Major Depressive Disorder

Major Depressive Disorder (MDD) is a common, serious, and potentially life-threatening mood disorder characterised by one or more major depressive episodes - discrete periods of at least 2 weeks of depressed mood and/or anhedonia accompanied by cognitive, neurovegetative, and behavioural symptoms. It is the leading cause of disability worldwide (WHO) and a major contributor to global disease burden. MDD affects approximately 10-15% of adults over their lifetime, with a 12-month prevalence of ~7%. References: DSM-5-TR, StatPearls (NCBI Bookshelf).

Major Depressive Disorder (MDD) is a common, serious, and potentially life-threatening mood disorder characterised by one or more major depressive episodes - discrete periods of at least 2 weeks of depressed mood and/or anhedonia accompanied by cognitive, neurovegetative, and behavioural symptoms. It is the leading cause of disability worldwide (WHO) and a major contributor to global disease burden. MDD affects approximately 10-15% of adults over their lifetime, with a 12-month prevalence of ~7%. It is classified under Depressive Disorders in DSM-5-TR and requires absence of prior manic or hypomanic episodes (distinguishing it from bipolar disorder). MDD is a recurrent illness in most individuals. This document is for psychoeducation only.

A major depressive episode requires >=5 of the following 9 symptoms, present nearly every day for >=2 weeks, with at least one being depressed mood or anhedonia:

• Depressed mood most of the day (in children/adolescents, may be irritable).
• Markedly diminished interest or pleasure in almost all activities (anhedonia).
• Significant weight or appetite change (loss or gain).
• Insomnia or hypersomnia.
• Psychomotor agitation or retardation (observable by others).
• Fatigue or loss of energy.
• Feelings of worthlessness or excessive or inappropriate guilt.
• Diminished concentration or indecisiveness.
• Recurrent thoughts of death, suicidal ideation, or a suicide attempt.

Additional features: anxiety, irritability, tearfulness, somatic complaints, social withdrawal. Psychotic features (mood-congruent delusions of worthlessness, guilt, disease, or ruin; hallucinations) may occur in severe episodes.

Biological: female sex (~2x risk after puberty); personal/family history of MDD or bipolar disorder; chronic medical illness (cardiovascular disease, diabetes, cancer, hypothyroidism, chronic pain); postpartum and perimenopausal hormonal changes. Genetic: heritability ~37-50% (twin studies); polygenic; serotonin transporter gene (5-HTTLPR) interaction with stress is historically cited; GWAS have identified multiple risk loci. Psychological: history of anxiety disorders (strongest predictor of later MDD); neuroticism; pessimistic attributional style; low self-esteem; history of trauma or childhood abuse (ACEs). Social/environmental: stressful life events (especially loss: bereavement, job loss, divorce); chronic social stressors; poverty; social isolation; lack of social support.

Monoamine hypothesis: reduced availability of serotonin (5-HT), norepinephrine (NE), and dopamine (DA). Though foundational and pharmacologically supported, this is an oversimplification. HPA axis dysregulation: hypercortisolism, cortisol non-suppression on DST, reduced negative feedback. Glucocorticoid excess may contribute to hippocampal volume loss. Neuroplasticity/BDNF: BDNF is reduced in MDD, impairing hippocampal and prefrontal neurogenesis. Antidepressants and exercise increase BDNF expression. Inflammatory hypothesis: elevated pro-inflammatory cytokines (IL-1beta, IL-6, TNF-alpha, CRP) present in ~30% of patients, contributing via kynurenine pathway serotonin depletion. Anti-inflammatory treatments may benefit inflamed-depression subtype. Glutamate/ketamine: ketamine's rapid antidepressant effect (via NMDA blockade and AMPA potentiation) has shifted understanding toward synaptic plasticity and neuroplasticity mechanisms, leading to FDA approval of esketamine (Spravato) for treatment-resistant depression.

Global: WHO estimates ~280 million people affected; 12-month prevalence ~7%; lifetime ~15-20%. USA: ~21 million adults (8.4%) experienced >=1 major depressive episode in 2020 (NIMH). Female:male ratio ~2:1; gender difference emerges at puberty. Median age of onset ~32 years; MDD occurs across all ages including children (~3%) and adolescents (~8%). Recurrence: after first episode 50-60% will have second; after two episodes 70-80%; after three episodes >90% risk of another. Comorbidity is common: ~60% with anxiety disorders, ~30% with substance use, and frequent medical comorbidities (cardiovascular disease, diabetes, cancer, chronic pain).

