Major depressive disorder (MDD), also known as clinical depression, is a mental disorder[10] characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. Introduced by a group of US clinicians in the mid-1970s,[11] the term was adopted by the American Psychiatric Association for this symptom cluster under mood disorders in the 1980 version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III), and has become widely used since. The disorder causes the second-most years lived with disability, after lower back pain.[12] The diagnosis of major depressive disorder is based on the person's reported experiences, behavior reported by family or friends, and a mental status examination.[13] There is no laboratory test for the disorder, but testing may be done to rule out physical conditions that can cause similar symptoms.[13] The most common time of onset is in a person's 20s,[3][4] with females affected about three times as often as males.[14] The course of the disorder varies widely, from one episode lasting months to a lifelong disorder with recurrent major depressive episodes. Those with major depressive disorder are typically treated with psychotherapy and antidepressant medication.[1] While a mainstay of treatment, the clinical efficacy of antidepressants is controversial.[15][16][17][18] Hospitalization (which may be involuntary) may be necessary in cases with associated self-neglect or a significant risk of harm to self or others. Electroconvulsive therapy (ECT) may be considered if other measures are not effective.[1] Major depressive disorder is believed to be caused by a combination of genetic, environmental, and psychological factors,[1] with about 40% of the risk being genetic.[5] Risk factors include a family history of the condition, major life changes, childhood traumas, environmental lead exposure,[19] certain medications, chronic health problems, and substance use disorders.[1][5] It can negatively affect a person's personal life, work life, or education, and cause issues with a person's sleeping habits, eating habits, and general health.[1][5] Signs and symptoms See also: Digital media use and mental health § Depression An 1892 lithograph of a woman diagnosed with melancholia A person having a major depressive episode usually exhibits a low mood, which pervades all aspects of life, and an inability to experience pleasure in previously enjoyable activities.[20] Depressed people may be preoccupied with or ruminate over thoughts and feelings of worthlessness, inappropriate guilt or regret, helplessness or hopelessness.[21] Other symptoms of depression include poor concentration and memory,[22] withdrawal from social situations and activities, reduced sex drive, irritability, and thoughts of death or suicide. Insomnia is common; in the typical pattern, a person wakes very early and cannot get back to sleep. Hypersomnia, or oversleeping, can also happen,[23] as well as day-night rhythm disturbances, such as diurnal mood variation.[24] Some antidepressants may also cause insomnia due to their stimulating effect.[25] In severe cases, depressed people may have psychotic symptoms. These symptoms include delusions or, less commonly, hallucinations, usually unpleasant.[26] People who have had previous episodes with psychotic symptoms are more likely to have them with future episodes.[27] A depressed person may report multiple physical symptoms such as fatigue, headaches, or digestive problems; physical complaints are the most common presenting problem in developing countries, according to the World Health Organization's criteria for depression.[28] Appetite often decreases, resulting in weight loss, although increased appetite and weight gain occasionally occur.[29] Major depression significantly affects a person's family and personal relationships, work or school life, sleeping and eating habits, and general health.[30] Family and friends may notice agitation or lethargy.[23] Older depressed people may have cognitive symptoms of recent onset, such as forgetfulness,[31] and a more noticeable slowing of movements.[32] Depressed children may often display an irritable rather than a depressed mood;[23] most lose interest in school and show a steep decline in academic performance.[33] Diagnosis may be delayed or missed when symptoms are interpreted as "normal moodiness".[34] Elderly people may not present with classical depressive symptoms.[35] Diagnosis and treatment is further complicated in that the elderly are often simultaneously treated with a number of other drugs, and often have other concurrent diseases.[35] Cause Further information: Biology of depression and Epigenetics of depression A cup analogy demonstrating the diathesis–stress model that under the same amount of stressors, person 2 is more vulnerable than person 1, because of their predisposition[36] The etiology of depression is not yet fully understood.