Objectives: To investigate whether the course of bipolar disorder (BD) type II is more depressive than that of BD I, and, if so, to explore the underlying factors that cause this difference.
Methods: In a prospective, naturalistic study of 191 secondary care psychiatric in- and outpatients diagnosed in an acute phase of BD I or II, 160 patients (85.1%) were followed for 18 months. Using a life chart, the exact timing of symptom states in follow-up was examined. Differences between BD I (n = 75) and II (n = 85) in duration of index phase and episode, time to full remission and recurrence, and time in any mood episode were investigated.
Results: Patients with BD II spent a higher proportion of time ill (47.5% versus 37.7%; p = 0.02) and in depressive symptom states (58.0% versus 41.7%; p = 0.003) than BD I patients. This was a result of the higher proportion (61.7% versus 48.6%; p = 0.03) and mean number (1.69 versus 1.11; p = 0.006) of depressive illness phases in BD II, rather than of differences in the duration of depressive phases. Type of index phase strongly predicted the outcome. In linear regression models, both BD II and type of index phase predicted more time spent in depressive symptom states.
Conclusions: In medium-term follow-up, BD II patients spend about 40% more time in depressive symptom states than BD I patients because a higher proportion of BD II patients have depressive phases and the frequency of these is higher. Differences in type of index phase may markedly confound differences in outcome between BD I and II.