Major Depressive Disorder (MDD) – High-Yield Study Guide for Medical Students

Definition

Major Depressive Disorder (MDD) is a common mood disorder characterized by one or more major depressive episodes, each lasting at least two weeks, involving depressed mood and/or anhedonia plus associated cognitive, somatic, and neurovegetative symptoms, resulting in clinically significant distress or functional impairment. Diagnostic criteria are defined by the Diagnostic and Statistical Manual of Mental Disorders (DSM-5).

Epidemiology

MDD is a highly prevalent psychiatric disorder with substantial individual and public health impact.

Lifetime prevalence: approximately 10–20% overall, with higher rates in women than men.
Age of onset: bimodal peaks in late adolescence/early adulthood and again in later life; onset can occur at any age.
Gender differences: women are affected about twice as often as men, likely due to a combination of biological, psychological, and sociocultural factors.
Course: often recurrent; many patients experience multiple depressive episodes across the lifespan.
Burden: a leading cause of disability worldwide and strongly associated with increased morbidity, mortality, and suicide risk.

Pathophysiology

The pathophysiology of MDD is multifactorial and incompletely understood. Current models emphasize interactions between genetic vulnerability, neurobiological changes, and environmental stressors.

Genetic factors: heritability is estimated at ~35–40%. First-degree relatives of individuals with MDD have a 2–3-fold increased risk. Multiple susceptibility genes with small effect sizes are implicated.
Monoamine hypothesis: dysregulation of central serotonin (5-HT), norepinephrine (NE), and dopamine (DA) pathways contributes to mood, reward, and cognitive symptoms. Many antidepressants target these systems.
Neuroplasticity and neurotrophic factors: decreased brain-derived neurotrophic factor (BDNF) and impaired synaptic plasticity, particularly in the hippocampus and prefrontal cortex, are associated with MDD; effective treatment appears to normalize some of these changes.
Neural circuitry: altered activity and connectivity in limbic and prefrontal networks, including the amygdala, hippocampus, anterior cingulate cortex, and dorsolateral prefrontal cortex. These changes are linked to impaired emotional regulation, negative bias, and rumination.
HPA axis dysregulation: chronic stress and elevated glucocorticoids can lead to hyperactivity of the hypothalamic–pituitary–adrenal (HPA) axis and structural changes (e.g., hippocampal volume loss).
Inflammation and immune factors: elevated inflammatory markers (e.g., CRP, IL-6) are associated with MDD in some patients; pro-inflammatory states may affect neurotransmission and neuroplasticity.
Psychosocial factors: adverse childhood experiences, chronic stress, interpersonal conflict, and loss events act as potent environmental triggers in vulnerable individuals.

Clinical Presentation

A major depressive episode is defined by a constellation of affective, cognitive, and somatic symptoms persisting for at least two weeks, present most of the day, nearly every day, and causing significant impairment.

Core Symptoms

Depressed mood: subjective sadness, emptiness, or hopelessness; in children and adolescents, this may present as irritable mood.
Anhedonia: markedly diminished interest or pleasure in nearly all activities.

Associated Symptoms (DSM-5 Criteria)

Five or more of the following, including at least depressed mood or anhedonia, during the same 2-week period:

Depressed mood.
Diminished interest or pleasure in most activities.
Significant weight loss or gain, or change in appetite (decreased or increased).
Insomnia (initial, middle, or terminal) or hypersomnia.
Psychomotor agitation or retardation observable by others.
Fatigue or loss of energy.
Feelings of worthlessness or excessive/inappropriate guilt.
Diminished ability to think or concentrate, or indecisiveness.
Recurrent thoughts of death, suicidal ideation, suicide plan, or suicide attempt.

Specifiers and Subtypes

With anxious distress: prominent anxiety symptoms, associated with higher suicide risk and poorer outcomes.
With melancholic features: profound anhedonia, early morning awakening, significant weight loss, excessive guilt, psychomotor changes.
With atypical features: mood reactivity, hypersomnia, hyperphagia, leaden paralysis, long-standing interpersonal rejection sensitivity.
With psychotic features: mood-congruent or mood-incongruent delusions and/or hallucinations in the context of severe depression.
With peripartum onset: onset during pregnancy or within 4 weeks postpartum (often clinically extended to 12 months postpartum).
With seasonal pattern: regular temporal relationship between depressive episodes and a particular time of year (classically fall/winter).

Functional Impact

Impaired academic, occupational, and social functioning.
Decreased self-care and adherence to medical treatments.
Increased risk of substance use as a maladaptive coping strategy.

Differential Diagnosis

Consider and systematically rule out the following:

Bipolar disorder: history of manic or hypomanic episodes; essential to exclude before starting antidepressant monotherapy.
Persistent depressive disorder (dysthymia): chronic depressed mood for ≥2 years (adults) with fewer symptoms and no symptom-free interval >2 months.
Adjustment disorder with depressed mood: depressive symptoms in response to an identifiable stressor, not meeting full MDD criteria and typically resolving within 6 months after the stressor ends.
Bereavement/grief: typically features waves of sadness tied to reminders of the deceased, preserved self-esteem, and a more reactive mood; can coexist with MDD.
Substance/medication-induced depressive disorder: due to substances such as alcohol, sedatives, corticosteroids, interferon, or other medications.
Depressive disorder due to another medical condition: hypothyroidism, anemia, neurologic disease (e.g., Parkinson disease, stroke), malignancy, chronic infections, autoimmune disease.
Other psychiatric conditions: anxiety disorders, obsessive-compulsive disorder, personality disorders, PTSD often present with overlapping symptoms.

