Bipolar disorders usually start in late adolescence and young adulthood. These illnesses, however, can appear any time between ages 5 and 50 and in rare cases beyond the age of 50. Research indicates that between 25 and 30% of the people who develop manic-depression as adults had one or more related symptoms before their 6th birthday. The more severe forms of bipolar illness (bipolar I disorder) are considered to be rare in prepubertal children. Only about 0.6% of adolescents are thought to have a bipolar I diagnosis, but estimates of teens with bipolar II disorder have reached as high as 10%.
Estimates of the number of people with a bipolar disorder vary. This occurs in part because of diagnostic difficulties and because of the fact that many people who have mild symptoms either do not seek or do not receive professional attention. At any given time, about 8% of America's population is at risk for developing a mood disorder. Most studies estimate that between 1 and 2.5% of the U.S. population has a bipolar disorder. A representative number of studies estimate that the prevalence of bipolar disorders is 36.5% of the U.S. population. Unlike unipolar depression, bipolar disorders are found equally in females and males. Between 5 and 20% of adult cases that are first diagnosed as unipolar depression over time will receive a reevaluated diagnosis of bipolar disorder.
Science has gained a large amount of information about manic-depressive illness but has been unable to identify specifically how the illness starts. There are most likely several causes for this syndrome of disorders. Studies of families, twins, and adoptions suggest that most cases of manic-depression are genetically inherited. There is general agreement among researchers that genetic components play a more significant role in transmitting bipolar I disorder than major depressive disorder, but this perspective continues to be debated. Family studies report that having a first-degree relative with bipolar I disorder increases the chances of developing manic-depressive illness by 8-18 times over families with no first-degree members having a bipolar disorder history. The likelihood of developing bipolar I disorder is 1.5-2.5 times greater if a first-degree family member has a major unipolar depressive disorder. Perhaps more illustrative of the genetic relationship to manic-depression is the fact that 50% of all people with a bipolar I disorder have at least one parent with a mood disorder. Additionally, a 25% probability exists of a child developing bipolar I disorder if one parent has this form of manic-depressive illness, and a 50-75% chance exists if both parents have bipolar I disorder. There is only a limited amount of information from adoption studies on bipolar disorders. The available data document that children adopted as infants from biological parents with a major mood disorder remain at an increased risk for developing bipolar disorders. The link between genetics and manic-depression has also been established through the study of twins. Monozygotic twins show a concordance rate for bipolar I disorder of 75%, whereas the concordance rate for bipolar I disorder in dizygotic twins drops to 20%. In addition, scientists have hypothesized that some cases of bipolar disorder may not stem from a genetic transmission. Researchers have found indications that bipolar disorders may be caused by in utero neuroviruses that attack the fetus' forming brain. There is currently growing interest in the role neuro-viruses and immunologic abnormalities play in the formation and development of major mental disorders, including manic-depressive illness and schizophrenia.
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