Major Depressive Disorder

Detailed Overview

MDD episodes must last for at least two consecutive weeks. It has profound impacts on sleep, appetite, energy, and cognition, often accompanied by feelings of worthlessness and suicidal ideation. Pathophysiologically, it involves dysregulation of monoamine neurotransmitters and structural changes in the hippocampus and prefrontal cortex. Management requires a combination of pharmacotherapy, psychotherapy, and in severe cases, neurostimulation.

Epidemiology & Demographics

Lifetime prevalence in the US is approximately 20%. It is roughly twice as common in females as in males. The peak onset is in the 20s, though it can occur at any age.

Etiological Mechanism

Multifactorial. Involves genetic vulnerability (heritability is ~35%), severe life stress or trauma (epigenetic changes), and neurobiological abnormalities.

Primary Causes

No single cause. Triggered by a combination of genetic predisposition, biochemical imbalances (serotonin, norepinephrine, dopamine depletion), and psychosocial stressors (loss, trauma).

• Family History: Having a first-degree relative with MDD increases risk 2- to 3-fold.
• Adverse Childhood Experiences: Trauma, abuse, or neglect in early life drastically increases vulnerability.
• Chronic Medical Illness: Conditions like Parkinson's, stroke, cancer, or chronic pain are heavily associated with secondary MDD.

The traditional Monoamine Hypothesis posits a deficiency in serotonin, norepinephrine, and dopamine. The Neurotrophic Hypothesis suggests stress-induced decreases in Brain-Derived Neurotrophic Factor (BDNF) lead to neuronal atrophy and decreased neurogenesis in the hippocampus and prefrontal cortex. Hyperactivity of the Hypothalamic-Pituitary-Adrenal (HPA) axis, leading to chronically elevated cortisol, also drives these neurotoxic structural brain changes and systemic inflammation.

Characteristic Clinical Presentation

• Depressed Mood: Feeling sad, empty, or hopeless most of the day, nearly every day.
• Anhedonia: Markedly diminished interest or pleasure in all, or almost all, daily activities.
• Neurovegetative Symptoms: Significant weight loss or gain, insomnia or hypersomnia, and profound fatigue or loss of energy.

Physical Examination Signs

• Psychomotor retardation (slowed speech, sluggish movement) or agitation
• Flat or restricted affect during mental status examination
• Poor personal hygiene or grooming in severe cases

• Suicide: Approximately 15% of patients with severe, untreated MDD die by suicide.
• Substance Use Disorders: High rate of self-medication with alcohol or illicit drugs, leading to dual diagnosis.

Diagnostic Criteria & Guidelines

DSM-5 Criteria: 5 or more of the following 9 symptoms present during the same 2-week period; at least one symptom must be depressed mood or anhedonia. Symptoms: 1. Depressed mood, 2. Anhedonia, 3. Weight/appetite change, 4. Sleep disturbance, 5. Psychomotor agitation/retardation, 6. Fatigue, 7. Feelings of worthlessness/guilt, 8. Diminished concentration, 9. Suicidal ideation. (Mnemonic: SIGECAPS).

Differential Diagnosis

• Bipolar Disorder (Depressive phase)
• Hypothyroidism
• Adjustment Disorder with Depressed Mood
• Substance-Induced Depressive Disorder

Laboratory Tests & Biomarkers

• TSH: Checked to rule out Hypothyroidism (must be normal for MDD diagnosis)
• Vitamin B12 / Folate: Normal (checked to rule out deficiency causing mood/cognitive issues)
• Urine Toxicology: Negative (rules out acute substance intoxication/withdrawal)

Imaging Modalities & Findings

• None: Imaging is not used for diagnosis, only to rule out organic brain disease (e.g., tumor, stroke) if focal neurological signs are present.

• Mild
  Meets criteria, but symptoms result in only minor impairment in social or occupational functioning.
• Moderate
  Symptoms are between mild and severe; noticeable impairment in daily life.
• Severe
  Number of symptoms is substantially in excess of what is required; severely interferes with functioning, possible psychotic features.

Second-Line & Adjunctive Therapy

If SSRI fails, switch to an SNRI (e.g., Venlafaxine 75-150 mg PO daily) or an atypical antidepressant (e.g., Bupropion 150 mg PO XL daily, especially useful for fatigue/anhedonia without sexual side effects). Augmentation with Aripiprazole 2-5 mg/day.

Surgical & Procedural Management

Electroconvulsive Therapy (ECT) is highly effective for treatment-resistant depression, severe suicidality, or depression with psychotic features.

Recommended Lifestyle Changes

• Regular aerobic exercise (minimum 150 minutes/week) has proven antidepressant effects.
• Strict sleep hygiene: consistent sleep/wake times, dark room, no screens 1 hr before bed.
• Eliminate alcohol and illicit substance use, which depress CNS function.

Patient Counseling & Advice

Educate that antidepressants take 4 to 6 weeks to show full effect, and side effects (nausea, jitteriness) often precede clinical benefit. Emphasize that depression is a medical illness, not a character flaw.

Follow-Up & Monitoring Schedule

Monitor weekly to bi-weekly upon initiating medication to assess for increased suicidality (especially in young adults < 25), tolerability, and early clinical response. Assess severity using PHQ-9.

Preventive Strategies

Maintenance therapy with an antidepressant for at least 6-12 months after the first episode, and indefinitely for patients with 3 or more episodes.

Highly treatable. 50% recover within 6 months. However, the recurrence rate is high; ~50% of patients will have a second episode, and those with two episodes have an 80% chance of a third.