Diagnosis

MDD is diagnosed clinically using DSM-5 criteria, informed by a thorough psychiatric and medical assessment.

DSM-5 Diagnostic Criteria for a Major Depressive Episode

Criterion A: Five or more of the specified symptoms (including depressed mood or anhedonia) present for at least 2 weeks, representing a change from previous functioning, causing clinically significant distress or impairment.
Criterion B: Symptoms are not attributable to the physiological effects of a substance or another medical condition.
Criterion C: The episode is not better explained by schizoaffective disorder, schizophrenia, schizophreniform disorder, delusional disorder, or other specified/unspecified schizophrenia spectrum and psychotic disorders.
Criterion D: There has never been a manic or hypomanic episode (unless substance- or medically induced).

Assessment Components

History: detailed psychiatric history (onset, duration, course, previous episodes and treatments), medical history, substance use, family psychiatric history, trauma and psychosocial stressors.
Mental status examination: affect, thought content (especially suicidality), thought process, cognition, psychomotor activity, insight, and judgment.
Suicide risk assessment: evaluate ideation, intent, plan, means, prior attempts, and protective factors.
Screening tools: use validated scales such as the Patient Health Questionnaire-9 (PHQ-9) or Hamilton Depression Rating Scale (HAM-D) to quantify severity and monitor treatment response.
Physical examination: focused exam to evaluate for medical contributors, including neurologic and endocrine signs.
Laboratory tests (as clinically indicated): CBC, CMP, thyroid function tests, vitamin B12/folate, pregnancy test, and others guided by history (e.g., HIV, syphilis, toxicology screen).

Management

Management of MDD integrates psychoeducation, pharmacotherapy, psychotherapy, and when indicated, neuromodulation or other somatic treatments. Treatment choice is guided by symptom severity, patient preference, prior response, comorbidities, and suicide risk.

General Principles

Psychoeducation: explain diagnosis, expected course, and treatment options; emphasize that MDD is a treatable medical condition.
Safety planning: address suicidal ideation, involve supports when appropriate, and develop crisis plans.
Collaborative decision-making: consider patient values, preferences, and prior treatment experiences.
Functional goals: target restoration of functioning, not just symptom reduction.

Pharmacotherapy

Antidepressants are first-line for moderate to severe MDD and may be considered for mild forms with significant impairment or failure of psychotherapy alone.

Selective Serotonin Reuptake Inhibitors (SSRIs) – First-Line

Examples: sertraline, fluoxetine, escitalopram, citalopram, paroxetine, fluvoxamine.
Mechanism: inhibit presynaptic serotonin reuptake, increasing synaptic 5-HT.
Advantages: favorable safety profile, relatively benign overdose risk, broad indication spectrum (including anxiety disorders).
Common adverse effects: GI upset, headache, sexual dysfunction, insomnia or sedation, initial activation/anxiety. Monitor for hyponatremia (SIADH), especially in older adults.

Serotonin–Norepinephrine Reuptake Inhibitors (SNRIs)

Examples: venlafaxine, desvenlafaxine, duloxetine.
Indications: useful when comorbid neuropathic pain, fibromyalgia, or when SSRI response is inadequate.
Adverse effects: similar to SSRIs plus potential blood pressure elevation, particularly at higher doses.

Atypical Antidepressants

Bupropion: norepinephrine–dopamine reuptake inhibitor; advantageous for low energy, hypersomnia, smoking cessation; relatively low sexual side-effect burden; avoid in patients with seizure disorders or eating disorders.
Mirtazapine: noradrenergic and specific serotonergic antidepressant; causes sedation and weight gain; useful in underweight or insomnia-predominant patients.

Tricyclic Antidepressants (TCAs) and Monoamine Oxidase Inhibitors (MAOIs)

Generally reserved for treatment-resistant cases due to side-effect burden and safety concerns in overdose.
TCAs: anticholinergic, antihistaminergic, and cardiotoxic effects; require caution in patients with cardiac disease and suicidal risk.
MAOIs: risk of hypertensive crisis with tyramine-rich foods and interactions with other serotonergic drugs; used mainly in refractory atypical depression or specific subtypes.

Dosing and Duration

Initiation: start at a low to moderate dose, titrate over 1–2 weeks based on tolerability and response.
Onset of action: partial response often emerges within 2–4 weeks; full response may take 6–8 weeks or longer.
Continuation treatment: continue the effective dose for at least 6–12 months after remission to reduce relapse risk.
Maintenance therapy: consider long-term treatment in patients with recurrent episodes, chronic depression, or high relapse risk.
Switching and augmentation: for partial or non-response, consider dose optimization, switching antidepressants, or augmenting with agents such as lithium, atypical antipsychotics, or thyroid hormone, individualized to patient profile.

