The number of people classified as suffering with Mania, Hypomania and other forms of MDI with recurrent mood changes depends on the diagnostic criteria used.
The DSM (Diagnostic and Statistical Manual), used to make the diagnosis requires a minimum duration of four days of manic symptoms for a diagnosis to be made, if the patient is not hospitalized.
In two large studies using DSM criteria, the rate of Mania and Hypomania was 1.2% and 1.6%. (-Regier, -Kessler)
When DSM duration criteria for Mania and Hypomania are removed, to include sub-threshold manifestations of MDI, rates of 5.0-8.3% have been reported. (-Lewinsohn, -Szadoczky, -Angst)
Among researchers, forms of illness that fall short of meeting all criteria are called sub-threshold. Inclusion in the broader umbrella of MDI of sub-threshold forms of Mania and Hypomania are justified by several similarities between patient meeting the duration criteria for these symptoms and those that do not. These similarities include:
-the presence of a positive family history of mood disorders,
-a history of suicide attempts and treatment for depression, and
-comorbidity with anxiety and substance abuse disorders.
The follow-up of subjects with Hypomania, Cyclothymia and other sub-syndromal forms -- not meeting the duration criteria of DSM, has revealed the continuity of these forms with those meeting the diagnostic criteria. The evidence appears to justify the use of a broader definition of MDI than the narrow one used by DSM.