The lifetime prevalence of Bipolar I Disorder in community samples has varied from 0.4% to 1.6%.
Average age at onset is 20 for both men and women. Bipolar I Disorder is a recurrent disorder—more than 90% of individuals who have a single Manic Episode go on to have future episodes. Roughly 60%-70% of Manic Episodes occur immediately before or after a Major Depressive Episode. Manic Episodes often precede or follow the Major Depressive Episodes in a characteristic pattern for a particular person.
The number of lifetime episodes (both Manic and Major Depressive) tends to be higher for Bipolar I Disorder compared with Major Depressive Disorder, Recurrent. Studies of the course of Bipolar I Disorder prior to lithium maintenance treatment suggest that, on average, four episodes occur in 10 years. The interval between episodes tends to decrease as the individual ages. There is some evidence that changes in sleep-wake schedule such as occur during time zone changes or sleep deprivation may precipitate or exacerbate a Manic, Mixed, or Hypomanic Episode. Approximately 5%-15% of individuals with Bipolar I Disorder have multiple (four or more) mood episodes (Major Depressive, Manic, Mixed, or Hypomanic) that occur within a given year. If this pattern is present, it is noted by the specifier With Rapid Cycling. A rapid-cycling pattern is associated with a poorer prognosis.
Although the majority of individuals with Bipolar I Disorder experience significant symptom reduction between episodes, some (20%-30%) continue to display mood lability and other residual mood symptoms. As many as 60% experience chronic interpersonal or occupational difficulties between acute episodes. Psychotic symptoms may develop after days or weeks in what was previously a nonpsychotic Manic or Mixed Episode. When an individual has Manic Episodes with psychotic features, subsequent Manic Episodes are more likely to have psychotic features. Incomplete interepisode recovery is more common when the current episode is accompanied by mood-incongruent psychotic features.
First-degree biological relatives of individuals with Bipolar I Disorder have elevated rates of Bipolar I Disorder (4%-24%), Bipolar II Disorder (1%-5%), and Major Depressive Disorder (4%-24%). Those individuals with Mood Disorder in their first-degree biological relatives are more likely to have an earlier age at onset. Twin and adoption studies provide strong evidence of a genetic influence for Bipolar I Disorder.