Mood disorders, also termed affective disorders, are characterized by disturbances in emotional state that cause significant distress and functional impairment. They are among the most prevalent and disabling mental health conditions worldwide, affecting hundreds of millions of people across the lifespan.

Sadness and grief — Depression is one of the most common mental health disorders worldwide, affecting over 280 million people. Source: Unsplash.

Classification Overview

The DSM-5 divides mood disorders into two broad categories. Depressive disorders include major depressive disorder, persistent depressive disorder (dysthymia), premenstrual dysphoric disorder, and disruptive mood dysregulation disorder. Their core feature is depressed mood and/or loss of interest or pleasure. Bipolar and related disorders include bipolar I disorder, bipolar II disorder, and cyclothymic disorder, characterized by manic or hypomanic episodes alternating with depressive episodes.

Major Depressive Disorder (MDD)

MDD is a common, recurrent, and debilitating condition affecting approximately 7% of the global population annually. Lifetime prevalence is 15–20%. It is twice as common in women as in men, with onset most frequently in young adulthood.

DSM-5 criteria. Five or more of the following symptoms must be present during the same two-week period, representing a change from previous functioning, and at least one symptom is either depressed mood or loss of interest/pleasure: depressed mood most of the day; markedly diminished interest or pleasure (anhedonia); significant weight loss or gain or appetite change; insomnia or hypersomnia; psychomotor agitation or retardation; fatigue or loss of energy; feelings of worthlessness or excessive guilt; diminished concentration or indecisiveness; recurrent thoughts of death or suicidal ideation. These symptoms must cause clinically significant distress or impairment and must not be attributable to a substance or medical condition.

Specifiers. Severity (mild, moderate, severe), psychotic features (mood-congruent or incongruent), melancholic features (profound anhedonia, early morning awakening, diurnal mood variation), atypical features (mood reactivity, increased appetite, hypersomnia, leaden paralysis, rejection sensitivity), catatonia, peripartum onset, and seasonal pattern (seasonal affective disorder).

Pathophysiology. The monoamine hypothesis — deficiency of serotonin, norepinephrine, and dopamine — has been the dominant neurochemical model, though it is an oversimplification. Current understanding emphasizes altered neuroplasticity: chronic stress reduces brain-derived neurotrophic factor (BDNF) and causes hippocampal atrophy, while antidepressants restore neurotrophic signaling. The hypothalamic-pituitary-adrenal (HPA) axis is dysregulated in many patients, with elevated cortisol and impaired feedback inhibition. Neuroimaging shows reduced hippocampal volume, hyperactive amygdala, and hypoactive prefrontal cortex. Inflammation (elevated IL-6, TNF-alpha, CRP) contributes to a significant subset of cases.

Treatment. First-line treatment for moderate to severe MDD is antidepressant medication (SSRIs and SNRIs are most commonly used), evidence-based psychotherapy (cognitive behavioral therapy, interpersonal therapy, behavioral activation), or a combination (which is more effective than either alone). Electroconvulsive therapy (ECT) is the most effective treatment for severe, treatment-resistant, or psychotic depression. Transcranial magnetic stimulation (TMS) and ketamine/esketamine are newer options for treatment-resistant depression.

⚠ Clinical Correlation

Approximately 30% of patients with MDD do not achieve remission after two adequate trials of antidepressants from different classes. When this occurs, systematic treatment strategies include optimizing the current medication dose, switching to a different class, augmenting with a non-antidepressant agent (aripiprazole, brexpiprazole, quetiapine, lithium, thyroid hormone), or combining two antidepressants. Esketamine (NMDA receptor antagonist) nasal spray is FDA-approved for treatment-resistant depression. ECT has a 70–85% response rate in treatment-resistant patients. TMS is non-invasive and well-tolerated but less effective for severe cases.

Persistent Depressive Disorder (Dysthymia)

A chronic, less severe form of depression with symptoms present for at least two years in adults (one year in children and adolescents). Depressed mood for most of the day, more days than not, accompanied by at least two of: poor appetite or overeating, insomnia or hypersomnia, low energy, low self-esteem, poor concentration, and hopelessness. Double depression refers to MDD superimposed on dysthymia.

