Intrathecal magnesium as analgesic adjuvant for spinal anesthesia: A meta-analysis of randomized trials

Background. The efficacy and safety of intrathecal magnesium as analgesic adjuvant has been tested by several clinical trials in recent years. We performed a meta-analysis of the available literature. Methods. Randomized clinical trials comparing a 50 to 100 mg dose of intrathecal magnesium sulfate versus placebo in addition to an intrathecal local anesthetic and/or opiate for a below-umbilicus procedure were included. Medline, LILACS, Cochrane Library and Google Scholar databases were searched. A random analysis was performed and heterogeneity was tested for. The size of the effect for quantitative outcomes was calculated as standard mean difference (SMD, neutral=0); and as odds ratio (OR, neutral= 1) for dichotomous outcomes. Results. Twelve studies totaling 817 patients were included. The "time to first analgesia request" was at least 35 minutes longer when intrathecal magnesium was included in the intervention (SDM 0.94, 95%CI 0.51 to 1.37, P<0.001). The "onset time to sensory block" (SDM 0.64, 95%CI 0.15 to 1.12, P=0.01) and the "time to maximal motor block" (SDM 0.97, 95%CI 0.28 to 1.67, P=0.006) were 2.4 minutes slower with intrathecal magnesium. There was no difference in "time to full motor recovery, incidence of pruritus, postoperative nausea and vomiting, bradicardia, low blood pressure and urinary retention". No cases of respiratory depression or neurotoxicity were recorded in these studies. Conclusion. The inclusion of 50 to 100 mg of intrathecal magnesium in a spinal anesthetic prolongs opiate analgesia duration; no safety concerns have been identified by the included clinical studies but additional evidence is advised.