The effect of magnesium sulfate on maternal serum electrolytes and parathyroid hormone in pre-eclampsia

To evaluate possible interactions of magnesium sulfate-the drug of choice in the management of pre-eclampsia/eclampsia-in response to a few case reports that revealed maternal electrolyte disturbances, especially symptomatic changes, following magnesium sulfate administration in pre-eclampsia.

Prospectively, women with pre-eclampsia were given 4 g of intravenous magnesium sulfate followed by a 2 g/h infusion up to 24 h after delivery. Sequential blood samples were drawn from each patient and used to measure the serum levels of sodium, potassium, calcium, phosphorus, magnesium, and parathyroid hormone.

A total of 30 pregnant women with pre-eclampsia were evaluated. They were aged between 20 and 41 years with median gestational age of 37.6 (interquartile range 35.4-38.9) weeks. Only five patients reached the therapeutic window of magnesium in at least one of our measuring intervals during magnesium sulfate infusion. Plasma magnesium concentrations increased significantly during magnesium sulfate administration and dropped during the next 12 and 24 h after infusion discontinuation (P < 0.05). Fifteen of 30 (50%) patients developed asymptomatic hypocalcemia, mainly at hour 24 of infusion. Negative moderate correlations were detected between the calcium and magnesium concentrations at 12 and 24 hours of infusion (ρ = -0.390, P = 0.044 and ρ = 0.315, P = 0.096, respectively). None of the patients with hypocalcemia reached the therapeutic level of magnesium or experienced parallel hyperphosphatemia. Eleven of 30 (36.6%) patients developed hyperphosphatemia mainly at 2 and 12 h of magnesium sulfate infusion.

Our study implies that magnesium sulfate could cause hypermagnesemia-induced hypocalcemia in women with pre-eclampsia, independent from parathyroid hormone. The negative correlations between calcium and magnesium concentrations could be indicative of dose-dependent associations between serum magnesium level and degree of hypocalcemia in our study.