Recommended Dose of Magnesium Sulphate for Seizure Prophylaxis in Preeclampsia
For seizure prophylaxis in preeclampsia, magnesium sulphate should be administered as a 4-6g IV loading dose over 20-30 minutes, followed by a maintenance infusion of 1-2g/hour for 24 hours postpartum, with the dosing regimens used in the Eclampsia and MAGPIE trials being the recommended standard. 1
Initial Loading Dose
- 4-5g IV in 250mL of 5% Dextrose or 0.9% Sodium Chloride infused over 20-30 minutes
- Alternatively, 4g IV (diluted to 10% or 20% concentration) over 3-4 minutes, plus 10g IM (5g in each buttock) simultaneously
Maintenance Regimen Options
IV Maintenance (Preferred in High-Resource Settings):
- 1-2g/hour continuous infusion via controlled pump for 24 hours postpartum
- 1g/hour has similar efficacy with fewer side effects compared to 2g/hour 2
IM Maintenance (Alternative):
- 5g IM every 4 hours in alternating buttocks
- Continue for 24 hours postpartum
Monitoring During Administration
- Deep tendon reflexes (patellar reflex): Loss indicates early toxicity (at 3.5-5 mmol/L)
- Respiratory rate: Should remain >12/min
- Urine output: Should exceed 30mL/hour
- Level of consciousness
- Target serum magnesium level: 1.8-3.0 mmol/L for therapeutic effect 3
Indications for MgSO₄ in Preeclampsia
- All women with preeclampsia in low-resource settings should receive MgSO₄
- In high-resource settings, selective use is reasonable for women with:
- Severe hypertension (≥160/110 mmHg) with ≥3+ proteinuria, OR
- BP ≥150/100 mmHg with ≥2+ proteinuria AND at least 2 signs of imminent eclampsia (headache, visual symptoms, clonus) 1
Duration of Treatment
While one study in Latin America suggested that women who received at least 8g before delivery may not need postpartum continuation, the International Society for the Study of Hypertension in Pregnancy (ISSHP) recommends continuing MgSO₄ for 24 hours postpartum until further studies confirm these findings in other populations 1. A recent systematic review also supports continuing magnesium sulfate for 24 hours postpartum for seizure prophylaxis 4.
Toxicity Management
- Respiratory depression (rate <12/min): Hold infusion
- Cardiac conduction abnormalities (>7.5 mmol/L): Prepare calcium gluconate
- Cardiac arrest risk (>12.5 mmol/L): Immediate intervention required
- For overdose: 10mL of 10% calcium gluconate IV over 3 minutes
Special Considerations
- In renal insufficiency: Maximum dose should not exceed 20g/48 hours with frequent serum magnesium monitoring 5
- Continuous use beyond 5-7 days can cause fetal abnormalities 5
- Total daily dose should not exceed 30-40g in 24 hours 5
Practical Implementation
Each unit should develop their own protocol for magnesium administration that incorporates appropriate monitoring, recognition of risks, and assessment of maternal and fetal outcomes 1. The evidence clearly shows that MgSO₄ prevents eclampsia, approximately halving the rate, with approximately 100 women needing treatment to prevent 1 seizure.