Structural changes: hippocampal volume reduction (proportional to episode duration and number), prefrontal cortex thinning, and changes in anterior cingulate cortex and amygdala; partly reversible with treatment. Neurotransmitter systems: serotonin, norepinephrine, and dopamine are all implicated. BDNF-TrkB signalling: reduced BDNF impairs synaptic plasticity and neurogenesis; antidepressants restore BDNF. Glutamate system: NMDA receptor dysfunction contributes; ketamine rapidly restores synaptic density via AMPA, BDNF, and mTOR signalling. Inflammatory pathways: elevated cytokines activate IDO (indoleamine 2,3-dioxygenase), shunting tryptophan away from serotonin synthesis toward kynurenine. HPA hyperactivation: chronic cortisol elevation causes excitotoxic hippocampal damage. Network-level: dysfunction in corticolimbic (PFC-amygdala-hippocampus) circuits and disrupted default mode network connectivity. Per StatPearls/NCBI, MDD reflects measurable neurobiological changes; treatment targets these mechanisms and can promote recovery.

DSM-5-TR criteria: A. >=5 of 9 symptoms for >=2 weeks representing change from baseline (must include depressed mood and/or anhedonia). B. Significant distress or impairment. C. Not attributable to substances or medical condition. D. Not better explained by psychotic spectrum disorder. E. No lifetime manic or hypomanic episode. Specifiers: Single episode or Recurrent; Severity (Mild/Moderate/Severe); With anxious distress; With mixed features; With melancholic features (anhedonia, morning worsening, early AM awakening, psychomotor changes, excessive guilt); With atypical features (mood reactivity, hypersomnia, leaden paralysis, rejection sensitivity); With mood-congruent or incongruent psychotic features; With catatonia; With peripartum onset; With seasonal pattern (SAD). ICD-11: Single episode depressive disorder (6A70) or Recurrent depressive disorder (6A71); graded by severity and presence/absence of psychotic symptoms.

SCID-5 or MINI for structured diagnosis. PHQ-9: sensitivity/specificity ~88% at cutoff >=10; tracks treatment response over time. MADRS: 10-item clinician-rated scale widely used in antidepressant trials. HAM-D-17: the classic research instrument. MDQ/HCL-32: screen for bipolar before antidepressant prescribing. Columbia Suicide Severity Rating Scale (C-SSRS): standardised suicide risk assessment. Cognitive assessment (MoCA, MMSE) when cognitive impairment is prominent in older adults. Labs: TSH, FBC, metabolic panel, B12/folate, iron studies; consider CRP/IL-6 for inflammatory subtype identification.

Bipolar disorder: critical - antidepressant monotherapy can precipitate mania; screen carefully for hypomanic/manic history, family history, mixed features. Persistent depressive disorder: milder but chronic (>=2 years); can co-occur (double depression). Grief/bereavement: normal grief lacks pervasive worthlessness, psychomotor disturbance, suicidality. Anxiety disorders: frequently comorbid; primary anxiety with secondary depression vs primary MDD. Hypothyroidism: fatigue, weight gain, cognitive slowing, depression-like symptoms; TSH diagnostic. Substance-induced depression: alcohol, opioids, cannabis, stimulant withdrawal; abstinence for >=4 weeks allows reassessment. Dementia: depression causes pseudodementia in older adults; neuropsychological testing differentiates. PTSD: overlaps in low mood, sleep disturbance, anhedonia; PTSD requires specific trauma exposure and symptom clusters. Schizoaffective disorder: psychotic symptoms persist outside mood episodes.

Treatment of Major Depressive Disorder (MDD) is guided by severity, patient preference, and clinical features. A combination of pharmacotherapy and psychotherapy is often most effective. Psychotherapy: CBT and Interpersonal Psychotherapy (IPT) have the strongest evidence base. CBT targets negative automatic thoughts, cognitive distortions, and behavioural withdrawal. IPT addresses interpersonal issues (grief, role transitions, interpersonal disputes, social isolation) that contribute to and maintain depression. Behavioural Activation focuses on re-engaging with rewarding activities. Mindfulness-Based Cognitive Therapy (MBCT) is effective for relapse prevention. Psychodynamic psychotherapy may be appropriate for patients with long-standing interpersonal patterns. For mild-to-moderate depression, psychotherapy alone may be sufficient. Pharmacotherapy: SSRIs (sertraline, escitalopram, fluoxetine) are first-line due to their efficacy, safety, and tolerability. SNRIs (venlafaxine, duloxetine) are alternatives. Other options include bupropion (useful when fatigue, weight gain, or sexual dysfunction are concerns), mirtazapine (helpful for insomnia and appetite loss), and atypical antidepressants (vortioxetine, vilazodone). Tricyclic antidepressants and MAOIs are effective but have more side effects and are typically reserved for treatment-resistant cases. An adequate trial is 4-8 weeks at therapeutic dose. Augmentation strategies for partial response include adding lithium, atypical antipsychotics (aripiprazole, quetiapine), or thyroid hormone. Treatment-Resistant Depression: Options include switching antidepressants, combining antidepressants from different classes, augmentation strategies, esketamine (intranasal, FDA-approved for treatment-resistant depression), electroconvulsive therapy (ECT, highly effective for severe or treatment-resistant depression), transcranial magnetic

stimulation (TMS), and vagus nerve stimulation. Maintenance: After remission, continuation treatment for 6-12 months reduces relapse risk. Patients with recurrent episodes may need long-term or indefinite maintenance treatment. Gradual tapering of antidepressants is recommended to avoid discontinuation syndrome.

