- Characterized by persistent low mood, loss of interest and enjoyment, and reduced energy.
- Common in primary care, affecting 5% to 10% of patients in this setting.
- Often have a personal or family history of depression; have experienced a recent stress, trauma, or loss; or have comorbid medical illness.
- Most patients respond well to psychotherapy, antidepressants, or a combination of both.
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 minor depression, dysthymia, and major depressive disorder, depending on the length and number of symptoms, the degree of functional impairment, and the severity of symptoms.   In bipolar disorder, major depressive disorder is accompanied by or interspersed with 1 or more manic or mixed episodes.
Minor depression is characterized by 2 to 4 depressive symptoms, including depressed mood or anhedonia, of greater than 2 weeks' duration.  Dysthymic disorder is characterized by at least 2 years of 3 or 4 dysthymic symptoms for more days than not.  Major depressive disorder is characterized by at least 5 symptoms 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. 
Depression in children and adolescents may have a more insidious onset than in adults. It may be characterized more by irritability than sadness, and occurs more often in association with other conditions such as anxiety, conduct disorder, hyperkinesis and learning problems.  
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. 
Symptoms generally develop within 4 weeks' postpartum and are present every day for 2 weeks or more. Characteristics of postpartum depression 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-IV TR 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.
Suicidal ideation 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.
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