What medications can be used for seizure prophylaxis in a patient with pre-eclampsia (high blood pressure during pregnancy) with severe features and myasthenia gravis (neuromuscular disorder)?

Seizure Prophylaxis in Pre-eclampsia with Severe Features and Myasthenia Gravis

Magnesium sulfate is the first-line medication for seizure prophylaxis in pre-eclampsia with severe features, even in patients with myasthenia gravis, but requires careful monitoring for neuromuscular blockade and respiratory depression. 1, 2

Magnesium Sulfate Protocol

Magnesium sulfate is the gold standard for preventing eclampsia in women with pre-eclampsia with severe features. The International Society for the Study of Hypertension in Pregnancy (ISSHP) strongly recommends its use 1.

Dosing in Myasthenia Gravis:

• Loading dose: 4g IV over 20-30 minutes (diluted in 250mL of fluid)
• Maintenance: Lower than standard - consider 0.5-1g/hour IV infusion (rather than standard 1-2g/hour) 2
• Duration: Continue for 24 hours postpartum 1

Special Monitoring Requirements for Myasthenia Gravis:

• Monitor respiratory function every 30 minutes for first 2 hours
• Check deep tendon reflexes hourly
• Monitor respiratory rate (should remain >12/min)
• Monitor oxygen saturation continuously
• Assess for signs of increasing muscle weakness
• Have calcium gluconate readily available as antidote

Alternative Options if Magnesium is Contraindicated

If magnesium sulfate is absolutely contraindicated due to severe myasthenic crisis or respiratory compromise:

Phenytoin:

• Loading dose: 15-20 mg/kg IV at rate not exceeding 50 mg/min
• Maintenance: 100 mg IV every 6-8 hours
• Monitor serum levels to maintain therapeutic range

Blood Pressure Management

Concurrent blood pressure control is essential:

• First-line: Labetalol IV (20mg initially, then 40mg, then 80mg every 10 minutes to maximum 220mg) 1
• Alternative: Hydralazine IV (5mg bolus, then 10mg every 20-30 minutes to maximum 25mg) 1
• Target BP: 140-150/90-100 mmHg 1

Important Considerations for Myasthenia Gravis

1. Increased sensitivity: Patients with myasthenia gravis are more sensitive to magnesium's neuromuscular blocking effects
2. Respiratory risk: Higher risk of respiratory depression requires closer monitoring
3. Drug interactions: Avoid medications that may worsen myasthenia (aminoglycosides, certain antibiotics)
4. Multidisciplinary approach: Involve neurology and critical care specialists
5. Delivery planning: Prepare for possible need for ventilatory support

Clinical Decision Algorithm

1. Assess myasthenia severity:

   • If well-controlled with minimal symptoms: Use magnesium sulfate with careful monitoring
   • If poorly controlled or in myasthenic crisis: Consider alternative anticonvulsants

2. Monitor for magnesium toxicity:

   • Loss of patellar reflex (3.5-5 mmol/L)
   • Respiratory depression (5-6.5 mmol/L)
   • Cardiac conduction changes (>7.5 mmol/L) 3

3. If signs of toxicity appear:

   • Stop infusion immediately
   • Administer calcium gluconate 1g IV over 3-5 minutes
   • Support respiration as needed

Delivery Considerations

Delivery is the definitive treatment for pre-eclampsia 1. Timing should be determined based on:

• Gestational age
• Maternal condition
• Fetal status
• Severity of pre-eclampsia

For severe pre-eclampsia at ≥34 weeks, delivery is generally recommended 1.

Pitfalls to Avoid

1. Avoid rapid administration of magnesium sulfate in myasthenia patients
2. Do not combine calcium channel blockers with magnesium sulfate as this may cause profound hypotension 1
3. Do not withhold magnesium sulfate solely due to myasthenia gravis diagnosis - benefits often outweigh risks with proper monitoring
4. Do not delay delivery when indicated, as it is the definitive treatment for pre-eclampsia
5. Do not use diazepam as it is inferior to magnesium sulfate for seizure prevention 4