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OB·CA-2

Eclamptic Seizure & Mag Toxicity

Navigate a sudden intraoperative seizure and the subsequent management of magnesium overdose.

The stem

A 32 y.o. G1P0 at 38 weeks with severe preeclampsia is undergoing a C-section under epidural anesthesia. Suddenly, she begins to have a tonic-clonic seizure. You administer a magnesium bolus, but shortly after, she becomes somnolent with shallow respirations and loss of patellar reflexes. PHYS: BP 140/90, HR 105, RR 6, O2 Sat 88%.

Focus

Seizure management in OB (Eclampsia), magnesium dosing, and treatment of magnesium toxicity.

Examination relevance

Examiners love to test the transition from one crisis (seizure) to another (medication error/overdose).

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Expert sample response

This is a dual crisis: an eclamptic seizure followed by acute magnesium toxicity. My first priority is protecting the patient's airway and providing 100% oxygen to prevent maternal and fetal hypoxia.

I will place the patient in the left lateral tilt position. For the seizure, I will ensure her magnesium bolus (4-6g) is completed, but given her current respiratory depression (RR 6) and loss of reflexes, I must immediately treat for magnesium toxicity. I will stop the magnesium infusion, provide positive pressure ventilation if needed, and administer 1 gram of Calcium Gluconate IV. I will also monitor the Fetal Heart Rate continuously and proceed with the C-section as soon as she is stabilized, likely converting to a general anesthetic to secure the airway if her mental status does not rapidly improve after the calcium.

Full walkthrough

What the Examiner Is Testing

This case tests whether you can pivot rapidly from treating one crisis to treating the complication of your treatment. The examiner specifically wants to see you recognize magnesium toxicity by its signs — loss of patellar reflexes first, then respiratory depression — and respond with calcium gluconate immediately.

The Board Trap

The trap is continuing the magnesium infusion while trying to "manage the airway" with supplemental oxygen. Once you've lost patellar reflexes and her RR is 6, more magnesium will kill her. The infusion must stop immediately. The second trap is giving more magnesium for seizure recurrence without first treating the toxicity.

Walk-Through: How This Case Plays Out

Examiner: She starts seizing mid-C-section under epidural. What do you do immediately?

Me: I would protect the airway and give supplemental oxygen first — 100% by face mask. I'd call for help. I'd administer the magnesium bolus — 4 to 6 grams IV over 15 to 20 minutes — to terminate the eclamptic seizure. I'd tilt her to left lateral tilt to relieve aortocaval compression. I'm monitoring fetal heart rate throughout and preparing to convert to general anesthesia if her mental status deteriorates.

Examiner: After the mag bolus, her respirations drop to 6 per minute and she loses her patellar reflexes. What's happening?

Me: She has magnesium toxicity. The magnesium level is too high and now I have respiratory depression. I would stop the magnesium infusion immediately — right now. I'd administer calcium gluconate 1 gram IV, which is the antidote and rapidly reverses magnesium's effects on calcium-dependent neuromuscular function. I'd be ready to support ventilation with a bag-mask or convert to GA and intubate if she doesn't improve quickly.

Examiner: After the calcium gluconate, her respirations improve to 14 per minute and she's more responsive. Do you still proceed with the C-section?

Me: Yes, but under general anesthesia now. Her mental status is not fully restored, she's had a convulsion, and I cannot rely on the epidural being adequate or her cooperation being consistent. I'd perform a modified RSI — video laryngoscope first-line given the obstetric airway — with etomidate and succinylcholine. The baby needs to come out given the fetal distress, and I need a secure airway to do that safely.

Examiner: The baby is delivered and the mag level comes back at 9.8 mEq/L. How do you manage post-op?

Me: Therapeutic magnesium is 4 to 8 mEq/L, so she's supratherapeutic. I'd hold further magnesium for now and recheck the level in 2 hours. She'll need a low-dose magnesium infusion to prevent recurrent seizures post-partum — eclampsia can occur up to 6 weeks post-delivery. I'd keep her intubated in the ICU until her mental status is fully restored and the magnesium level is in the therapeutic range.

Key Phrases That Score Points

  • "Stop the magnesium infusion immediately — respiratory depression and loss of reflexes means toxicity, not under-treatment."
  • "Calcium gluconate 1 gram IV — it's the antidote for magnesium toxicity."
  • "Patellar reflex check is my bedside magnesium monitor — when reflexes are lost, toxicity is imminent."
  • "Convert to GA for the C-section — she's had a seizure and her mental status is unreliable."
  • "Eclampsia can occur up to 6 weeks post-partum — she needs continued magnesium post-op in the therapeutic range."

Why This Case Appears on the Boards

This case is a test of pivoting from crisis management to complication management. Examiners love the two-problem scenario because it reveals whether you can recognize when your treatment is causing harm and act on that immediately — rather than escalating the dose of the thing that's hurting the patient.