Bipolar Disorder

Bipolar disorder is a common, severe, and persistent mental illness characterized by recurrent episodes of mania and depression and associated with substantial morbidity and mortality. Other mental disorders and general medical conditions are more prevalent in patients with bipolar disorder than in the general population, with cardiometabolic conditions such as cardiovascular disease, diabetes, and obesity representing major contributors to morbidity and mortality. ^[1]  Bipolar disorder is a chronic, lifelong condition associated with significant functional impairment, increased risk of suicide, and reduced quality of life, although appropriate treatment can substantially improve outcomes. ^[2]

Individuals with bipolar disorder have a reduced life expectancy of approximately 12–14 years and a 1.6- to 2-fold increase in cardiovascular mortality, often occurring earlier than in the general population. Elevated rates of cardiometabolic risk factors (including metabolic syndrome, obesity, and type 2 diabetes) and a markedly increased risk of suicide contribute to this excess mortality. ^[2]

Bipolar disorder is characterized by recurrent episodes of depression and periods of elevated or irritable mood (mania). In bipolar I disorder (BPI), patients experience at least one manic episode, which may occur with or without depressive episodes. Bipolar II disorder (BPII) is defined by recurrent depressive episodes with hypomania, a less severe form of mania that does not include psychosis or marked functional impairment. ^[3] Cyclothymic disorder is characterized by chronic fluctuating hypomanic and depressive symptoms that do not meet full criteria for mania, hypomania, or major depression.

Symptoms of mania include decreased need for sleep, pressured speech, increased libido, impulsivity or risk-taking behavior, grandiosity, and, in some cases, psychosis. Between episodes, many patients may return to baseline functioning, particularly with appropriate treatment.

Unipolar (major depressive) disorder and bipolar disorder share depressive features; however, bipolar disorder is defined by the presence of manic or hypomanic episodes. Bipolar disorder is highly impairing and associated with substantial direct and indirect costs, including reduced productivity, unemployment, and increased mortality. ^[4]

Early-onset bipolar disorder (childhood or adolescence) is associated with a more severe, chronic course and higher rates of psychiatric comorbidity.

A diagnosis of bipolar disorder is based on clinical criteria defined in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision and requires the presence of manic, hypomanic, and/or major depressive episodes. ^[5]

Bipolar I disorder (BPI) is defined by at least one manic episode, which may occur with or without depressive episodes. Bipolar II disorder (BPII) requires at least one hypomanic episode and one major depressive episode, without a history of mania.

A manic episode consists of a distinct period of abnormally elevated, expansive, or irritable mood and increased activity or energy lasting at least 1 week (or any duration if hospitalization is required), accompanied by symptoms such as decreased need for sleep, pressured speech, grandiosity, distractibility, and increased goal-directed or risk-taking behavior. Symptoms cause marked functional impairment or include psychotic features.

A hypomanic episode is similar but lasts at least 4 days and is not associated with marked impairment, hospitalization, or psychosis.

A major depressive episode consists of at least 2 weeks of depressed mood or loss of interest, with associated cognitive and somatic symptoms that impair functioning.

Mixed features are common and are specified using the DSM-5-TR “mixed features” specifier. Additional specifiers (eg, anxious distress, rapid cycling, psychotic features, catatonia, peripartum onset, seasonal pattern) may further characterize the episode.

Prevalence

Bipolar disorder is a relatively common psychiatric condition with substantial global burden. Worldwide, an estimated 37 million people (approximately 0.5% of the global population) were living with bipolar disorder in 2021. ^[6]

Lifetime prevalence estimates vary depending on diagnostic criteria and inclusion of subthreshold forms. Broad bipolar spectrum disorders may affect up to ~2%–3% of the population globally. ^{[7, 8]}

In the United States, the 12-month prevalence is approximately 2.8%, and the lifetime prevalence is approximately 4.4% among adults.

Bipolar disorder occurs with similar frequency in males and females, although some subtypes (eg, bipolar II disorder and rapid cycling) are more commonly reported in females. ^[9]

Age at onset

Bipolar disorder most commonly begins in adolescence or early adulthood, with a typical age of onset between 15 and 25 years and a peak in late adolescence. ^[10] Diagnosis in children and adolescents can be challenging because of overlapping symptoms with other psychiatric conditions and developmental variability  ^[11] Early-onset bipolar disorder is associated with a more severe course, greater comorbidity, and increased risk of recurrence.

Global trends and burden

Bipolar disorder is a leading cause of disability worldwide, particularly among young people, and is associated with substantial functional impairment and reduced quality of life.

Epidemiologic studies suggest that the global incidence and burden of bipolar disorder have increased modestly over recent decades, particularly among adolescents and young adults. ^[7]

Bipolar disorder is a multifactorial illness resulting from the interaction of genetic, neurobiologic, and environmental factors.

Genetic factors

Bipolar disorder, particularly BPI, has a strong genetic component. Genome-wide association studies have identified susceptibility loci in genes such as ANK3 and CACNA1C, which are involved in neuronal signaling and calcium channel regulation. ^{[12, 13]}  First-degree relatives of affected individuals have a substantially increased risk of developing bipolar disorder, and offspring of affected parents are at increased risk of mood and other psychiatric disorders. ^[14]

Twin and family studies further support a genetic contribution, with high concordance rates in monozygotic twins, indicating an important role for both genetic and environmental influences.

Early-onset bipolar disorder is associated with greater familial loading and higher rates of comorbid conditions, including attention-deficit/hyperactivity disorder (ADHD), anxiety disorders, and substance use disorders.

