Magnesium sulfate is a drug commonly used in obstetrics as an agent to stop contractions for patients who have pre-term labor (a “tocolytic” drug) and also for patients with pre-eclampsia to prevent seizures.
Lately, magnesium has also been used for an additional benefit, neuroprotection of the the infant, for patients who are less than 32 weeks gestation near imminent delivery. (This is a fairly new application for magnesium that was discovered fortuitously. In general, pre-term babies may be at risk for intracranial bleeding, known as intra-ventricular hemorrhage., because their blood vessels in their brain tend to be more fragile. Studies had shown that babies whose moms had been given magnesium, whether to stop pre-term labor or for pre-eclampsia, incidentally had a decreased risk of fetal intracranial bleeding. Because of this we now routinely give it to our patients who are less than 32 weeks if delivery is thought to be soon.)
The unique properties of magnesium make it an ideal agent for our patients, when indicated. Magnesium relaxes the smooth muscles of the body, which makes it a perfect agent for pre-term labor. By stopping the contractions of the uterine muscle, preterm delivery may be prevented. For our pre-eclamptic patients, who are at risk for seizures because of central nervous system irritability, magnesium decreases this irritability and helps to prevent seizures from occurring. Finally, magnesium “stabilizes” the fragile blood vessels in the brain of the preterm infant. This “stabilization” allows the blood vessels to withstand the stressors of life outside the uterus, if necessary, and may prevent the intraventricular hemorrhage that is so concerning for our smallest patients.
So how long does a patient need to be on magnesium sulfate and at what dose? For patients in pre-term labor, I tend to give a higher dosing regimen; basically whatever is needed, within safety profiles, to stop uterine contractions. Sometimes, the amount required is fairly low, but for some patients, a higher dose is necessary. We typically will keep these patients on magnesium for 48 hours total to maximize the effects of steroids, such as beta-methasone, for fetal lung maturation. In contrast, preeclamptic patients tend to need less magnesium. They are dosed based on their serum blood levels of magnesium until it reaches a particular level. These patients are maintained on magnesium throughout labor, delivery and generally for 24 hours postpartum as well. Finally for our patients who find themselves with emergent delivery under 32 weeks, we simply give a dose immediately prior to delivery. Patients who receive magnesium for pre-term labor or for neuroprotection do not need magnesium after the delivery of the infant, unless there was also concerns for pre-eclampsia.
What are the maternal effects? I explain to my patients that magnesium effects are “dose dependent”, meaning that unlike some drugs where side effects are fairly common to every patient regardless of dose, magnesium’s effects can be variable, depending on the amount given. Unfortunately, these side effects may be uncomfortable. One patient told me it feels like being “under water” and hard to move.
and that is a direct byproduct of the mechanism of magnesium. Remember, it is used to relax the smooth muscles of the body and will affect various areas, including your eyes and gross motor skills. Patients may be nauseated during the initial bolus and feel a “flushing” feeling, they may have difficulty focusing as the eye muscles become less responsive. The effects are certainly not pleasant, but the benefit to the infant can be significant, especially in cases of pre-term delivery.