Rochon PA et al. (2005) Atypical antipsychotics and parkinsonism. Arch Intern Med 165: 1882–1888

A recent retrospective cohort study using data from Ontario, Canada found that, contrary to previous suggestions, atypical antipsychotic drugs are not always associated with a lower risk of parkinsonism than typical antipsychotics.

Rochon and colleagues identified 57,838 Ontario residents aged 66 years or older who had dementia with no evidence of schizophrenia or major depressive disorder. Of these, 25,769 were being treated with antipsychotic therapy—11,571 (45%) were receiving an atypical antipsychotic (e.g. risperidone) and 14,198 (55%) a typical antipsychotic (e.g. haloperidol). The subjects were assessed for evidence of parkinsonism during a period of continuous antipsychotic use lasting up to 1 year; those not receiving antipsychotic treatment acted as controls. The researchers used Cox proportional hazard models to study whether there was an association between the occurrence of parkinsonism and the type, dose and potency of the drug being taken by a patient.

Although patients receiving a typical antipsychotic were 30% more likely overall to develop parkinsonism than those receiving an atypical antipsychotic (adjusted hazard ratio 1.30; 95% CI 1.04–1.58), when the researchers looked at those treated with a high-dose atypical antipsychotic, they found that there was a similar risk of parkinsonism development for these patients and those receiving a typical antipsychotic (P = 0.7).

The authors conclude that the dispensing of atypical antipsychotics, particularly when given at a high dose, should be undertaken with caution because of the risk of developing parkinsonism.