[37][38][39] The biopsychosocial model proposes that biological, psychological, and social factors all play a role in causing depression.[5][40] The diathesis–stress model specifies that depression results when a preexisting vulnerability, or diathesis, is activated by stressful life events. The preexisting vulnerability can be either genetic,[41][42] implying an interaction between nature and nurture, or schematic, resulting from views of the world learned in childhood.[43] American psychiatrist Aaron Beck suggested that a triad of automatic and spontaneous negative thoughts about the self, the world or environment, and the future may lead to other depressive signs and symptoms.[44][45] Genetics Genes play a major role in the development of depression.[46] Family and twin studies suggest that genetic factors account for nearly 40% of the variation in risk for major depressive disorder. Like most psychiatric disorders, major depression is likely shaped by a combination of many individual genetic influences.[47] In 2018, a genome-wide association study discovered 44 genetic variants linked to risk for major depression;[48] a 2019 study found 102 variants in the genome linked to depression.[49] However, it appears that major depression is less heritable compared to bipolar disorder and schizophrenia.[50][51] Research focusing on specific candidate genes has been criticized for its tendency to generate false positive findings.[52] There are also other efforts to examine interactions between life stress and polygenic risk for depression.[53] Other health problems Depression can also arise after a chronic or terminal medical condition, such as HIV/AIDS or asthma, and may be labeled "secondary depression".[54][55] It is unknown whether the underlying diseases induce depression through effect on quality of life, or through shared etiologies (such as degeneration of the basal ganglia in Parkinson's disease or immune dysregulation in asthma).[56] Depression may also be iatrogenic (the result of healthcare), such as drug-induced depression. Therapies associated with depression include interferons, beta blockers,[57] isotretinoin,[58] contraceptives,[57] cardiac agents,[59] anticonvulsants,[60] and hormonal agents.[61] Celiac disease is another possible contributing factor.[62] Substance use in early age is associated with increased risk of developing depression later in life.[63] Depression occurring after giving birth is called postpartum depression and is thought to be the result of hormonal changes associated with pregnancy.[64] Seasonal affective disorder, a type of depression associated with seasonal changes in sunlight, is thought to be triggered by decreased sunlight.[65] Vitamin B2, B6 and B12 deficiency may cause depression in females.[66] A 2025 study found that, among more than 172,500 adults in the UK aged 39 and older, those with a history of depression experienced the onset of chronic illnesses approximately 30% earlier than those without depression.[67] Environmental Adverse childhood experiences (incorporating childhood abuse, neglect and family dysfunction) markedly increase the risk of major depression, especially if more than one type.[68] Childhood trauma also correlates with severity of depression, poor responsiveness to treatment and length of illness.[69] Some are more susceptible than others to developing mental illness such as depression after trauma, and various genes have been suggested to control susceptibility.[70] Couples in unhappy marriages have a higher risk of developing clinical depression.[71] There appears to be a link between air pollution and depression and suicide. There may be an association between long-term PM2.5 exposure and depression, and a possible association between short-term PM10 exposure and suicide.[72] Living alone has been found to increase the risk of depression by 42%.[6] Pathophysiology Further information: Biology of depression and Epigenetics of depression The pathophysiology of depression is not completely understood, but current theories center around monoaminergic systems, the circadian rhythm, immunological dysfunction, HPA-axis dysfunction, and structural or functional abnormalities of emotional circuits. Derived from the effectiveness of monoaminergic drugs in treating depression, the monoamine theory posits that insufficient activity of monoamine neurotransmitters is the primary cause of depression. Evidence for the monoamine theory comes from multiple areas. First, acute depletion of tryptophan—a necessary precursor of serotonin and a monoamine—can cause depression in those in remission or relatives of people who are depressed, suggesting that decreased serotonergic neurotransmission is important in depression.