Last Updated: 2013-12-10
Depression is a mental state characterized by persistent low mood, loss of interest and enjoyment in everyday activities, neurovegetative disturbance, and reduced energy, causing varying levels of social and occupational dysfunction. Depressive disorders are common, and they are the fourth-highest cause of disability worldwide.   In people aged 18 to 44 years, depression is the leading cause of disability and premature death, and it is predicted to be the second-leading cause of disability in people of all ages by the year 2020. 
Depressive disorders may be divided into a number of disorders, depending on the length and number of symptoms, the degree of functional impairment, and the severity of symptoms.   In bipolar disorder, a manic episode may have been preceded by and may be followed by hypomanic or major depressive episodes. 
Major depressive disorder is characterized by at least 5 symptoms during the same 2-week period and may be further categorized as mild, moderate, or severe. The greater the number of symptoms, the more severe the depression.  Psychotic features (e.g., hallucinations or paranoia) may be present. 
Dysthymia (persistent depressive disorder) is characterized by at least 2 years of 3 or 4 dysthymic symptoms for more days than not. The symptoms are not as severe as those in major depressive disorder. 
The Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) does not recognize postpartum depression as a separate diagnosis; rather, patients must meet the criteria for a major depressive episode and the criteria for the peripartum-onset specifier. The definition is therefore a major depressive episode with an onset during pregnancy or within 4 weeks of delivery. 
Symptoms generally develop during pregnancy or within 4 weeks postpartum and are present every day for 2 weeks or more. Characteristics of postpartum depression may include guilt about the depressive symptoms, ambivalent feelings toward the infant, impaired bonding, and obsessive ruminations, including intrusive thoughts about harming the infant. Postpartum depression should be distinguished from postpartum blues ("baby blues"), in which the symptoms generally resolve within 2 weeks.
Premenstrual syndrome (PMS) is characterized by cyclical physical and behavioral symptoms occurring in the luteal phase of the menstrual cycle (the period between ovulation and onset of menstruation). Premenstrual dysphoric disorder (PMDD) is a more severe variant that includes at least 1 affective symptom. Depression may coexist with PMS or PMDD in up to 50% of cases. A diagnosis of PMS or PMDD may predate a diagnosis of depression. 
Seasonal affective disorder (SAD) is a subtype of major depression, occurring with seasonal change. SAD occurs more commonly in high latitudes. Lifetime estimates for depressive and bipolar disorders with a seasonal pattern average between 0.4% and 2.9% in US, Canadian, and UK community studies.    Some estimates may be as high 9.7%.  However, these differences are probably due to differences in the sampling and diagnostic criteria used.
Bipolar affective disorder is major depressive disorder accompanied by, or interspersed with, 1 or more manic or mixed episodes. Repeated episodes of hypomania or mania only are also classified as bipolar disorder. 
The DSM-5 does not distinguish in its definition between bipolar disorders in adults and children.  The prevalence in children is lower than in adults, though the disease can be more severe and the cycles between mania and depression much quicker.
Suicide risk management may constitute part of a number of mental disorders, or it may be more isolated as a symptom of an emotional state.  Suicide is one of the leading causes of death in the US, Canada, the UK, Australia, and New Zealand. In people ages 15 to 44 years, self-inflicted injury is the fourth-leading cause of death and the sixth-leading cause of ill health and disability worldwide, making suicide a significant public health concern.