Which medication is commonly used for seizure prophylaxis in severe preeclampsia?

Magnesium sulfate toxicity is a life-threatening complication that can occur during treatment for severe preeclampsia or eclampsia. Magnesium sulfate depresses neuromuscular transmission and central nervous system activity, and toxicity manifests with hyporeflexia, respiratory depression, and cardiac arrest. The antidote is calcium gluconate, which antagonizes magnesium’s effects at the neuromuscular junction and restores muscle contractility. Calcium gluconate is administered IV, typically 1 g over 3 minutes. Magnesium is renally excreted, so toxicity risk increases with renal impairment. Therapeutic serum magnesium levels range from 4.8 to 8.4 mg/dL.

Rationale for correct answer

2. Calcium gluconate is the antidote for magnesium sulfate toxicity. It restores neuromuscular function by competing with magnesium at calcium channels and reversing respiratory depression. It is administered IV and acts rapidly to prevent progression to cardiac arrest.

Rationale for incorrect answers

1. Sodium bicarbonate is used to correct metabolic acidosis, not magnesium toxicity. It does not antagonize magnesium at the neuromuscular junction and has no role in reversing respiratory depression caused by magnesium.

3. Potassium chloride is used to treat hypokalemia, not magnesium toxicity. It can worsen cardiac conduction abnormalities if given during magnesium-induced bradycardia or heart block. It does not reverse magnesium’s neuromuscular effects.

4. Furosemide is a loop diuretic used to promote renal excretion of magnesium, but it is not an antidote. It acts slowly and is not suitable for acute reversal of toxicity. It may be used adjunctively after stabilization.

Take home points

• Calcium gluconate is the antidote for magnesium sulfate toxicity.
• Magnesium toxicity presents with hyporeflexia, respiratory depression, and cardiac arrest.
• Renal impairment increases the risk of magnesium accumulation.

Furosemide may aid magnesium excretion but is not an emergency antidote.

Severe preeclampsia is a hypertensive disorder of pregnancy characterized by blood pressure ≥160/110 mmHg, proteinuria ≥0.3 g/24h, and signs of end-organ dysfunction. It poses a high risk for eclampsia, defined by new-onset seizures. The cornerstone of seizure prophylaxis is magnesium sulfate, which acts as a central nervous system depressant by blocking neuromuscular transmission and reducing cerebral vasospasm. It is not an antihypertensive but is essential for preventing convulsions. Therapeutic serum magnesium levels range from 4.8 to 8.4 mg/dL. Toxicity presents with hyporeflexia, respiratory depression, and cardiac arrest, especially in renal impairment.

Rationale for correct answer

3. Magnesium sulfate is the drug of choice for seizure prophylaxis in severe preeclampsia. It reduces the risk of progression to eclampsia by stabilizing neuronal membranes and decreasing excitability. It does not lower blood pressure but is critical in preventing maternal morbidity from seizures.

Rationale for incorrect answers

1. Labetalol is a beta-blocker used to manage acute hypertension in preeclampsia. It reduces systemic vascular resistance but has no anticonvulsant properties. It is not used for seizure prevention and does not affect neuronal excitability.

2. Hydralazine is a direct vasodilator used to lower diastolic blood pressure in hypertensive emergencies. It is effective for blood pressure control but lacks central nervous system depressant effects. It does not prevent seizures and is not the standard for eclampsia prophylaxis.

4. Nifedipine is a calcium channel blocker used for blood pressure reduction in pregnancy. It relaxes vascular smooth muscle but does not cross the blood-brain barrier to exert anticonvulsant effects. It is not indicated for seizure prevention in preeclampsia.

Take home points

• Magnesium sulfate is the first-line agent for seizure prophylaxis in severe preeclampsia.
• Antihypertensives like labetalol, hydralazine, and nifedipine manage blood pressure but do not prevent seizures.
• Eclampsia is defined by new-onset seizures in a preeclamptic patient.
• Magnesium toxicity presents with hyporeflexia and respiratory depression.