Depression

Depression, in its clinical sense as a depressive disorder, is a common mood disorder defined by a sustained period of depressed mood and/or loss of interest or pleasure, together with cognitive, somatic, and functional disturbances. It is one of the most frequently encountered mental health conditions in primary care and a leading worldwide cause of disability.

Definition

A depressive disorder is characterized by the presence of depressed mood or loss of interest or pleasure for a sustained period, accompanied by additional symptoms such as changes in sleep, appetite, energy, concentration, and feelings of worthlessness, that cause clinically significant distress or impairment, in the absence of a manic or hypomanic history (which would indicate a bipolar rather than a unipolar disorder).

Scope

The entry covers the clinical concept of unipolar depressive disorder as defined in standard classifications, its core features, hypothesized mechanisms, and population burden. It treats depression as a reference topic within the family-medicine mental-health area and does not provide diagnostic instruments, dosing, or individualized treatment plans.

Core questions

• What distinguishes a depressive disorder from ordinary low mood or grief?
• How is unipolar depression separated from bipolar disorder?
• What is known about the biological and psychosocial mechanisms underlying depression?

Key concepts

• Depressed mood and anhedonia
• Major depressive episode
• Unipolar versus bipolar distinction
• Recurrence and chronicity
• Monoamine and neuroplasticity hypotheses
• Stress-diathesis model
• Functional impairment and disability

Mechanisms

Depression is understood as multifactorial. Classic accounts emphasized disturbances in monoaminergic neurotransmission (serotonin, noradrenaline, dopamine), while later work broadened the picture to include hypothalamic-pituitary-adrenal axis dysregulation, impaired neuroplasticity and neurotrophic signaling, inflammatory processes, and genetic vulnerability interacting with psychosocial stressors. Reviews emphasize that no single mechanism fully explains the disorder and that a stress-diathesis framework, in which predisposition interacts with adverse life events, captures its heterogeneity.

Clinical relevance

Depression is among the conditions most often seen and followed in primary care, where it frequently co-occurs with chronic physical illness and with anxiety and substance-use disorders. This entry describes the clinical concept and the evidence base for understanding it; it is educational reference material and is not a tool for diagnosing or treating an individual patient.

Epidemiology

Depressive disorders are highly prevalent across regions and rank among the leading causes of years lived with disability in the Global Burden of Disease analyses. Onset can occur across the lifespan, the disorder is more frequently diagnosed in women, and a substantial proportion of cases follow a recurrent or chronic course.

Evidence & guidelines

The disorder is defined in the DSM-5 and in the ICD-11 (where unipolar depressive disorders fall under code 6A70 and related categories). Narrative reviews in major medical journals summarize its clinical features, mechanisms, and course, and the Global Burden of Disease studies quantify its population impact. Condition-specific management recommendations are issued by national bodies and are outside the scope of this reference entry.

History

Melancholia was described in classical and early-modern medicine, but the modern concept of depressive disorder was shaped by twentieth-century psychiatric nosology. The monoamine hypothesis arose from mid-century observations on the mood effects of certain drugs, and successive DSM and ICD editions progressively operationalized the diagnostic criteria, while more recent reviews have integrated neurobiological and psychosocial perspectives.

Debates

Where is the boundary between clinical depression and normal sadness?

How far symptom-count thresholds capture a meaningful disorder, as opposed to medicalizing understandable distress, remains debated, and successive classification revisions have revisited criteria such as the handling of bereavement.

Seminal works

• belmaker-agam-2008
• kupfer-2012
• vos-2022

Frequently asked questions

How is depression different from ordinary sadness?

Sadness is a normal emotion, whereas a depressive disorder is a sustained syndrome of depressed mood or loss of interest together with other symptoms that persist over time and cause clinically significant distress or impairment in functioning.

What separates unipolar depression from bipolar disorder?

Unipolar depression involves depressive episodes without any history of mania or hypomania; the presence of manic or hypomanic episodes places a person in the bipolar spectrum, which is classified and managed differently.