[73] Second, the correlation between depression risk and polymorphisms in the 5-HTTLPR gene, which codes for serotonin receptors, suggests a link.[74] Third, decreased size of the locus coeruleus, reduced activity of tyrosine hydroxylase, increased density of alpha-2 adrenergic receptor, and evidence from rat models suggest decreased adrenergic neurotransmission in depression.[75] Furthermore, decreased levels of homovanillic acid, altered response to dextroamphetamine, responses of depressive symptoms to dopamine receptor agonists, decreased dopamine receptor D1 binding in the striatum,[76] and polymorphism of dopamine receptor genes implicate dopamine, another monoamine, in depression.[77][78] Lastly, increased activity of monoamine oxidase, which degrades monoamines, has been associated with depression.[79] However, the monoamine theory is inconsistent with observations that serotonin depletion does not cause depression in healthy persons, that antidepressants instantly increase levels of monoamines but take weeks to work, and the existence of atypical antidepressants which can be effective despite not targeting this pathway.[80] One proposed explanation for the therapeutic lag, and further support for the deficiency of monoamines, is a desensitization of self-inhibition in raphe nuclei by the increased serotonin mediated by antidepressants.[81] However, disinhibition of the dorsal raphe has been proposed to occur as a result of decreased serotonergic activity in tryptophan depletion, resulting in a depressed state mediated by increased serotonin. Further countering the monoamine hypothesis is the fact that rats with lesions of the dorsal raphe are not more depressive than controls; the finding of increased jugular 5-HIAA in people who are depressed that normalized with selective serotonin reuptake inhibitor (SSRI) treatment, and the preference for carbohydrates in people who are depressed.[82] Already limited, the monoamine hypothesis has been further oversimplified when presented to the general public.[83] A 2022 review found no consistent evidence supporting the serotonin hypothesis linking serotonin levels and depression.[84] HPA-axis abnormalities have been suggested in depression given the association of CRHR1 with depression and the increased frequency of dexamethasone test non-suppression in people who are depressed. However, this abnormality is not adequate as a diagnosis tool because its sensitivity is only 44%.[85] These stress-related abnormalities are thought to be the cause of hippocampal volume reductions seen in people who are depressed.[86] Furthermore, a meta-analysis yielded decreased dexamethasone suppression, and increased response to psychological stressors.[87] Further abnormal results have been obscured with the cortisol awakening response, with increased response being associated with depression.[88] There is also a connection between the gut microbiome and the central nervous system, otherwise known as the Gut-Brain axis, which is a two-way communication system between the brain and the gut. Experiments have shown that microbiota in the gut can play an important role in depression, as people with MDD often have gut-brain dysfunction. One analysis showed that those with MDD have different bacteria in their guts. Bacteria Bacteroidetes and Firmicutes were most affected in people with MDD, and they are also impacted in people with irritable bowel syndrome.[89] Another study showed that people with IBS have a higher chance of developing depression, which shows the two are connected.[90] There is even evidence suggesting that altering the microbes in the gut can have regulatory effects on developing depression.[89] Theories unifying neuroimaging findings have been proposed. The first model proposed is the limbic-cortical model, which involves hyperactivity of the ventral paralimbic regions and hypoactivity of frontal regulatory regions in emotional processing.[91] Another model, the cortico-striatal model, suggests that abnormalities of the prefrontal cortex in regulating striatal and subcortical structures result in depression.[92] Another model proposes hyperactivity of salience structures in identifying negative stimuli and hypoactivity of cortical regulatory structures resulting in a negative emotional bias and depression, consistent with emotional bias studies.[93] Immune pathogenesis theories on depression The newer field of psychoneuroimmunology, the study between the immune system and the nervous system and emotional state, suggests that cytokines may impact depression. Immune system abnormalities have been observed, including increased levels of cytokines -cells produced by immune cells that affect inflammation- involved in generating sickness behavior, creating a pro-inflammatory profile in MDD.