BACKGROUND AND PURPOSE: Pilot clinical trials using magnesium sulfate in 
patients with acute aneurysmal subarachnoid hemorrhage have reported trends 
toward improvement in clinical outcomes. This Phase III study aimed to compare 
intravenous magnesium sulfate infusion with saline placebo among such patients.
METHODS: We recruited patients with aneurysmal subarachnoid hemorrhage within 48 
hours of onset from 10 participating centers. The patients were randomly 
assigned to magnesium sulfate infusion titrated to a serum magnesium 
concentration twice the baseline concentration or saline placebo for 10 to 14 
days. Patients and assessors were blinded to treatment allocation. The study is 
registered at www.strokecenter.org/trials (as Intravenous Magnesium Sulphate for 
Aneurysmal Subarachnoid Hemorrhage [IMASH]) and www.ClinicalTrials.gov 
(NCT00124150).
RESULTS: Of the 327 patients recruited, 169 were randomized to receive treatment 
with intravenous magnesium sulfate and 158 to receive saline (placebo). The 
proportions of patients with a favorable outcome at 6 months (Extended Glasgow 
Outcome Scale 5 to 8) were similar, 64% in the magnesium sulfate group and 63% 
in the saline group (OR, 1.0; 95% CI, 0.7 to 1.6). Secondary outcome analyses 
(modified Rankin Scale, Barthel Index, Short Form 36, and clinical vasospasm) 
also showed no significant differences between the 2 groups. Predefined 
subgroups included age, admission World Federation of Neurological Surgeons 
grade, pre-existing hypertension, intracerebral hematoma, intraventricular 
hemorrhage, location of aneurysm, size of aneurysm, and mode of aneurysm 
treatment. In none of the subgroups did the magnesium sulfate group show a 
better outcome at 6 months.
CONCLUSIONS: The results do not support a clinical benefit of intravenous 
magnesium sulfate infusion over placebo infusion in patients with acute 
aneurysmal subarachnoid hemorrhage.