Before reviewing the differential diagnosis of bipolar disorder, it is important to understand the epidemiology, incident, and prevalence of the disorder as well as to divide it by type: type I, type II, or the phenomenon of not otherwise specified, which is defined as subthreshold mania or hypomania. Subthreshold mania or hypomania entails fewer than the requisite number of Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition symptoms of mania or hypomania, or an insufficient duration of time with which to make a diagnosis of a syndrome of mania or hypomania. Currently, the most reliable epidemiology of prevalence rates were reported in a replication study of 9,282 respondents to the National Comorbidity Survey conducted by Merikangas and colleagues.
Those data show there is a lifetime prevalence rate of bipolar I disorder of 1 % and bipolar II disorder of 1.1%. In this study, not otherwise specified was labeled subthreshold bipolar disorder as the illness duration was not long enough or the number of symptoms was not extensive enough to constitute syndromal mania or hypomania. This type had a prevalence rate of 2.4%, for a total lifetime prevalence of ~4.4%. This rate is somewhat higher than has been described in the past, but included the subdivision that researchers believe comprises the bipolar spectrum.
There has been much underdiagnosis or lack of diagnostic accuracy of bipolar disorder, with relative overdiagnosis of major depressive disorder, which historically has been well documented. However, in the last 10 years, there has been a substantial increase in the making of diagnoses of bipolar disorder in various clinical settings. One dataset examined ambulatory medical care survey data and found that among adults over an ~10-year period, there was approximately a doubling in the incidence of diagnosing bipolar disorder of any type (Slide 1).