Diagnostic conversion from depression to bipolar disorders: results of a long-term prospective study of hospital admissions.

OBJECTIVES: To analyse the time course and some risk factors for a diagnostic change from major depression to bipolar disorders (BP) over an average of 20 years from the onset of the disorders.
METHODS: Patients (406) with major mood disorders hospitalised at some time between 1959 and 1963 were followed-up until 1985. The analysis also included the course prior to hospitalisation. Survival analyses and Cox regression models were applied.
RESULTS: A diagnostic change from depression to bipolar I occurred in about 1% of the patients per year and to bipolar II disorders in about 0.5% per year. Risk factors for a change from depression to BP-I disorder were male sex and an early onset of the disorder; risk factors for a change from depression to BP-II disorder were female sex, a later onset of the disorder and a positive family history of mania.
CONCLUSIONS: Across the entire lifetime, every new episode of depression brings a new risk for mania; more than half of our severe mood disorder cases became bipolars. The risk of depression developing into bipolar disorder remains constant lifelong. LIMITATIONS: The diagnostic classification of ICD-9 met RDC criteria for bipolar disorder in only 90% of cases. Part of the data collected in retrospect may be less reliable; the prospective data were only collected every 5 years from 1965 to 1985 using multiple sources; mild manifestations between the follow-ups may have been partially missed. The sample of subsequent hospital admissions for major depression and mania represents a severe group of patients and generalisations to ambulatory cases may not be possible. Not all risk factors for diagnostic conversion described in the literature could be assessed in this study.