ADHD

Key Mechanism 1: Prefrontal Dopaminergic and Noradrenergic Dysfunction

• Reduced synaptic DA and NE availability due to increased catecholamine reuptake via dopamine transporter (DAT) and norepinephrine transporter (NET)
• Altered activity of catecholaminergic neurons originating from the ventral tegmental area (VTA) and locus coeruleus (LC)
• Genetic polymorphisms in genes encoding DAT, catechol-O-methyltransferase (COMT), and the DA D4 receptor (DRD4) that affect catecholamine signaling efficiency
• Dysregulation of post-synaptic dopamine receptors (D1 and D2) and adrenergic receptors in the PFC

Key Mechanism 2: Limbic System Hyperactivity and Impaired Top-Down Inhibition

• Enhanced sensitivity to immediate rewards with temporal discounting (preference for smaller, immediate rewards over larger, delayed rewards)
• Reduced amygdala-to-PFC connectivity, impairing top-down emotional regulation
• Hyperactive ventral striatal responses to salient stimuli, increasing distractibility
• Impaired functioning of the default mode network (DMN), which normally suppresses during task-directed attention, leading to mind-wandering and inattention

Key Mechanism 3: Striatal and Anterior Cingulate Abnormalities

• Reduced activation during error-monitoring tasks (decreased error-related negativity on event-related potentials)
• Impaired ability to adjust behavior following errors
• Reduced white matter integrity in connections between ACC and lateral PFC

Additional Mechanisms

• Neurotransmitter dysregulation beyond catecholamines: Abnormalities in serotonergic, glutamatergic, and GABAergic systems contribute to emotional dysregulation and impulsivity
• Developmental timing: Critical periods for brain development (particularly puberty and late adolescence) explain symptom evolution and gender-specific presentation patterns
• Sleep dysregulation: Altered circadian rhythm signaling and reduced melatonin production contribute to sleep onset insomnia and non-restorative sleep
• Adenosine signaling: Reduced adenosine-mediated sedation may contribute to hyperactivity
• Inflammatory and metabolic factors: Emerging evidence suggests mild neuroinflammation and mitochondrial dysfunction may contribute in some cases

Genetic Factors (Primary Etiology)

• Catecholaminergic gene polymorphisms: DAT1 (SLC6A3) 10-repeat allele, COMT Val158Met polymorphism, DRD4 7-repeat allele, and DRD5 variants influence dopamine signaling efficiency
• Noradrenergic genes: Variants in SLC6A2 (NET gene) and ADRA2A affect norepinephrine function
• Genome-wide association studies (GWAS) have identified additional loci affecting neurotransmitter synthesis and synaptic plasticity genes
• SNAP25, DDC, HTR1B, and other neurodevelopmental genes contribute to disease susceptibility
• Family history: First-degree relatives have 2-8 fold increased risk; approximately 30-40% of children with ADHD have an affected parent

Prenatal & Perinatal Risk Factors

• Maternal substance exposure: Prenatal tobacco, alcohol, and cocaine use increase ADHD risk by up to 2-fold
• Low birth weight and prematurity: <2500g birth weight or <37 weeks gestation associated with increased prevalence
• Maternal infections during pregnancy: Second and third trimester infections may disrupt fetal neurodevelopment
• Nutritional deficiencies: Severe prenatal malnutrition and micronutrient deficiencies (iron, zinc, omega-3 fatty acids)
• Maternal stress and psychopathology: Gestational diabetes, preeclampsia, and untreated maternal psychiatric illness increase risk

Postnatal Environmental Factors

• Lead exposure: Particularly during critical developmental windows (ages 0-3); blood lead levels >10 μg/dL associated with increased ADHD risk
• Early psychosocial adversity: Institutional deprivation, neglect, and chaotic home environment increase risk by 2-3 fold
• Head injury: Significant traumatic brain injury (TBI) during childhood may precipitate ADHD-like symptoms
• Sleep disorders: Untreated obstructive sleep apnea (OSA) and restless leg syndrome (RLS) can mimic or exacerbate ADHD symptoms
• Screen time exposure: Excessive early childhood exposure (>2 hours daily) associated with increased inattention symptoms

Medical Conditions Associated with Secondary ADHD-like Symptoms

• Thyroid disorders: Hypothyroidism and hyperthyroidism can present with attention and behavioral dysregulation
• Seizure disorders: Absence seizures may be misdiagnosed as inattention; anticonvulsants may improve or worsen symptoms
• Sleep-disordered breathing: OSA in children frequently presents with ADHD-like hyperactivity and inattention
• Iron deficiency anemia: Severe deficiency associated with attention problems and restless behavior
• Hearing impairment: Undiagnosed hearing loss leads to inattention and behavioral problems

