Hypocalcaemia may reduce the beneficial effect of magnesium treatment in aneurysmal subarachnoid haemorrhage.

To assess whether magnesium treatment in patients with subarachnoid haemorrhage (SAH) is associated with hypocalcaemia and whether hypocalcaemia in these patients is associated with an increased risk of delayed cerebral ischemia (DCI) and poor outcome. All 137 patients randomized in the clinically controlled "Magnesium in Aneurysmal Subarachnoid Haemorrhage" trial were included. The relationship between mean serum magnesium and mean serum calcium during treatment was assessed with linear regression. The relationship between hypocalcaemia (serum calcium < 2.0 mmol/L) during treatment and the occurrence of DCI and poor outcome was studied with the Cox proportional hazards method and logistic regression, respectively. There was a statistically significant inverse relation between elevated serum magnesium and hypocalcaemia (B = -0.27; 95% CI, -0.33 to -0.20; p < 0.001). Patients with hypocalcaemia during study treatment had an increased frequency of DCI (HR 2.1; 95% CI, 1.0 to 4.3), and an increased risk for poor outcome (OR 2.9; 95% CI, 1.4 to 6.4), but this effect attenuated in the multivariable analysis (OR 1.9; 95% CI, 0.8 to 4.7). In conclusion, prolonged elevated serum magnesium is associated with hypocalcaemia. Hypocalcaemia is associated with an increased risk of DCI and poor outcome and may therefore reduce the potential beneficial effect of magnesium treatment in SAH.