[94][95][96] Some people with depression have increased levels of pro-inflammatory cytokines and some have decreased levels of anti-inflammatory cytokines.[97] Research suggests that treatments can reduce pro-inflammatory cell production, like the experimental treatment of ketamine with treatment-resistant depression.[98] With this, in MDD, people will more likely have a Th-1 dominant immune profile, which is a pro-inflammatory profile. This suggests that there are components of the immune system affecting the pathology of MDD.[99] Another way cytokines can affect depression is in the kynurenine pathway, and when this is overactivated, it can cause depression. This can be due to too much microglial activation and too little astrocytic activity. When microglia get activated, they release pro-inflammatory cytokines that cause an increase in the production of COX2. This, in turn, causes the production of PGE2, which is a prostaglandin, and this catalyzes the production of indolamine, IDO. IDO causes tryptophan to get converted into kynurenine, and kynurenine becomes quinolinic acid.[100] Quinolinic acid is an agonist for NMDA receptors, so it activates the pathway. Studies have shown that the post-mortem brains of patients with MDD have higher levels of quinolinic acid than people who did not have MDD. With this, researchers have also seen that the concentration of quinolinic acid correlates to the severity of depressive symptoms.[101] Diagnosis Assessment Further information: Rating scales for depression Caricature of a man with depression A diagnostic assessment may be conducted by a suitably trained general practitioner, or by a psychiatrist or psychologist,[30] who records the person's current circumstances, biographical history, current symptoms, family history, and alcohol and drug use. The assessment also includes a mental state examination, which is an assessment of the person's current mood and thought content, in particular the presence of themes of hopelessness or pessimism, self-harm or suicide, and an absence of positive thoughts or plans.[30] Specialist mental health services are rare in rural areas, and thus diagnosis and management is left largely to primary-care clinicians.[102] This issue is even more marked in developing countries.[103] Rating scales are not used to diagnose depression, but they provide an indication of the severity of symptoms for a time period, so a person who scores above a given cut-off point can be more thoroughly evaluated for a depressive disorder diagnosis. Several rating scales are used for this purpose;[104] these include the Hamilton Rating Scale for Depression,[105] the Beck Depression Inventory[106] or the Suicide Behaviors Questionnaire-Revised.[107] Primary-care physicians have more difficulty with underrecognition and undertreatment of depression compared to psychiatrists. These cases may be missed because for some people with depression, physical symptoms often accompany depression. In addition, there may also be barriers related to the person, provider, and/or the medical system. Non-psychiatrist physicians have been shown to miss about two-thirds of cases, although there is some evidence of improvement in the number of missed cases.[108] A doctor generally performs a medical examination and selected investigations to rule out other causes of depressive symptoms. These include blood tests measuring TSH and thyroxine to exclude hypothyroidism; basic electrolytes and serum calcium to rule out a metabolic disturbance; and a full blood count including ESR to rule out a systemic infection or chronic disease.[109] Adverse affective reactions to medications or alcohol misuse may be ruled out, as well. Testosterone levels may be evaluated to diagnose hypogonadism, a cause of depression in men.[110] Vitamin D levels might be evaluated, as low levels of vitamin D have been associated with greater risk for depression.[111] Subjective cognitive complaints appear in older depressed people, but they can also be indicative of the onset of a dementing disorder, such as Alzheimer's disease.[112][113] Cognitive testing and brain imaging can help distinguish depression from dementia.[114] A CT scan can exclude brain pathology in those with psychotic, rapid-onset or otherwise unusual symptoms.[115] No biological tests confirm major depression.[116] In general, investigations are not repeated for a subsequent episode unless there is a medical indication. DSM and ICD criteriaMajor depressive disorder (MDD), also known as clinical depression, is a mental disorder[10] characterized by at least two weeks of pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. Introduced by a group of US clinicians in the mid-1970s,[11] the term was adopted by the American Psychiatric Association for this symptom cluster under mood disorders in the 1980 version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III), and has become widely used since. The disorder causes the second-most years lived with disability, after lower back pain.