Cardinal Symptom 1: Inattention (Predominantly in Inattentive or Combined Type)

• Careless mistakes in schoolwork due to failure to pay close attention to details
• Difficulty maintaining focus during conversations, lectures, or reading (child frequently loses track of what was being said)
• Easy distractibility by extraneous stimuli—a hallmark that distinguishes ADHD inattention from intellectual disability or learning disorders
• Frequent misplacing of necessary items (homework, pencils, keys) suggesting impaired working memory and organizational executive function
• Apparent mind-wandering with reduced engagement in the present task
• Poor academic performance despite adequate intellectual ability, with grades lower than IQ would predict

Cardinal Symptom 2: Hyperactivity-Impulsivity (Predominantly in Hyperactive-Impulsive or Combined Type)

• Constant fidgeting with hands or feet, difficulty remaining seated during class or meals
• Excessive talking (children often talk excessively or blurt out answers before questions are completed)
• Running or climbing at inappropriate times in younger children; in adolescents and adults, may present as subjective feeling of restlessness
• Difficulty playing quietly or engaging in silent activities
• Being "on the go" constantly, driven by an internal motor

• Blurting out answers before questions are completed (reflects failure of response inhibition)
• Difficulty waiting turns in games, conversations, or lines
• Interrupting or intruding on others' conversations or activities
• Difficulty inhibiting responses even when given explicit instructions to "think before acting"
• Increased accident proneness due to failure to consider safety consequences
• High-risk behaviors in adolescents (reckless driving, substance experimentation)

Physical Examination Findings

1. 1.Neurological Exam Nonspecific Abnormalities

• Soft neurological signs: Minor abnormalities in motor coordination, rapid alternating movements, or graphesthesia (though these are nonspecific)
• Postural instability or difficulty maintaining steady stance with eyes closed
• Hyperreflexia is occasionally noted but not diagnostic

1. 1.Behavioral Observations During Examination

• Increased motor activity: Child difficulty sitting still during office visit, fidgeting in chair
• Impulsive responses: Answering questions before they are fully asked; inability to wait for instructions
• Reduced eye contact and inattention: Difficulty sustaining attention to examiner-led conversation
• Oppositional or defiant behavior: Some children may demonstrate reduced compliance with requests during exam (suggesting possible comorbid oppositional defiant disorder)

Important Clinical Variants and Context-Dependent Presentations

1. 1.Predominantly Inattentive Type (Most Common, ~50% of Cases)

• Often underrecognized in girls due to less disruptive presentation
• May be misdiagnosed as anxiety or depressive disorder ("spacey," withdrawn behavior)
• Often goes undiagnosed until higher academic demands (middle or high school) unmask deficits
• Frequently comorbid with learning disorders

1. 1.Predominantly Hyperactive-Impulsive Type (10-15% of Cases)

• More commonly recognized in early childhood due to overtly disruptive behavior
• High risk for accidental injury and disciplinary problems
• May meet criteria for oppositional defiant disorder (ODD) concurrently
• Risk of social ostracism by peers

1. 1.Combined Type (Most Functionally Impaired, ~35% of Cases)

• Most common presentation in clinic-referred samples
• Worst academic and social outcomes
• Highest rates of comorbid disorders (oppositional defiant disorder, anxiety, mood disorders)

1. 1.Presentation Variations by Age

• Preschool (Age 3-5): Hyperactivity predominates; inattention difficult to assess; symptoms must cause functional impairment in multiple settings
• School-age (Age 6-11): Full symptom spectrum evident; academic impact becomes apparent; peer relationship problems emerge
• Adolescence: Hyperactivity may manifest as subjective restlessness and fidgeting rather than overt motor activity; risk behaviors emerge
• Adulthood: Inattention, time management problems, and emotional dysregulation predominate; hyperactivity may be subtle

1. 1.Gender-Specific Presentation

• Girls: More likely to have inattentive type; present with emotional dysregulation and anxiety rather than behavioral problems; may "mask" symptoms through social camouflage, leading to underdiagnosis
• Boys: More likely to have combined or hyperactive-impulsive type with overt behavioral symptoms prompting referral

