Role of Magnesium in Preventing Seizures
Magnesium sulfate is primarily indicated for the prevention and treatment of seizures in preeclampsia and eclampsia, but has limited evidence supporting its use in other seizure disorders. 1
Mechanism of Action
Magnesium prevents or controls convulsions through several mechanisms:
- Blocks neuromuscular transmission
- Decreases acetylcholine release at the motor nerve end-plate
- Has a depressant effect on the central nervous system
- Acts as a vasodilator, potentially relieving cerebral vasoconstriction
- Protects the blood-brain barrier and limits cerebral edema formation 2, 3
Clinical Applications
Preeclampsia and Eclampsia
Magnesium sulfate is the first-line agent for seizure prophylaxis and treatment in preeclampsia and eclampsia:
Dosing regimen:
Efficacy: Significantly superior to phenytoin and diazepam for preventing recurrent seizures in eclamptic patients 4, 5
Therapeutic levels: 2.5-7.5 mEq/L (effective anticonvulsant range) 2
Other Seizure Disorders
- Limited evidence supports magnesium's role in other types of seizures
- May have potential benefit in infantile spasms when combined with ACTH 6
- Theoretical benefit in patients with epilepsy who have low magnesium levels, but insufficient evidence for routine use 6
Monitoring and Safety
Signs of Magnesium Toxicity
- Early signs (4-5 mEq/L): Loss of deep tendon reflexes, sedation, muscle weakness
- Moderate toxicity (6-10 mEq/L): Respiratory depression, hypotension, bradycardia
- Severe toxicity (>12 mEq/L): Respiratory paralysis, heart block, potentially fatal 2, 1
Required Monitoring
- Deep tendon reflexes (patellar reflex)
- Respiratory rate (should remain >12/min)
- Urine output (should remain >30 mL/hour)
- Serum magnesium levels when available 1
Contraindications and Precautions
- Important: Should not be given concomitantly with calcium channel blockers due to risk of severe hypotension 7
- Use with caution in patients with renal impairment (reduce dose by 50%) 1
- Have calcium gluconate (10%) 10mL or calcium chloride (10%) 5-10mL immediately available as an antidote 1
Special Considerations
Hypomagnesemia
- May lower seizure threshold in patients with epilepsy
- Consider supplementation in patients with documented hypomagnesemia who have seizures 6
- For severe hypomagnesemia: Up to 250mg/kg IM within a four-hour period or 5g IV infusion over three hours 1
Peripartum Management
- Magnesium sulfate is recommended for women with severe preeclampsia or eclampsia during labor and for 24 hours postpartum 7
- Monitor for signs of magnesium toxicity, particularly in patients with renal dysfunction 1
Conclusion
Magnesium sulfate has a well-established role in preventing and treating seizures in preeclampsia and eclampsia. Its effectiveness in other seizure disorders remains theoretical with limited clinical evidence. When using magnesium for seizure prevention, careful monitoring is essential to prevent toxicity.