Incidence of delirium in older adults newly prescribed lithium or valproate: a population-based cohort study

Background — The use of lithium carbonate for the treatment of mood disorders in old age has decreased at a dramatic rate in favor of valproate. Because of lithium's narrow therapeutic range, neurotoxicity can be an important complication in lithium therapy and potentially influence prescription patterns. Therefore, we compared the incidence of delirium in older adults with mood disorders who were newly dispensed either lithium or valproate.

Method — Using 4 population-based administrative databases from the province of Ontario, Canada (the Ontario Drug Benefit program, the Canadian Institute for Health Information, the Ontario Health Insurance Plan, and the Registered Persons Data Base), we were able to identify a cohort of mood disorder patients 66 years and older who were newly dispensed lithium or valproate over an 8-year period (1993-2001). Measures were taken to ensure that the sample was composed of mood disorder patients. As a comparator, we included a known deliriogenic drug, benztropine. The main outcome measure was a new diagnosis of delirium on a hospitalization record during 1 year of follow-up.

Results — Our study cohort consisted of 2422 new users of lithium and 2918 new users of valproate over an 8-year period. There was no statistically significant difference in the incidence of delirium between lithium (2.8 per 100 person-years) and valproate (4.1 per 100 person-years). Compared with patients who received lithium, patients who received benztropine had a significantly higher risk of delirium (p < .001).

Conclusion — The incidence of hospitalizations with delirium was similar in patients treated with lithium and valproate. These findings add to the evidence suggesting that the shift away from the use of lithium carbonate to manage mood disorders in older adults is not justified on the basis of concerns of neurotoxicity.