1. 1.Context-Dependent Symptoms

• Symptoms dramatically improve in one-on-one settings with novel activities and immediate rewards (e.g., office visit, one-on-one tutoring)
• Symptoms worsen with fatigue, hunger, or emotionally provocative situations
• Medication effects: Symptoms should improve with stimulant medications if diagnosis is accurate (though symptom improvement with stimulants does not confirm diagnosis)

Diagnostic Approach Overview

Detailed History Elements

1. 1.Symptom-Specific History

• Onset timing: When were symptoms first noticed? (Must be before age 12 for diagnosis; DSM-5 specifies onset before age 12, though full criteria may not be met until later)
• Longitudinal course: Are symptoms present consistently across time or episodic? Have they changed with development?
• Functional impairment: Specific examples of how symptoms impact academics, behavior, social relationships, and daily functioning
• School performance: Current grades relative to ability; teacher reports; presence of learning disorders; need for special services
• Developmental history: Prenatal exposure, birth weight, developmental milestones, early temperament
• Family psychiatric history: Particularly ADHD in parents, siblings, or other first-degree relatives

1. 1.Setting-Specific Symptoms and Context

• School performance: Teacher ratings (Vanderbilt Rating Scale or Conners scales); academic struggles; disciplinary records
• Home behavior: Organization skills, completion of chores, family meal/sleep interactions
• Social functioning: Peer relationships, participation in structured activities, social skills quality
• Medication trials: Previous stimulant or non-stimulant trials with dose, duration, and response

1. 1.Red Flags and Alternative/Comorbid Diagnoses to Assess

• Mood symptoms: Irritability, sadness, anxiety (assessing for mood disorder)
• Oppositional patterns: Defiance, arguing, vindictiveness (ODD symptoms)
• Sleep history: Sleep onset insomnia, sleep-disordered breathing, restless sleep
• Medical history: Seizures, head injury, hearing impairment, thyroid disease

Physical Examination

• General: Growth parameters (height/weight for stimulant baseline; growth monitoring during treatment)
• Cardiovascular: Blood pressure and heart rate (baseline for stimulant therapy); cardiac history
• Neurological: Cranial nerves, motor strength, coordination, reflexes, gait (to rule out neurological disorders)
• Behavioral observations: Motor activity level, impulse control, ability to follow commands during exam
• Vision and hearing screening: To rule out sensory deficits mimicking inattention

Rating Scales and Questionnaires (Highly Recommended for Diagnosis)

1. 1.Vanderbilt ADHD Diagnostic Rating Scale (VADRS)

• Sensitivity 90%, specificity 72% for ADHD
• 9-item inattention subscale and 9-item hyperactivity-impulsivity subscale
• Scores ≥6 items (with symptom intensity ≥3 on 0-3 scale) in either domain suggest ADHD
• Completed by teachers and parents separately; mismatch between settings important
• Administration: Takes 5-10 minutes; available in Spanish

1. 1.Conners Rating Scale (3rd Edition)

• Parent and teacher versions with good psychometric properties
• Provides T-scores comparing to age-standardized norms

Age-stratified first-line therapy (AAP 2019 Clinical Practice Guideline for ADHD in Children and Adolescents)

• Ages 4–5 (preschool): Parent training in behavior management (PTBM) and/or behavioral classroom intervention first. Medication is added only if behavior therapy fails and impairment persists — methylphenidate is the preferred agent at this age (best evidence base in preschoolers).
• Ages 6–11: FDA-approved medication plus PTBM/behavioral classroom intervention; the combination outperforms either alone. School supports under a 504 plan or IEP (IDEA) are part of the plan.
• Ages 12–18: FDA-approved medication with the adolescent's assent, plus behavioral/organizational-skills training.

Drug classes in order

• Stimulants (first-line): methylphenidate or amphetamine (e.g., lisdexamfetamine, mixed amphetamine salts). Methylphenidate blocks DAT/NET reuptake; amphetamines additionally displace vesicular catecholamines via VMAT2 and reverse transporter flow — both raise prefrontal DA/NE tone. Effect is same-day; titrate to symptom control weighed against appetite and sleep.
• Non-stimulants (second-line, or first if tics, substance-misuse/diversion risk, or intolerable stimulant effects):
• Selective norepinephrine reuptake inhibitor: atomoxetine (also viloxazine ER) — takes several weeks for full effect.
• Alpha-2A adrenergic agonists: guanfacine ER or clonidine ER, FDA-approved as monotherapy and as stimulant adjuncts; useful with tics, aggression, or sleep-onset insomnia.
• Bupropion and tricyclics are off-label alternatives.

