglia, cortex | Learning, memory | Not primary in ADHD | **Mnemonic:** **DARN** — **D**opamine, **A**drenaline (noradrenaline), **R**educed in **N**ADHD prefrontal cortex.