Neurobiologic and biochemical factors

Multiple neurobiologic pathways are implicated in bipolar disorder, including dysregulation of monoaminergic neurotransmitters (dopamine, norepinephrine, and serotonin), glutamatergic signaling, and intracellular calcium regulation. ^{[8, 15]}

Neuroimaging studies demonstrate structural and functional abnormalities in brain regions involved in emotional regulation, particularly within cortical-limbic networks. ^[16]  White matter abnormalities and altered connectivity patterns have also been observed and may reflect underlying disease vulnerability.

Neuroendocrine dysregulation, including alterations in the hypothalamic-pituitary-adrenal axis, may further contribute to mood instability.

Environmental and psychosocial factors

Environmental stressors may precipitate or exacerbate mood episodes in individuals with underlying vulnerability. These include psychosocial stress, substance use, sleep disruption, and life events such as pregnancy or seasonal changes in light exposure. ^{[17, 18]}

Environmental factors may also influence risk. One study reported an association between geographic latitude, solar exposure, and bipolar I disorder risk, with higher risk observed in individuals with a family history of mood disorders living at higher latitudes and lower risk near the equator, particularly among females. ^[19]

Substance use is strongly associated with bipolar disorder and may both mimic and trigger mood episodes. In some cases, substance-induced psychosis may precede the development of bipolar disorder or schizophrenia. ^[20]

Pharmacologic agents, particularly antidepressants and dopaminergic medications, may precipitate manic or hypomanic episodes in susceptible individuals. ^[21]

Developmental and course-related factors

Neurodevelopmental and early-life factors may contribute to the onset and trajectory of bipolar disorder. Childhood anxiety, sleep disturbances, and subthreshold mood symptoms have been associated with increased risk of later bipolar disorder. ^[22]

Early-onset illness is associated with a more severe and recurrent course, higher rates of comorbidity, and greater functional impairment.

The pathophysiology of bipolar disorder is not fully understood but is thought to involve dysregulation of neural circuits that regulate mood, reward processing, and emotional reactivity. ^[8]

Abnormal connectivity between the prefrontal cortex and limbic structures, including the amygdala and hippocampus, contributes to impaired emotional regulation, with reduced top-down control and increased limbic reactivity during mood episodes. ^[16]

Neurotransmitter systems implicated in bipolar disorder include dopamine, serotonin, and norepinephrine, as well as glutamate and gamma-aminobutyric acid (GABA). Dysregulation of these systems contributes to mood instability, with dopaminergic hyperactivity associated with mania and relative hypoactivity associated with depressive states. ^[8]

Alterations in intracellular signaling pathways, including calcium signaling, may contribute to neuronal excitability and mood dysregulation and are supported by genetic findings. ^{[12, 15]}

Evidence also suggests abnormalities in white matter integrity and myelination, which may impair communication between brain regions involved in emotional processing. ^[16]

Neuroplasticity and cellular resilience appear to be altered in bipolar disorder, with changes in neurotrophic signaling and cell survival pathways contributing to disease progression. ^[15]

Circadian rhythm disruption plays an important role, with abnormalities in sleep-wake regulation and clock gene function contributing to mood episode onset and recurrence. ^[8]

The kindling hypothesis proposes that recurrent mood episodes may become progressively autonomous over time, with decreasing dependence on external stressors, contributing to illness progression and recurrence. ^[23]

Bipolar disorder is associated with substantial morbidity and increased mortality. Suicide risk is significantly elevated, with approximately 25%–50% of individuals attempting suicide during their lifetime and 15%–20% dying by suicide. ^[2]

Individuals with bipolar disorder also have higher rates of medical comorbidity and premature mortality compared with the general population, including increased risk of cardiovascular and respiratory disease and metabolic abnormalities. ^{[24, 25]}

The clinical course is typically recurrent. Most patients experience multiple mood episodes over time, and recurrence is common, particularly within the first few years after an initial episode. The frequency of episodes may increase over time, and interepisode recovery may become less complete.

Early-onset bipolar disorder (childhood or adolescence) is associated with a more severe and chronic course, higher rates of comorbidity, and greater functional impairment.

Bipolar disorder is also associated with a broad range of comorbid medical conditions, including neurologic, cardiometabolic, and respiratory disorders. ^[26]

The kindling hypothesis suggests that recurrent mood episodes may become progressively autonomous over time, with decreasing dependence on external stressors, contributing to illness progression and recurrence.

Functional outcomes vary, but many patients experience persistent psychosocial impairment, including difficulties with employment, relationships, and overall quality of life, contributing to substantial economic burden and reduced productivity, with indirect costs accounting for the majority of total disease burden. ^[27]

Factors associated with poorer prognosis include:

• Early age of onset

• Substance use disorders

• Psychotic features

• High episode frequency or rapid cycling

• Persistent subthreshold depressive symptoms

• Comorbid medical or psychiatric conditions

Factors associated with better prognosis include:

• Later age of onset

• Fewer prior episodes

• Good treatment adherence

• Strong psychosocial support

• Absence of psychotic features or substance use

Patient and family education is an essential component of bipolar disorder management. Psychoeducation has been shown to reduce relapse rates, improve medication adherence, and decrease hospitalizations. ^[28]

Self-management strategies, including maintaining regular sleep-wake cycles, structured daily routines, physical activity, and social support, are important for long-term stability. ^[29]

Despite these benefits, psychoeducational interventions do not appear to significantly alter symptom severity or overall functional outcomes and should be used as an adjunct to pharmacologic treatment. ^[28]