Major depressive disorder is the most common serious psychiatric diagnosis in the world. Roughly one in six people will meet criteria during their lifetime; current point prevalence runs around 5-7%. The disorder costs more disability-adjusted life years globally than nearly any other illness. And — clinically important — it is treatable.
The DSM-5 criteria require at least five symptoms over a 2-week period, with at least one being either depressed mood or anhedonia. The remaining symptoms cluster as the SIGECAPS mnemonic: Sleep disturbance, loss of Interest, Guilt or worthlessness, low Energy, Concentration difficulty, Appetite or weight change, Psychomotor change, Suicidal ideation. Significant functional impairment must be present.
The neuroanatomical signature, by now familiar from Volume 1: hyperactive default mode network producing ruminative self-referential thinking; dysregulated HPA axis with elevated and flattened-diurnal cortisol; hippocampal volume loss measurable on MRI in patients with chronic or recurrent disease; reduced top-down regulation from prefrontal cortex over amygdala reactivity. All four findings are partially reversible with effective treatment.
First-line treatments converge from different angles on the same circuits. SSRIs and SNRIs (citalopram, escitalopram, sertraline, fluoxetine, venlafaxine, duloxetine) work over 4-8 weeks through plasticity changes downstream of monoamine elevation. Cognitive-behavioral therapy works by training disengagement from DMN ruminative content and rebuilding cognitive control. Behavioral activation directly counters the avoidance and withdrawal patterns. Aerobic exercise has effect sizes comparable to medication and is the most underused intervention in primary care. Combinations outperform single modalities.
For partial response, augmentation strategies help: bupropion add-on (sexual side effects offset, energy gain), aripiprazole at low dose, lithium augmentation, thyroid augmentation. For treatment-resistant depression: ketamine and esketamine offer rapid mechanism through NMDA modulation and downstream glutamatergic plasticity; ECT remains highly effective for severe and TRD cases; TMS has FDA approval for medication-resistant cases. The treatment toolkit is broad and continues to expand.
Important diagnostic moves before treatment: screen for bipolar disorder (family history, prior hypomania, age at first episode, postpartum psychosis history); screen thyroid (TSH); screen substance use; screen sleep apnea in treatment-resistant patients; screen vitamin D deficiency in northern climates. Many "treatment-resistant" cases turn out to be incompletely treated underlying conditions.
When you next see a depressed patient, picture not a moral failure or a character defect but a measurable circuit failure with multiple convergent treatment routes. The hippocampus can regrow. The DMN can normalize. The HPA axis can recalibrate. You are intervening in a treatable disorder of the brain — and the intervention often works.