To grasp some of the complexities of MDI it is best to focus our attention on the following aspects of this illness.
Polarity of MDI is the predominant clinical feature. Mania and Depression are the ones that have been studied most extensively.
Mixed States are the result of coexisting and overlapping symptoms of Mania and Depression. One can observe forms of MDI where the polarity alternates from Mania to Depression, others where the presentation is consistently of the same polarity (i.e., chronic Mania/Hypomania, recurrent Depression or recurrent Mixed States).
Severity of the symptomatic and functional impairment determines the severity of MDI. One can encounter forms where the symptoms are so mild that might go undetected while the extent of the disability produced is severe. Also, common forms are where extremely severe symptoms during episodes remit completely, causing limited disability.
Duration of symptomatic phases can range from minutes, hours, days, months or years. In children and adolescents it is common to observe very rapid shifts of mood and energy lasting only minute. While in older patients, phases of MDI can last for years. Modern treatments have significantly affected the duration of MDI’s phases and its course.
Cyclicity describes the tendency of MDI to manifest in cycles of episodes or phases, with or without intervals, and to recur with different frequency. Some forms only recur every few years. Others recur regularly with a seasonal pattern or follow a rapid-cycling pattern (four or more episodes in 12 months).
Polarity and Severity are symptomatic variables.
In the course of a manic or depressive episode, one can experience symptoms of different severity causing different degrees of disability.
Duration and Cyclicity are course variables.
Short phases tend to occur in a rapid cycling course, as opposed to phases lasting for months in those with a seasonal course of illness.