50-60% respond to first antidepressant (>=50% symptom reduction); only 30-40% achieve full remission on first medication. STAR*D trial: cumulative remission rate ~67% after 4 sequential treatment steps; relapse common even after remission. Recurrence is the norm: each episode increases risk of subsequent episodes. Residual symptoms (especially cognitive impairment) predict early relapse. Functional recovery often lags symptom recovery. Chronic non-remitting course in 15-20%. Suicide: MDD is the most common psychiatric diagnosis in completed suicide; lifetime risk ~2-8%. Early identification, adequate treatment, and maintenance strategies significantly improve long-term outcomes.

Suicide and self-harm: MDD carries 20-fold increased suicide risk vs general population. Social/occupational impairment: relationship breakdown, marital instability, divorce, work disability, unemployment. Medical comorbidity progression: MDD worsens cardiac prognosis (2-4x post-MI mortality), diabetes, stroke, and cancer outcomes; impairs immune function. Cognitive impairment: depression-associated cognitive dysfunction may persist even in remission and may accelerate dementia. Substance use: self-medication with alcohol/drugs complicates treatment and worsens prognosis. Untreated episodes may worsen neurobiology (allostatic damage hypothesis), increasing episode frequency and severity over time.

Prevention: exercise has strongest evidence for prevention of first and subsequent episodes; resilience-building programs; early treatment of anxiety (which precedes MDD in many cases). Relapse prevention: continuation and maintenance pharmacotherapy; MBCT (40% relapse reduction in recurrent MDD); psychoeducation about warning signs. Patient education: MDD is a medical illness, not a character flaw or weakness. Antidepressants are not addictive. Full treatment courses (minimum 6-9 months after remission) are necessary to prevent relapse. Exercise, sleep hygiene, social engagement, and stress management are important adjuncts. Suicidal thoughts are a medical emergency - encourage patients to contact their provider or emergency services immediately.

Ancient: Hippocrates described 'melancholia' (excess black bile) as a distinct illness. 2nd century CE: Aretaeus linked mania and melancholia as phases of one disease. 19th century: Emil Kraepelin systematised manic-depressive insanity vs dementia praecox. 1952: iproniazid (MAOI) antidepressant effect discovered accidentally in TB treatment. 1957: imipramine (first TCA) identified. Monoamine hypothesis of depression formulated in the 1960s. 1987: first SSRI (fluoxetine/Prozac) approved, transforming treatment. DSM-III (1980): MDD formalised as discrete diagnostic category. DSM-5 (2013): bereavement exclusion removed; dimensional specifiers added.

Despite MDD being the most common serious mental illness, stigma remains a major barrier to treatment-seeking. Only ~50% of people with MDD receive any treatment in high-income countries; <10% in low-income countries (WHO treatment gap). Celebrity advocacy and public awareness campaigns have increased help-seeking. The 'antidepressant controversy' (including the 2004 FDA black box warning on suicidality in under-18s) created significant barriers; updated guidance emphasises monitoring rather than avoidance. The 2018 Cipriani Lancet meta-analysis (21 antidepressants vs placebo) definitively confirmed efficacy of antidepressants, addressing longstanding media-driven scepticism.

Ketamine/esketamine: mechanism (NMDA antagonism, AMPA potentiation, BDNF, mTOR, synaptogenesis) and optimisation of dosing protocols for TRD. Psilocybin: FDA Breakthrough Therapy designation for TRD and MDD; Phase II trials show large effect sizes with durable remission. Inflammation: anti-inflammatory strategies (celecoxib, IL-6 inhibitors) for inflamed-depression subtype (~30% of MDD). Pharmacogenomics: CYP2D6/CYP2C19 testing for antidepressant metabolism; combinatorial pharmacogenomic decision support (GeneSight). Neuroimaging biomarkers: brain connectivity subtypes to predict antidepressant vs psychotherapy response. Digital therapeutics and AI: CBT-based apps, AI-assisted depression monitoring. Peri-partum depression: brexanolone (FDA-approved 2019 for post-partum depression) and zuranolone.

APA. (2022). DSM-5-TR. APA Publishing. StatPearls. MDD. NCBI Bookshelf NBK559078.

StatPearls: Major Depressive Disorder (NCBI Bookshelf)

NIMH: Depression

WHO: Depression fact sheet

Crisis resources: 988 (US Suicide & Crisis Lifeline); Crisis Text Line: text HOME to 741741.