Commentary

Delirium as a result of psychotropic medication in the elderly can be a toxic side effect that makes caring for these patients more difficult or even medically dangerous. In recent years (as reviewed by the authors), the rate of prescribing valproate to elderly people with psychiatric illness has greatly increased, while the rate of prescription of lithium to this population has decreased. This change in prescription rates may reflect physician concern that lithium is more likely than valproate to result in delirium.

The literature on this issue is mixed, with Patten et al reporting an association between lithium and a higher risk of delirium, whereas Ritchie et al found no such increase. The current study made use of a large database with a good capture of population data derived from a single payer healthcare model in Ontario. They found that valproate and lithium did not significantly differ in the risk of incident cases of delirium, and both agents were much less likely associated with delirium than the known deliriogenic agent benztropine. Because of the other benefits of lithium, for example suicide risk reduction, avoidance of lithium in elderly mood disordered people might not be justified solely on the basis of concern of increased delirium risk.

The main clinical implications of this study, if replicated, may be that lithium (when carefully dosed and closely monitored) could be safely used in elderly people with mood disorders, in whom it may have a neuroprotective effect as well as a mood stabilisation effect. In addition, valproate may not be as free of delirium risk as previously thought. Certainly, both of these medications should be used cautiously in older people with mood disorders, with careful monitoring of serum drug levels and other laboratory parameters, and appropriate screening for delirium with acute mental status changes. Avoidance of benztropine in elderly patients on the basis of delirium risk is to be encouraged.