Bipolar I Disorder

Bipolar I disorder is defined by the occurrence of at least one manic episode. Manic episodes are distinct periods of abnormally and persistently elevated, expansive, or irritable mood and abnormally and persistently increased goal-directed activity or energy, lasting at least one week (or requiring hospitalization). During a manic episode, three or more of the following are present: inflated self-esteem or grandiosity, decreased need for sleep, pressured speech, flight of ideas or racing thoughts, distractibility, increase in goal-directed activity or psychomotor agitation, and excessive involvement in activities with high potential for painful consequences (spending sprees, sexual indiscretions, unwise investments). The episode causes marked impairment in social or occupational functioning or requires hospitalization to prevent harm.

Bipolar mood — Bipolar disorder involves alternating episodes of mania and depression requiring long-term mood stabilization. Source: Unsplash.

Most patients with bipolar I also experience major depressive episodes, though this is not required for diagnosis. The course is chronic, with frequent recurrences. Lifetime prevalence is approximately 1%.

Bipolar II Disorder

Bipolar II disorder is defined by the occurrence of at least one hypomanic episode and at least one major depressive episode. Hypomanic episodes are similar to manic episodes but shorter (at least four consecutive days) and less severe — they do not cause marked impairment in function and do not require hospitalization. Between episodes, functioning may be near normal. Diagnosis is frequently delayed because the hypomanic phase is often experienced as a period of pleasant productivity, and patients may only seek treatment during depressive episodes.

Bipolar II is not a milder form of bipolar I. The depressive episodes in bipolar II are typically more frequent, more prolonged, and more disabling than in bipolar I. Patients spend three times as many days in depression as in hypomania. Suicide risk is substantial, comparable to that of bipolar I.

★ Key Concept

The cornerstone of bipolar disorder treatment is mood stabilization. Lithium remains the gold standard — it reduces manic and depressive episodes and has unique anti-suicidal effects (reducing suicide risk by 60% compared to other treatments). Valproate and carbamazepine are alternatives, particularly for acute mania. Atypical antipsychotics (quetiapine, olanzapine, aripiprazole, risperidone, lurasidone, cariprazine) have mood-stabilizing properties. Antidepressants are used cautiously in bipolar depression due to the risk of inducing mania (switching) or accelerating mood cycles. Lamotrigine is effective for bipolar depression and maintenance. Long-term adherence is challenging but essential for preventing relapse.

Cyclothymic Disorder

A chronic, fluctuating mood disturbance with numerous periods of hypomanic symptoms and numerous periods of depressive symptoms that do not meet full criteria for hypomanic or major depressive episodes. Symptoms persist for at least two years (one year in children). The prevalence is 0.4–1.0%. A substantial proportion of individuals with cyclothymia eventually develop bipolar I or II disorder.

Suicide Risk in Mood Disorders

Mood disorders carry the highest suicide risk of any mental health condition. Lifetime suicide risk is approximately 15% for bipolar disorder and 6–10% for MDD. Assessment of suicidal ideation, plan, intent, access to means, and protective factors is essential at every clinical encounter. Acute risk requires emergency intervention — hospitalization, removal of means, and initiation or intensification of treatment. Chronic risk is managed through ongoing treatment of the underlying mood disorder, psychotherapy (particularly dialectical behavior therapy and cognitive behavioral therapy), and maintaining a strong therapeutic alliance.

Summary

Mood disorders span from unipolar depression to the bipolar spectrum. MDD is highly prevalent and treatable but frequently recurrent. Bipolar disorders require long-term mood stabilization, with lithium as the cornerstone. Accurate diagnosis — distinguishing unipolar from bipolar depression — is critical because antidepressant monotherapy can destabilize bipolar patients. Suicide risk must be assessed routinely. With appropriate treatment, most individuals with mood disorders achieve significant improvement in symptoms and quality of life.