Long-term depression versus episodic major depression: results from the prospective Zurich study of a community sample

BACKGROUND: Clinical studies have demonstrated a great clinical relevance of long-term depression (LTD). Our study aims to characterise LTD in comparison with episodic (non-chronic) major depressive episodes (MDE) on the basis of data from a community sample. METHOD: 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). LTD was assessed from age 27/28 to 40/41 and defined as being symptomatic more days than not over 2 years plus the presence of work impairment. MDE and dysthymia were defined by DSM-III-R criteria. RESULTS: The cumulative incidence of LTD was 5.7%, and of episodic MDE 20.9%. In both groups we found a similar preponderance of women. LTD subjects reported disturbed memory, feelings of inferiority, hopelessness, fear of everyday tasks, fear of being alone and thoughts of dying significantly more often than subjects with episodic MDE. Subjects with LTD had an earlier age of onset. 82% of them were treated over lifetime for depression compared to 61% with MDE. LTD subjects were less often married, less often in fulltime employment, more often unemployed, and more often receiving social benefits. LTD was comorbid with cardiac and respiratory syndromes, and LTD subjects were more frequently treated for insomnia and pain. They suffered significantly more from social phobia and benzodiazepine abuse; there was also a statistical trend to greater comorbidity with panic attacks, agoraphobia, obsessive-compulsive syndrome, binge eating and neurasthenia. Somatic and psychological well-being were also reduced. CONCLUSIONS: LTD is common, clinically more serious than episodic MDE and highly comorbid. LIMITATIONS: The sample is relatively small with an attrition rate of 38.5% over 20 years. The results cannot be generalised to persons over 40 years of age and may be dependent on the definition of LTD