Magnesium Sulfate: A Treatment for Preterm Labor and a Method of Neuroprotection

Magnesium sulfate is an inorganic salt that has a plethora of uses within the medical community. Magnesium sulfate is particularly useful during pregnancy, especially when preterm birth is expected. There are two very important uses of magnesium sulfate during the antenatal period:

• Magnesium sulfate can be used to suppress preterm labor, preventing or delaying preterm birth for several days.
• Magnesium sulfate can help protect the fetal brain before the onset of preterm birth, lessening the risk of hypoxic-ischemic encephalopathy, cerebral palsy, and other brain injuries.

The uses, administration, risks, and indications for magnesium sulfate use during pregnancy will be discussed in detail below.

Preterm labor, or labor beginning before 37 weeks of gestation, increases the risk of birth complications and injuries as compared to babies born at term. Magnesium sulfate is a tocolytic, a medication used to suppress preterm labor, and can be used to help slow or inhibit contractions to delay the birth of a preterm baby. Magnesium sulfate lowers the amount of calcium in the uterine muscles which, in turn, encourages muscle relaxation.

The use of magnesium sulfate to prevent or delay preterm birth can give medical professionals the time they need to prepare a preterm baby for the stress of the delivery process. During this time, physicians may administer prenatal steroids like betamethasone, which acts as a neuroprotector and helps the baby’s lungs develop in preparation for life outside the womb.

In order to prevent or delay preterm labor, magnesium sulfate should be administered at a dose of about 6 grams, over a span of 15 to 30 minutes. Each hour a maintenance dose of 2 grams should be administered.

Magnesium sulfate can also be used to protect the fetal brain before premature birth. Because premature babies are at increased risk for injuries like HIE and cerebral palsy, neuroprotective agents like magnesium sulfate can be administered before birth in order to reduce the risk of these injuries. Studies show that when a very preterm baby is exposed to magnesium sulfate before birth, the incidence and severity of cerebral palsy and severe motor dysfunction is significantly reduced.

The mechanism by which magnesium sulfate protects the fetal brain is not completely understood. Professionals, however, hypothesize that magnesium sulfate administration may help the baby in the following ways:

• Stabilize cell membranes, blood pressure, and cerebral blood flow
• Decrease the levels of inflammatory or antioxidant molecules in the baby’s brain
• Increase blood flow to the baby’s brain
• Reduce levels of excitotoxicity (the process in which neurons are destroyed due to overactive neural receptors)

When a premature birth is expected to occur within 24 hours, magnesium sulfate should be given to women if indicated, whether or not their membranes have ruptured. A 4 gram dose should be given to begin, and then each hour a 1 gram maintenance dose will likely be administered. If physicians are trying to prevent preterm labor (tocolysis) concurrently, indomethacin should be used because other tocolytics may have adverse side effects when used in conjunction with magnesium sulfate.

Magnesium sulfate is also widely used for the prevention of eclampsia, so most providers are familiar with common maternal side effects, which may include sweating, flushing, nausea, headache, and magnesium toxicity. Other maternal complications which may be associated with magnesium sulfate include the following:

• Respiratory arrest (loss of breathing, which is life-threatening)
• Pulmonary edema (fluid in the lungs, which is life-threatening)
• Effects on the central nervous system
• Hypersomnolence (excessive sleeping)
• Muscle weakness
• Visual changes/blurred vision
• Loss of reflexes
• Chest tightness

Research shows that these adverse events may occur due to unfamiliarity with safe dose ranges and signs of toxicity, inadequate patient monitoring, incorrect preparations of the drug, wrong administration technique, and mix-ups between magnesium sulfate and oxytocin. When physicians are skilled and careful with magnesium administration, these side effects are very uncommon.

With regard to the baby, magnesium freely crosses the placenta, so the concentration of magnesium in the baby’s cord blood is about the same as the concentration in the mother’s blood. A slight decrease in fetal heart rate and variability may be observed, but this is not clinically significant.

The potential benefits of magnesium therapy should outweigh the potential risks. Magnesium therapy is not advised when the mother has myasthenia gravis (a neuromuscular disease), compromised heart function, or heart conduction defects. Since magnesium is eliminated by the kidneys, mothers with impaired kidney function may develop magnesium toxicity at the usual infusion doses. Thus, maintenance dosing must be adjusted or eliminated for these women.

Doctors should closely monitor urine output, deep tendon reflexes, and other reactions to prevent magnesium toxicity. The maintenance phase of treatment should be continued only if a patellar (knee) reflex is present, breathing rate exceeds 12 breaths per minute, and urine output exceeds 100 mL in four hours.