Magnesium Bier's block for treatment of chronic limb pain: a randomised, double-blind, cross-over study.

Magnesium is a physiological antagonist of both calcium and the NMDA receptor. Patients with chronic pain of a limb (>1 month's duration) were treated with two Bier's blocks (250 mmHg, 10 m) in a randomised, double-blind, cross-over design. They received once 20% magnesium sulphate (500 mg) + lignocaine 1% (75 mg), and once physiological saline + lignocaine 1% (75 mg). The volume of both treatments was 10 ml. Efficacy data from 49 treatments (25 magnesium, 24 placebo) could be analysed. With magnesium-lignocaine, the duration of pain relief as reported by the patients was on average 23 days (95% CI 8-38) compared with 6 days (95% CI 2-10) with lignocaine alone (P = 0.043). With magnesium-lignocaine, 54.2% of patients had more than 50% pain relief compared with 25% with lignocaine alone (number-needed-to-treat 3.5, P = 0.075). With magnesium-lignocaine, 20% of patients had a treatment failure (i.e. pain relief <24 h) compared with 50% with lignocaine alone (number needed-to-treat 3.3, 95% CI 1.8-29, P = 0.038). The magnesium-lignocaine treatment was painful in 52% of patients compared with 8% with lignocaine alone (number-needed-to-harm 2.3, 95% CI 1.5-4.5, P =0.0008). For patients with chronic limb pain, the addition of magnesium to a Bier's block with lignocaine improves and prolongs pain relief and reduces the number of treatment failures. The optimal dose of lignocaine to prevent magnesium-induced aching of the treated limb needs to be established. Bier's block with magnesium-lignocaine may provide a possible treatment alternative in these patients.