[12] The diagnosis of major depressive disorder is based on the person's reported experiences, behavior reported by family or friends, and a mental status examination.[13] There is no laboratory test for the disorder, but testing may be done to rule out physical conditions that can cause similar symptoms.[13] The most common time of onset is in a person's 20s,[3][4] with females affected about three times as often as males.[14] The course of the disorder varies widely, from one episode lasting months to a lifelong disorder with recurrent major depressive episodes. Those with major depressive disorder are typically treated with psychotherapy and antidepressant medication.[1] While a mainstay of treatment, the clinical efficacy of antidepressants is controversial.[15][16][17][18] Hospitalization (which may be involuntary) may be necessary in cases with associated self-neglect or a significant risk of harm to self or others. Electroconvulsive therapy (ECT) may be considered if other measures are not effective.[1] Major depressive disorder is believed to be caused by a combination of genetic, environmental, and psychological factors,[1] with about 40% of the risk being genetic.[5] Risk factors include a family history of the condition, major life changes, childhood traumas, environmental lead exposure,[19] certain medications, chronic health problems, and substance use disorders.[1][5] It can negatively affect a person's personal life, work life, or education, and cause issues with a person's sleeping habits, eating habits, and general health.[1][5] Signs and symptoms See also: Digital media use and mental health § Depression An 1892 lithograph of a woman diagnosed with melancholia A person having a major depressive episode usually exhibits a low mood, which pervades all aspects of life, and an inability to experience pleasure in previously enjoyable activities.[20] Depressed people may be preoccupied with or ruminate over thoughts and feelings of worthlessness, inappropriate guilt or regret, helplessness or hopelessness.[21] Other symptoms of depression include poor concentration and memory,[22] withdrawal from social situations and activities, reduced sex drive, irritability, and thoughts of death or suicide. Insomnia is common; in the typical pattern, a person wakes very early and cannot get back to sleep. Hypersomnia, or oversleeping, can also happen,[23] as well as day-night rhythm disturbances, such as diurnal mood variation.[24] Some antidepressants may also cause insomnia due to their stimulating effect.[25] In severe cases, depressed people may have psychotic symptoms. These symptoms include delusions or, less commonly, hallucinations, usually unpleasant.[26] People who have had previous episodes with psychotic symptoms are more likely to have them with future episodes.[27] A depressed person may report multiple physical symptoms such as fatigue, headaches, or digestive problems; physical complaints are the most common presenting problem in developing countries, according to the World Health Organization's criteria for depression.[28] Appetite often decreases, resulting in weight loss, although increased appetite and weight gain occasionally occur.[29] Major depression significantly affects a person's family and personal relationships, work or school life, sleeping and eating habits, and general health.[30] Family and friends may notice agitation or lethargy.[23] Older depressed people may have cognitive symptoms of recent onset, such as forgetfulness,[31] and a more noticeable slowing of movements.[32] Depressed children may often display an irritable rather than a depressed mood;[23] most lose interest in school and show a steep decline in academic performance.[33] Diagnosis may be delayed or missed when symptoms are interpreted as "normal moodiness".[34] Elderly people may not present with classical depressive symptoms.[35] Diagnosis and treatment is further complicated in that the elderly are often simultaneously treated with a number of other drugs, and often have other concurrent diseases.[35] Cause Further information: Biology of depression and Epigenetics of depression A cup analogy demonstrating the diathesis–stress model that under the same amount of stressors, person 2 is more vulnerable than person 1, because of their predisposition[36] The etiology of depression is not yet fully understood.[37][38][39] The biopsychosocial model proposes that biological, psychological, and social factors all play a role in causing depression.[5][40] The diathesis–stress model specifies that depression results when a preexisting vulnerability, or diathesis, is activated by stressful life events. The preexisting vulnerability can be either genetic,[41][42] implying an interaction between nature and nurture, or schematic, resulting from views of the world learned in childhood.