Eclampsia and pre-eclampsia are characterized by the new onset of hypertension and proteinuria after 20 weeks of gestation in a previously normotensive woman. One of the major causes of morbidity to both mother & fetus is the occurrence of seizures. Fortunately, decades of experience with intravenous magnesium have led to a generally safe and effective use of this medication as an anticonvulsant. iv Mg is indicated both as a seizure prophylactic agent (in severe pre-eclampsia) as well as for the prevention of recurrent eclamptic seizures.
Generally, iv magnesium is given first as a loading dose (e.g., 6 grams iv bolus given over 15-20 minutes) followed by a continuous infusion of 2 grams per hour. An important caveat to be aware of as nephrologists is that as magnesium is excreted by the kidneys, individuals with acute or chronic renal failure are especially susceptible to hypermagnesemia and its toxic effects. This is not so uncommon, as thrombotic microangiopathy is often seen in women with advanced pre-eclampsia/eclampsia. Up-To-Date recommends giving the full loading dose (6 grams) but a reduced continuous infusion rate of 1gram per hour in individuals with moderately reduced GFR (Cr less than 2.5 mg/dL) and NO continuous infusion rate for individuals with a severely-reduced GFR (Cr greater than 2.5 mg/dL). Although the measured serum Mg concentration does not totally correlate with symptoms, the suggested therapeutic range is between 4.8 – 8.4 mg/dL.
Early magnesium toxicity can be detected as a loss of deep tendon reflexes (typically occuring at levels between 10-12 mg/dL) with more severe symptoms (respiratory paralysis, cardiac arrest) occuring at Mg levels greater than 12 mg/dL. Calcium gluconate can be given as a cardiac stabilizing agent in these instances. Mg therapy may also transiently suppress PTH release and can result in a mild hypocalcemia.