Does the risk of developing dementia increase with the number of episodes in patients with depressive disorder and in patients with bipolar disorder?

Objective: Several findings suggest that some patients with depressive or bipolar disorder may be at increased risk of developing dementia. The present study aimed to investigate whether the risk of developing dementia increases with the number of affective episodes in patients with depressive disorder and in patients with bipolar disorder. Methods: This was a case register study including all hospital admissions with primary affective disorder in Denmark during 1970-99. The effect of the number of prior episodes leading to admission on the rate of readmission with a diagnosis of dementia following the first discharge after 1985 was estimated. A total of 18 726 patients with depressive disorder and 4248 patients with bipolar disorder were included in the study. Results: The rate of a diagnosis of dementia on readmission was significantly related to the number of prior affective episodes leading to admission. On average, the rate of dementia tended to increase 13% with every episode leading to admission for patients with depressive disorder and 6% with every episode leading to admission for patients with bipolar disorder, when adjusted for differences in age and sex. Conclusion: On average, the risk of dementia seems to increase with the number of episodes in depressive and bipolar affective disorders.