[43] American psychiatrist Aaron Beck suggested that a triad of automatic and spontaneous negative thoughts about the self, the world or environment, and the future may lead to other depressive signs and symptoms.[44][45] Genetics Genes play a major role in the development of depression.[46] Family and twin studies suggest that genetic factors account for nearly 40% of the variation in risk for major depressive disorder. Like most psychiatric disorders, major depression is likely shaped by a combination of many individual genetic influences.[47] In 2018, a genome-wide association study discovered 44 genetic variants linked to risk for major depression;[48] a 2019 study found 102 variants in the genome linked to depression.[49] However, it appears that major depression is less heritable compared to bipolar disorder and schizophrenia.[50][51] Research focusing on specific candidate genes has been criticized for its tendency to generate false positive findings.[52] There are also other efforts to examine interactions between life stress and polygenic risk for depression.[53] Other health problems Depression can also arise after a chronic or terminal medical condition, such as HIV/AIDS or asthma, and may be labeled "secondary depression".[54][55] It is unknown whether the underlying diseases induce depression through effect on quality of life, or through shared etiologies (such as degeneration of the basal ganglia in Parkinson's disease or immune dysregulation in asthma).[56] Depression may also be iatrogenic (the result of healthcare), such as drug-induced depression. Therapies associated with depression include interferons, beta blockers,[57] isotretinoin,[58] contraceptives,[57] cardiac agents,[59] anticonvulsants,[60] and hormonal agents.[61] Celiac disease is another possible contributing factor.[62] Substance use in early age is associated with increased risk of developing depression later in life.[63] Depression occurring after giving birth is called postpartum depression and is thought to be the result of hormonal changes associated with pregnancy.[64] Seasonal affective disorder, a type of depression associated with seasonal changes in sunlight, is thought to be triggered by decreased sunlight.[65] Vitamin B2, B6 and B12 deficiency may cause depression in females.[66] A 2025 study found that, among more than 172,500 adults in the UK aged 39 and older, those with a history of depression experienced the onset of chronic illnesses approximately 30% earlier than those without depression.[67] Environmental Adverse childhood experiences (incorporating childhood abuse, neglect and family dysfunction) markedly increase the risk of major depression, especially if more than one type.[68] Childhood trauma also correlates with severity of depression, poor responsiveness to treatment and length of illness.[69] Some are more susceptible than others to developing mental illness such as depression after trauma, and various genes have been suggested to control susceptibility.[70] Couples in unhappy marriages have a higher risk of developing clinical depression.[71] There appears to be a link between air pollution and depression and suicide. There may be an association between long-term PM2.5 exposure and depression, and a possible association between short-term PM10 exposure and suicide.[72] Living alone has been found to increase the risk of depression by 42%.[6] Pathophysiology Further information: Biology of depression and Epigenetics of depression The pathophysiology of depression is not completely understood, but current theories center around monoaminergic systems, the circadian rhythm, immunological dysfunction, HPA-axis dysfunction, and structural or functional abnormalities of emotional circuits. Derived from the effectiveness of monoaminergic drugs in treating depression, the monoamine theory posits that insufficient activity of monoamine neurotransmitters is the primary cause of depression. Evidence for the monoamine theory comes from multiple areas. First, acute depletion of tryptophan—a necessary precursor of serotonin and a monoamine—can cause depression in those in remission or relatives of people who are depressed, suggesting that decreased serotonergic neurotransmission is important in depression.[73] Second, the correlation between depression risk and polymorphisms in the 5-HTTLPR gene, which codes for serotonin receptors, suggests a link.[74] Third, decreased size of the locus coeruleus, reduced activity of tyrosine hydroxylase, increased density of alpha-2 adrenergic receptor, and evidence from rat models suggest decreased adrenergic neurotransmission in depression.