Yearly zoledronic acid in postmenopausal osteoporosis.

To the Editor: In their report on the Health Outcomes and Reduced Incidence with Zoledronic Acid Once Yearly (HORIZON) Pivotal Fracture Trial, Black et al. (May 3 issue)1 conclude that a onceyearly infusion of zoledronic acid reduces the risk of vertebral, hip, and other fractures. Although this is a well-designed and well-done study, we are concerned about the increase in atrial fibrillation in patients treated with zoledronic acid. Hypocalcemia as a cause was ruled out. There is no report of hypokalemia during the study. Another electrolyte imbalance, hypomagnesemia, is reported as a side effect of zoledronic acid2 and can cause cardiac arrhythmias.3,4 It would be interesting to know the incidence of hypomagnesemia during the HORIZON study, mainly among the patients with atrial fibrillation. Forty-nine patients in the zoledronic-acid group had transient hypocalcemia at days 9 to 11 after the infusion. There are several case reports describing hypocalcemia during treatment with zoledronic acid, inducing a compensatory increase in parathyroid hormone.5 In the study by Black et al., however, we are not informed about the mean change in calcium and parathyroid hormone concentrations during treatment.