Clinical studies have demonstrated the great clinical relevance of long-term depression (LTD). Our study aims to characterise long-term bipolar disorder (LTBP) in comparison with episodic (non-chronic) bipolar major depressive episodes (EBP) on the basis of data from a community sample.

The Zurich Cohort Study is a prospective study of young adults followed from age 20/21 to 40/41 with six interviews. The stratified sample consisted of two thirds high scorers and one third lower scorers on the Symptom Checklist-90 R (SCL-90-R). LTBP, which was assessed from age 27/28 to 40/41, was defined as threshold or subthreshold depressive syndrome plus hypomanic symptoms, present for more days than not over the past year, plus work or social impairment.

The cumulative incidence of LTBP was 6.3%, and of episodic BP 15.3%. The two groups did not differ in mean age of onset (13.7 vs. 13.1 years). Age of onset was significantly earlier in subjects with a positive FH of depression and was inversely correlated with the somatisation score of the SCL-90R as assessed at age 19/20. The age of onset increased with increasing levels of childhood family problems in subjects with a positive FH, but decreased in subjects without an FH of depression. Chronic BP was associated with early childhood adversity and low self-esteem and, at a trend-level, with no family history of depression.

The results are surprising and counter-intuitive, a genetic disposition seeming to be correlated more with periodicity than with chronicity.