Role of Intravenous Magnesium Sulphate in Eclampsia Management
Intravenous magnesium sulphate is the first-line treatment for eclampsia, both for treating active seizures and preventing recurrent seizures, with clear evidence showing it approximately halves the seizure rate compared to other anticonvulsants. 1
Indications and Mechanism
Magnesium sulphate is indicated for:
- Treatment of active eclamptic seizures
- Prevention of recurrent seizures in women with eclampsia
- Seizure prophylaxis in severe pre-eclampsia
Magnesium works primarily through:
- Central nervous system depression
- Peripheral neuromuscular blockade
- Cerebral vasodilation
Dosing Regimen
The FDA-approved dosing for eclampsia includes 2:
Initial Loading Dose
- 4-5g IV in 250mL of 5% Dextrose or 0.9% Sodium Chloride infused over 15-20 minutes
Maintenance Options
IV Continuous Infusion:
- 1-2g/hour by controlled infusion pump
- Continue until 24 hours after delivery or last seizure
IM Regimen (Pritchard):
- 10g IM initially (5g in each buttock) following IV loading dose
- Then 5g IM every 4 hours in alternating buttocks
The ISSHP recommends using dosing regimens from the Eclampsia and MAGPIE trials 1.
Therapeutic Monitoring
Careful monitoring is essential to prevent toxicity 2, 3:
- Therapeutic range: 1.8-3.0 mmol/L (4.5-7.5 mg/dL)
- Monitor:
- Deep tendon reflexes (patellar reflex) - first sign of toxicity when lost
- Respiratory rate (should be >12/min)
- Urine output (should be >30 mL/hour)
- Level of consciousness
Toxicity Warning Signs
| Serum Mg Level | Clinical Manifestation |
|---|---|
| 3.5-5 mmol/L | Loss of patellar reflex |
| 5-6.5 mmol/L | Respiratory depression |
| >7.5 mmol/L | Cardiac conduction abnormalities |
| >12.5 mmol/L | Cardiac arrest |
Duration of Treatment
- Continue MgSO₄ for 24 hours after delivery or last seizure 1
- Some evidence suggests that shorter duration may be sufficient if the patient has received at least 8g before delivery, but most guidelines still recommend 24 hours postpartum 1
Concurrent Management
- Control blood pressure with IV labetalol or oral nifedipine to maintain BP <160/105 mmHg 1
- Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema 1
- Monitor fetal heart rate (caution with labetalol: cumulative dose should not exceed 800 mg/24h to prevent fetal bradycardia) 1
- Consider delivery after maternal stabilization 1
Efficacy Compared to Other Agents
Magnesium sulphate is superior to other anticonvulsants:
- Compared to phenytoin: 66% reduction in recurrent seizures (RR 0.34) 4
- Better maternal and fetal outcomes than diazepam or phenytoin 4, 5
Common Pitfalls and Caveats
Renal function monitoring: Magnesium is primarily excreted by kidneys; dose adjustment needed in renal impairment 2
Calcium gluconate availability: Always have 10% calcium gluconate (10mL) readily available as antidote for magnesium toxicity
Prolonged use risks: Continuous administration beyond 5-7 days can cause fetal bone abnormalities 2
Aluminum toxicity: Contains aluminum that may reach toxic levels with prolonged administration in patients with impaired kidney function 2
Drug interactions: Enhanced effects with CNS depressants, muscle relaxants, and calcium channel blockers
Monitoring gaps: Failure to monitor reflexes, respiration, and urine output can lead to undetected toxicity
Alternative regimens: Low-dose IV regimens (0.6g/hour) may be equally effective with potentially fewer side effects 6
Magnesium sulphate remains the cornerstone of eclampsia management, with strong evidence supporting its efficacy and safety when properly administered and monitored.