Depression is a mental state characterised 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 in people of all ages and may be classified depending on the duration, severity, and number of symptoms, and the degree of functional impairment.  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 characterised by at least 5 symptoms and can be classified along a spectrum of mild to severe. Severe episodes may include psychotic symptoms such as paranoia, hallucinations, or functional incapacitation. 
One of the most common paediatric psychiatric disorders, especially in girls during adolescence. Depression in children and adolescents may have a more insidious onset than in adults. It may be characterised more by irritability than sadness, and it often occurs in association with other conditions such as anxiety.  
Disruptive mood dysregulation disorder is a category of depressive disorders first diagnosed at 6 to 18 years of age, with age of onset before 10 years. It is characterised by the DSM-5 as severe and persistent irritability or angry mood nearly every day, and severe and recurrent temper outbursts on average 3 or more times per week for at least 1 year. 
Common form of depression, but lasting longer than acute major depressive disorder. The DSM-5 developed new diagnostic criteria for persistent depressive disorder that included both chronic major depressive disorder and the previous category of dysthymic disorder (dysthymia), or chronic low-grade depression. The DSM-5 included specifiers to identify different pathways to the diagnosis of persistent depressive disorder and various presentations based on severity and clinical characteristics.
The DSM-5 does not recognise 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. Therefore, according to the DSM-5, the definition is a major depressive episode with an onset in pregnancy or within 4 weeks of delivery,  although definitions of the length of the postnatal period vary and may encompass up to 12 months following delivery.
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 characterised by cyclical physical and behavioural 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 and bipolar disorder, occurring with seasonal change over at least a 2-year period. Most commonly presents with onset of depression in the autumn or winter, and full remission of symptoms over the spring or summer. 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.
A recurrent and sometimes chronic mental illness, bipolar affective disorder is marked by alternating periods of mood elevation (mania or hypomania) and depression, associated with a change or impairment in functioning. The long-term course of illness is characterised by a predominance of depression, although a history of at least one manic, hypomanic, or mixed episode is required to make the diagnosis of a bipolar disorder.
Bipolar disorder, type I: at least one manic or mixed episode.
Bipolar disorder, type II: has never had a full manic episode; at least one hypomanic episode and at least one major depressive episode. 
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, but becomes more frequent in teenagers. In children and adolescents, the disease can be more severe and the cycles between mania and depression much quicker. The diagnosis can be controversial, as criteria overlap with other childhood conditions such as ADHD and oppositional defiant disorder.
Suicide is an important causes of death globally. In people aged 15 to 44 years, suicide 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. Suicide risk management refers to the identification, assessment, and treatment of a person exhibiting suicidal behaviour. It is an ongoing process in the treatment of a person who has a mental disorder, but may also be relevant for those with no previous psychiatric diagnosis.
BMJ Publishing Group
This overview has been compiled using the information in existing sub-topics.
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