[75] Furthermore, decreased levels of homovanillic acid, altered response to dextroamphetamine, responses of depressive symptoms to dopamine receptor agonists, decreased dopamine receptor D1 binding in the striatum,[76] and polymorphism of dopamine receptor genes implicate dopamine, another monoamine, in depression.[77][78] Lastly, increased activity of monoamine oxidase, which degrades monoamines, has been associated with depression.[79] However, the monoamine theory is inconsistent with observations that serotonin depletion does not cause depression in healthy persons, that antidepressants instantly increase levels of monoamines but take weeks to work, and the existence of atypical antidepressants which can be effective despite not targeting this pathway.[80] One proposed explanation for the therapeutic lag, and further support for the deficiency of monoamines, is a desensitization of self-inhibition in raphe nuclei by the increased serotonin mediated by antidepressants.[81] However, disinhibition of the dorsal raphe has been proposed to occur as a result of decreased serotonergic activity in tryptophan depletion, resulting in a depressed state mediated by increased serotonin. Further countering the monoamine hypothesis is the fact that rats with lesions of the dorsal raphe are not more depressive than controls; the finding of increased jugular 5-HIAA in people who are depressed that normalized with selective serotonin reuptake inhibitor (SSRI) treatment, and the preference for carbohydrates in people who are depressed.[82] Already limited, the monoamine hypothesis has been further oversimplified when presented to the general public.[83] A 2022 review found no consistent evidence supporting the serotonin hypothesis linking serotonin levels and depression.[84] HPA-axis abnormalities have been suggested in depression given the association of CRHR1 with depression and the increased frequency of dexamethasone test non-suppression in people who are depressed. However, this abnormality is not adequate as a diagnosis tool because its sensitivity is only 44%.[85] These stress-related abnormalities are thought to be the cause of hippocampal volume reductions seen in people who are depressed.[86] Furthermore, a meta-analysis yielded decreased dexamethasone suppression, and increased response to psychological stressors.[87] Further abnormal results have been obscured with the cortisol awakening response, with increased response being associated with depression.[88] There is also a connection between the gut microbiome and the central nervous system, otherwise known as the Gut-Brain axis, which is a two-way communication system between the brain and the gut. Experiments have shown that microbiota in the gut can play an important role in depression, as people with MDD often have gut-brain dysfunction. One analysis showed that those with MDD have different bacteria in their guts. Bacteria Bacteroidetes and Firmicutes were most affected in people with MDD, and they are also impacted in people with irritable bowel syndrome.[89] Another study showed that people with IBS have a higher chance of developing depression, which shows the two are connected.[90] There is even evidence suggesting that altering the microbes in the gut can have regulatory effects on developing depression.[89] Theories unifying neuroimaging findings have been proposed. The first model proposed is the limbic-cortical model, which involves hyperactivity of the ventral paralimbic regions and hypoactivity of frontal regulatory regions in emotional processing.[91] Another model, the cortico-striatal model, suggests that abnormalities of the prefrontal cortex in regulating striatal and subcortical structures result in depression.[92] Another model proposes hyperactivity of salience structures in identifying negative stimuli and hypoactivity of cortical regulatory structures resulting in a negative emotional bias and depression, consistent with emotional bias studies.[93] Immune pathogenesis theories on depression The newer field of psychoneuroimmunology, the study between the immune system and the nervous system and emotional state, suggests that cytokines may impact depression. Immune system abnormalities have been observed, including increased levels of cytokines -cells produced by immune cells that affect inflammation- involved in generating sickness behavior, creating a pro-inflammatory profile in MDD.[94][95][96] Some people with depression have increased levels of pro-inflammatory cytokines and some have decreased levels of anti-inflammatory cytokines.[97] Research suggests that treatments can reduce pro-inflammatory cell production, like the experimental treatment of ketamine with treatment-resistant depression.[98] With this, in MDD, people will more likely have a Th-1 dominant immune profile, which is a pro-inflammatory profile. This suggests that there are components of the immune system affecting the pathology of MDD.