Major Depressive Disorder

Major depressive disorder (MDD) is a mood disorder defined by one or more discrete major depressive episodes — periods of at least two weeks with depressed mood or loss of interest and pleasure (anhedonia), accompanied by changes in sleep, appetite, energy, concentration, self-worth, and, in some cases, thoughts of death. It is a leading cause of disability worldwide and is distinguished from bipolar disorder by the absence of mania or hypomania.

Definition

Major depressive disorder is a unipolar mood disorder characterised by one or more major depressive episodes — at least two weeks of depressed mood and/or anhedonia plus additional neurovegetative and cognitive symptoms causing clinically significant distress or impairment — in the absence of any history of mania or hypomania.

Scope

This entry covers the definition and core clinical features of MDD, its diagnostic criteria as set out in standard nosologies, its epidemiology, the broad understanding of its mechanisms, and the evidence base for its course and treatment as described in the literature. It is a reference and educational overview and does not provide diagnostic or treatment instructions.

Core questions

• What symptoms and duration define a major depressive episode?
• How is MDD distinguished from bipolar disorder, persistent depressive disorder, and normal grief?
• What is known about its causes, course, and response to treatment?

Key concepts

• Major depressive episode
• Anhedonia
• Neurovegetative symptoms (sleep, appetite, energy)
• Recurrence and relapse
• Treatment resistance
• Specifiers (e.g. melancholic, atypical, with anxious distress, seasonal)
• Remission and response

Mechanisms

MDD is understood as a heterogeneous disorder with no single cause. Reviews describe interacting genetic vulnerability, dysregulation of mood-regulating neural circuits and the stress-response (hypothalamic-pituitary-adrenal) system, monoaminergic and other neurotransmitter changes, neuroplasticity and inflammatory mechanisms, and psychosocial precipitants such as adverse life events. The relative contribution of these factors differs across individuals and clinical presentations, which is reflected in the disorder's diagnostic specifiers.

Clinical relevance

MDD is one of the most common psychiatric diagnoses encountered across health care, and many depressive disorders studied in research are operationalised through its criteria. This entry explains how the disorder is defined and measured so that its literature can be read critically; it is not a basis for self-diagnosis and does not give individualised treatment advice.

Epidemiology

MDD is common and recurrent. Population data from the United States estimate a 12-month prevalence of adult DSM-5 MDD of roughly 10% and a lifetime prevalence around 20%, with higher rates in women than men and substantial associated role impairment. Globally, depressive disorders are a leading contributor to years lived with disability.

Evidence & guidelines

MDD is diagnosed using standardised criteria (DSM-5-TR; ICD-11 code 6A70) and is the subject of extensive treatment-outcome research. The STAR*D effectiveness study documented that remission often requires sequential treatment steps and that cumulative remission rates fall with each successive step, informing how treatment-resistant depression is conceptualised. Specific assessment and management follow current clinical guidelines and are outside the scope of this reference entry.

History

The modern concept of major depressive disorder was formalised with the operationalised criteria introduced in DSM-III (1980), which separated discrete depressive episodes from chronic low-grade depression and from bipolar illness. Subsequent DSM and ICD revisions refined the criteria, specifiers, and the handling of bereavement, while large pragmatic trials such as STAR*D shaped understanding of its course and treatment.

Debates

How should the boundary between MDD and normal sadness or grief be drawn?

Successive classifications have debated whether and when distress after bereavement or major loss should qualify as a major depressive episode, balancing the risk of pathologising ordinary grief against the risk of missing treatable depression.

Is MDD one disorder or many?

The marked heterogeneity of presentations, biology, and treatment response has led to debate over whether MDD is a single entity or a collection of distinct conditions sharing a symptom profile, motivating work on subtypes and specifiers.

Seminal works

• kupfer-2012
• malhi-mann-2018
• rush-stard-2006
• hasin-2018

Frequently asked questions

How long must symptoms last for a major depressive episode?

Standard criteria require that depressed mood or loss of interest, together with the requisite associated symptoms, be present for at least two weeks and represent a change from previous functioning.

What is the difference between major depressive disorder and persistent depressive disorder?

MDD is defined by discrete major depressive episodes, whereas persistent depressive disorder (dysthymia) is defined by chronic depressed mood lasting at least two years; the two can co-occur, and the distinction turns largely on chronicity and symptom threshold.