BACKGROUND AND PURPOSE: Magnesium reverses cerebral vasospasm and reduces 
infarct volume after experimental subarachnoid hemorrhage (SAH) in rats. We 
aimed to assess whether magnesium reduces the frequency of delayed cerebral 
ischemia (DCI) in patients with aneurysmal SAH.
METHODS: Patients were randomized within 4 days after SAH. Magnesium sulfate 
therapy consisted of a continuous intravenous dose of 64 mmol/L per day, to be 
started within 4 days after SAH and continued until 14 days after occlusion of 
the aneurysm. The primary outcome DCI (defined as the occurrence of a new 
hypodense lesion on computed tomography compatible with clinical features of 
DCI) was analyzed according to the "on-treatment" principle. For the secondary 
outcome measures "poor outcome" (Rankin >3) and "excellent outcome" (Rankin 0), 
we used the "intention-to-treat" principle.
RESULTS: A total of 283 patients were randomized. Magnesium treatment reduced 
the risk of DCI by 34% (hazard ratio, 0.66; 95% CI, 0.38 to 1.14). After 3 
months, the risk reduction for poor outcome was 23% (risk ratio, 0.77; 95% CI, 
0.54 to 1.09). At that time, 18 patients in the treatment group and 6 in the 
placebo group had an excellent outcome (risk ratio, 3.4; 95% CI, 1.3 to 8.9).
CONCLUSIONS: This study suggests that magnesium reduces DCI and subsequent poor 
outcome, but the results are not yet definitive. A next step should be a phase 
III trial to confirm the beneficial effect of magnesium therapy, with poor 
outcome as primary outcome.