[99] Another way cytokines can affect depression is in the kynurenine pathway, and when this is overactivated, it can cause depression. This can be due to too much microglial activation and too little astrocytic activity. When microglia get activated, they release pro-inflammatory cytokines that cause an increase in the production of COX2. This, in turn, causes the production of PGE2, which is a prostaglandin, and this catalyzes the production of indolamine, IDO. IDO causes tryptophan to get converted into kynurenine, and kynurenine becomes quinolinic acid.[100] Quinolinic acid is an agonist for NMDA receptors, so it activates the pathway. Studies have shown that the post-mortem brains of patients with MDD have higher levels of quinolinic acid than people who did not have MDD. With this, researchers have also seen that the concentration of quinolinic acid correlates to the severity of depressive symptoms.[101] Diagnosis Assessment Further information: Rating scales for depression Caricature of a man with depression A diagnostic assessment may be conducted by a suitably trained general practitioner, or by a psychiatrist or psychologist,[30] who records the person's current circumstances, biographical history, current symptoms, family history, and alcohol and drug use. The assessment also includes a mental state examination, which is an assessment of the person's current mood and thought content, in particular the presence of themes of hopelessness or pessimism, self-harm or suicide, and an absence of positive thoughts or plans.[30] Specialist mental health services are rare in rural areas, and thus diagnosis and management is left largely to primary-care clinicians.[102] This issue is even more marked in developing countries.[103] Rating scales are not used to diagnose depression, but they provide an indication of the severity of symptoms for a time period, so a person who scores above a given cut-off point can be more thoroughly evaluated for a depressive disorder diagnosis. Several rating scales are used for this purpose;[104] these include the Hamilton Rating Scale for Depression,[105] the Beck Depression Inventory[106] or the Suicide Behaviors Questionnaire-Revised.[107] Primary-care physicians have more difficulty with underrecognition and undertreatment of depression compared to psychiatrists. These cases may be missed because for some people with depression, physical symptoms often accompany depression. In addition, there may also be barriers related to the person, provider, and/or the medical system. Non-psychiatrist physicians have been shown to miss about two-thirds of cases, although there is some evidence of improvement in the number of missed cases.[108] A doctor generally performs a medical examination and selected investigations to rule out other causes of depressive symptoms. These include blood tests measuring TSH and thyroxine to exclude hypothyroidism; basic electrolytes and serum calcium to rule out a metabolic disturbance; and a full blood count including ESR to rule out a systemic infection or chronic disease.[109] Adverse affective reactions to medications or alcohol misuse may be ruled out, as well. Testosterone levels may be evaluated to diagnose hypogonadism, a cause of depression in men.[110] Vitamin D levels might be evaluated, as low levels of vitamin D have been associated with greater risk for depression.[111] Subjective cognitive complaints appear in older depressed people, but they can also be indicative of the onset of a dementing disorder, such as Alzheimer's disease.[112][113] Cognitive testing and brain imaging can help distinguish depression from dementia.[114] A CT scan can exclude brain pathology in those with psychotic, rapid-onset or otherwise unusual symptoms.[115] No biological tests confirm major depression.[116] In general, investigations are not repeated for a subsequent episode unless there is a medical indication. DSM and ICD criteria
Major depressive disorder ( MDD ), also known as clinical depression , is a mental disorder [ 10 ] characterized by at least two weeks of pervasive low mood , low self-esteem , and loss of interest or pleasure in normally enjoyable activities. Introduced by a group of US clinicians in the mid-1970s, [ 11 ] the term was adopted by the American Psychiatric Association for this symptom cluster under mood disorders in the 1980 version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III), and has become widely used since. The disorder causes the second-most years lived with disability , after lower back pain . [ 12 ] The diagnosis of major depressive disorder is based on the person's reported experiences, behavior reported by family or friends, and a mental status examination . [ 13 ] There is no laboratory test for the diso...