Magnesium has anticonvulsant and neuroprotective efects in several animal models. Some proposed mechanisms of action include: (1) reduced presynaptic release of the neurotransmitter glutamate, (2) blockade of glutamatergic N-methyl-D-aspartate (NMDA) receptors, (3) potentiation of adenosine action, (4) improved calcium bufering by mitochondria, and (5) blockage of calcium entry via voltage-gated channels (Arango, 2008; Wang, 2012).
In the uterus, relatively high serum magnesium concentrations depress myometrial contractility both in vivo and in vitro. With the suggested regimen, myometrial depression has not been observed, except for a transient decline in activity during and immediately ater the initial intravenous loading dose (Leveno,
TABLE 40-12. Randomized Comparative Trials of Magnesium Sulfate Versus Phenytoin and Diazepam to Prevent Recurrent Eclamptic Convulsions
Recurrent seizuresa 60/453 (1n3%) 126/452 (28%) 22/388 (5.6%) 66/389 (1n7%) Matenal deathsb 10/388 (2.6%) 20/387 (5.2%) 17/453 (3.8%) 24/452 (5.3%) aA11 comparisons p < 0.01n.
blndividual comparisons nonsignificant, combined comparison p < .05. Data from Eclampsia Trial Collaborative Group, 1995.
1998; Szal, 1999; Witlin, 1997). Blood loss at delivery is not increased by standard magnesium treatment (Graham, 2016). However, inhibition of uterine contractility is magnesium dose dependent, and serum levels of at least 8 to 10 mEq/L are necessary to inhibit uterine contractions (Watt-Morse, 1995).
Magnesium administered parenterally promptly crosses the placenta to achieve equilibrium in fetal serum and less so in amnionic fluid (Hallak, 1993). Levels in amnionic luid rise with the duration of maternal infusion (Gortzak-Uzan, 2005). Magnesium sulfate has small but signiicant efects on the fetal heart rate pattern-specifically beat-to-beat variability (Hallak, 1999). Duy and associates (2012) reported a lower heart rate baseline that was within the normal range; decreased variability; and fewer prolonged decelerations. They noted no adverse outcomes.
Overall, maternal magnesium therapy appears safe for perinates (Drassinower, 2015). One MFMU Network study of more than 1500 exposed preterm neonates found no association between the need for neonatal resuscitation and cord blood magnesium levels Qohnson, 2012). Still, a few neonatal adverse events are associated with its use. In a Parkland Hospital study of 6654 mostly term, exposed newborns, 6 percent had hypotonia (Abbassi-Ghanavati, 2012). In addition, exposed neonates had lower 1-and 5-minute Apgar scores, a higher intubation rate, and more admissions to the special care nursery. The study showed that neonatal depression occurs only if hypermagnesemia at delivery is severe.