Psychotherapy

Psychotherapy is a core component of treatment and can be first-line for mild to moderate MDD or combined with medication for moderate to severe cases.

Cognitive Behavioral Therapy (CBT): focuses on identifying and modifying maladaptive thoughts and behaviors; robust evidence base and widely used.
Interpersonal Therapy (IPT): targets interpersonal issues (e.g., grief, role transitions, interpersonal disputes) contributing to depression.
Behavioral Activation: emphasizes increasing engagement in rewarding and meaningful activities to counteract withdrawal and inactivity.
Psychodynamic therapies: explore underlying conflicts and relational patterns; may be particularly useful in chronic or recurrent depression.

Somatic and Neuromodulation Treatments

Electroconvulsive Therapy (ECT): highly effective for severe, psychotic, or treatment-resistant depression, and when rapid response is required (e.g., high suicide risk, refusal to eat/drink). Typically administered 2–3 times per week over several weeks.
Repetitive Transcranial Magnetic Stimulation (rTMS): noninvasive focal brain stimulation; indicated for treatment-resistant depression; generally well tolerated.
Ketamine and esketamine: NMDA receptor antagonists with rapid antidepressant effects in treatment-resistant depression; administered intravenously (ketamine) or intranasally (esketamine) under controlled conditions.
Vagus nerve stimulation and deep brain stimulation: options in highly treatment-resistant cases, typically in specialized centers.

Lifestyle and Adjunctive Interventions

Sleep hygiene: structured sleep schedules and behavioral interventions for insomnia or hypersomnia.
Exercise: regular aerobic physical activity has antidepressant and anxiolytic effects and is recommended as an adjunctive treatment.
Nutrition: balanced diet; address deficiencies such as vitamin D, B12, and folate if present.
Substance use: counsel on reducing or abstaining from alcohol and other substances that may worsen mood or interact with medications.
Social support: encourage engagement with supportive relationships, peer groups, and community resources.

Prognosis and Course

The course of MDD is variable, ranging from single episodes with full remission to chronic, recurrent illness.

Many patients experience recurrent episodes; risk of recurrence increases with each episode.
Residual symptoms after apparent remission predict higher relapse rates.
Comorbid anxiety, substance use, personality disorders, and chronic medical conditions are associated with poorer outcomes.
Timely diagnosis, adequate treatment intensity and duration, and strong therapeutic alliance improve long-term prognosis.

Key Clinical Pearls for Medical Students

Always screen for suicidality: ask directly about suicidal thoughts, plans, and prior attempts; do not rely solely on patient spontaneity.
Rule out bipolar disorder: carefully assess for past manic or hypomanic episodes before initiating antidepressant monotherapy to avoid antidepressant-induced mood switching.
Think medically: evaluate for underlying general medical conditions (e.g., hypothyroidism, anemia, neurologic disease) and review medications that may contribute to depressive symptoms.
Set expectations: explain that antidepressants typically take several weeks to show benefit and that adherence is critical.
Monitor early: schedule follow-up within a few weeks of initiating or changing treatment to assess response, side effects, and safety.
Combination therapy is common: moderate to severe depression often responds best to a combination of pharmacotherapy and psychotherapy.
Functional recovery matters: aim for full remission and restoration of functioning, not just partial symptom improvement.
Address stigma: normalize depression as a medical condition; psychosocial support and therapeutic alliance are powerful components of care.
Use rating scales: tools like the PHQ-9 can guide diagnosis, gauge severity, and track treatment progress over time.
Consider maintenance therapy: long-term antidepressant treatment can substantially reduce relapse risk in patients with recurrent or severe depression.

Exam Tips and High-Yield Points

Diagnostic hallmark: ≥2 weeks of depressed mood and/or anhedonia plus associated symptoms causing functional impairment.
First-line pharmacologic treatment: SSRIs (e.g., sertraline, fluoxetine, escitalopram) for most patients without contraindications.
First-line psychotherapy: CBT and IPT have strong evidence, alone or combined with medication.
Treatment-resistant or severe cases: consider ECT, rTMS, or ketamine/esketamine, particularly when suicidality or psychotic features are present.
Specify subtypes: melancholic, atypical, psychotic, peripartum, and seasonal patterns appear frequently in exam questions and affect treatment choices.
Duration of therapy: continue antidepressants for at least 6–12 months after remission for a first episode; longer for recurrent episodes.

Summary

Major Depressive Disorder is a highly prevalent and potentially disabling mood disorder characterized by episodes of depressed mood and/or anhedonia with associated cognitive and somatic symptoms. Accurate diagnosis requires careful assessment using DSM-5 criteria and exclusion of bipolar disorder, medical causes, and substance effects. Effective management is typically multimodal, combining pharmacotherapy, psychotherapy, and lifestyle interventions, with neuromodulation reserved for severe or refractory cases. Early recognition, adequate treatment duration, and close monitoring significantly improve long-term outcomes and reduce morbidity and mortality.