Contraindicated or to avoid

• MAOIs within 14 days of a stimulant — hypertensive crisis.
• Stimulants in symptomatic structural/arrhythmogenic cardiac disease or untreated hyperthyroidism; amphetamines in narrow-angle glaucoma.
• Routine screening ECG or neuroimaging is not recommended before starting stimulants (AAP/AHA) — obtain an ECG or cardiology input only for a concerning cardiac history, exam, or family history of sudden death.

Complications of untreated/undertreated ADHD

• Academic underachievement and grade retention: executive-function failure, not low IQ; signal is grades far below measured ability.
• Injury and motor vehicle crashes: impulsivity plus failure of response inhibition; adolescents show excess traffic citations and crashes — a major driver of the recommendation to continue treatment through driving years.
• Substance use disorder: mediated largely by comorbid conduct disorder and self-medication; notably, adequately treated ADHD does not increase — and may reduce — later SUD risk.
• Comorbid psychiatric illness: ODD/conduct disorder, anxiety, depression, learning disorders; new-onset persistent sadness or self-harm statements warrants urgent psychiatric evaluation.
• Social rejection and low self-esteem: from intrusive, impulsive peer interactions.

Stimulant adverse effects

• Appetite suppression and weight loss: central catecholamine-mediated anorexia; signal is downward crossing of weight percentiles. Attenuated height velocity may occur with sustained use — monitor growth at every visit; consider dose reduction, medication holidays, or calorie-dense meals off-peak.
• Sleep-onset insomnia: dose too late in the day; shift the last dose earlier or switch formulation.
• Increased heart rate and blood pressure: modest and dose-dependent; check vitals each visit. Sudden cardiac death is exceedingly rare but is why symptomatic cardiac disease is screened for by history.
• Tic exacerbation: dopaminergic; tics are usually not a reason to stop — alpha-2 agonists treat both.
• Stimulant-induced psychosis or mania: new hallucinations (classically tactile — formication) or grandiosity — stop the drug; this is an emergency-level evaluation.
• Misuse and diversion: especially immediate-release forms in adolescents; prodrug/extended-release formulations mitigate.
• Priapism (reported with methylphenidate and atomoxetine): a urologic emergency.

Non-stimulant adverse effects

• Atomoxetine: black-box warning for suicidal ideation in children/adolescents; rare hepatotoxicity — jaundice, dark urine, or rising transaminases mandates discontinuation.
• Alpha-2 agonists: sedation, hypotension, bradycardia; abrupt clonidine withdrawal causes rebound hypertension — taper.

• The single best next step in an undiagnosed child is to obtain standardized rating scales from at least two settings (e.g., Vanderbilt from parent and teacher) — not neuroimaging, EEG, or continuous performance testing. DSM-5-TR requires symptoms before age 12, in ≥2 settings, for ≥6 months, with functional impairment.
• Preschooler (age 4–5) with hyperactivity → parent training in behavior management first, medication only if behavior therapy fails (AAP 2019). Reflexively starting a stimulant is the classic distractor.
• Rule out the mimics before labeling ADHD: obstructive sleep apnea (snoring, tonsillar hypertrophy, restless sleep), absence seizures (abrupt staring with eyelid flutter, no postictal state, hyperventilation-provoked, 3-Hz generalized spike-and-wave on EEG), hearing loss, lead exposure, iron deficiency, and thyroid disease. Staring spells that end abruptly and cannot be interrupted point to absence, not inattention.
• Mechanism examiners love: methylphenidate blocks DAT/NET reuptake, while amphetamines also drive vesicular release via VMAT2 and reverse transporters — both increase prefrontal dopamine/norepinephrine. Stimulants work the same day; atomoxetine takes weeks — do not call it a treatment failure at one week.
• Growth and vitals are the monitoring answer: height, weight, heart rate, and blood pressure at every follow-up. A child crossing weight percentiles downward on a stimulant is expected teaching, not a reason for a metabolic work-up.
• Comorbid tics or Tourette syndrome: not an absolute contraindication to stimulants; an alpha-2A agonist (guanfacine ER) elegantly treats both.
• Substance-misuse or diversion risk in an adolescent → choose a non-stimulant (atomoxetine) or a prodrug/extended-release stimulant such as lisdexamfetamine.
• Hard contraindication to remember: a stimulant within 14 days of an MAOI → hypertensive crisis.
• Common distractor: routine screening ECG before stimulants — not recommended (AAP/AHA) absent cardiac symptoms, abnormal exam, or family history of sudden death.