In a patient with myasthenia gravis and hypomagnesemia, is intravenous magnesium sulfate appropriate?

Magnesium Sulfate is Contraindicated in Myasthenia Gravis

Intravenous magnesium sulfate should NOT be administered to patients with myasthenia gravis, even in the presence of hypomagnesemia, due to the high risk of precipitating myasthenic crisis and respiratory failure. 1, 2, 3

Mechanism of Harm

Magnesium acts at the neuromuscular junction through two mechanisms that directly worsen myasthenia gravis:

• Inhibits presynaptic acetylcholine release, further reducing the already compromised neurotransmitter availability in myasthenia gravis 4
• Desensitizes the postsynaptic membrane, compounding the antibody-mediated receptor dysfunction 4
• The FDA drug label explicitly states magnesium causes "excessive neuromuscular block" when combined with neuromuscular transmission defects 3

Clinical Evidence of Severe Harm

Multiple case reports document catastrophic outcomes when magnesium is given to myasthenia gravis patients:

• A 62-year-old woman with myasthenia gravis developed acute respiratory failure requiring intubation and mechanical ventilation after standard IV magnesium replacement, resulting in an extended ICU stay 4
• A woman in her 90s developed acute respiratory failure requiring intubation after receiving IV magnesium for atrial fibrillation; she experienced respiratory failure on three separate occasions after repeated magnesium doses before the connection was recognized 5
• A 70-year-old woman with undiagnosed myasthenia gravis (presenting only with dysphagia) went into myasthenic crisis after magnesium infusion, requiring 18 days of mechanical ventilation and 25-day hospital stay 6
• A patient with no prior neuromuscular symptoms became virtually quadriplegic after parenteral magnesium for preeclampsia at a serum level of only 3.0 mEq/L (normally well-tolerated), revealing previously undiagnosed myasthenia gravis 7

Guideline-Based Contraindication

The American College of Neurology explicitly lists IV magnesium among medications that must be strictly avoided in myasthenia gravis patients 1, 2. This contraindication is absolute and applies regardless of magnesium levels.

The complete list of contraindicated medications includes:

• Intravenous magnesium 1, 2
• β-blockers 1, 2
• Fluoroquinolone antibiotics 1, 2
• Aminoglycoside antibiotics 1, 2
• Macrolide antibiotics 1, 2

Management of Hypomagnesemia in Myasthenia Gravis

If Hypomagnesemia Must Be Corrected:

Oral magnesium supplementation is the only potentially safe route, though even this requires extreme caution and close monitoring:

• Use oral magnesium oxide or magnesium citrate in divided doses 3
• Start with the lowest possible dose (e.g., 200-400 mg elemental magnesium daily in divided doses)
• Monitor closely for any signs of worsening weakness, particularly:
  • Respiratory function (vital capacity, negative inspiratory force) 1, 2
  • Bulbar symptoms (speech, swallowing) 1
  • Proximal muscle strength 1
  • Diplopia or ptosis worsening 1

Critical Monitoring Parameters:

Before and during any magnesium replacement:

• Pulmonary function testing with negative inspiratory force and vital capacity 1, 2
• Daily neurologic assessments for any sign of weakness progression 2
• Immediate availability of rescue therapy: IVIG (2 g/kg over 5 days) or plasmapheresis 1, 2
• ICU-level monitoring if any respiratory symptoms present 2

When Hypomagnesemia is Mild or Asymptomatic:

Consider tolerating mild hypomagnesemia rather than risking myasthenic crisis 3. The risk-benefit analysis strongly favors avoiding magnesium administration in most scenarios.

Special Clinical Scenario: Preeclampsia

In pregnant myasthenia gravis patients with preeclampsia, magnesium sulfate for seizure prophylaxis is absolutely contraindicated 8:

• Use levetiracetam as alternative anticonvulsant for seizure prophylaxis 8
• For hypertension management, avoid β-blockers and calcium channel blockers (also contraindicated in myasthenia gravis) 8
• Consider intravenous labetalol with extreme caution and close monitoring, or alternative agents like hydralazine 8

Common Pitfall to Avoid

The most dangerous error is administering "standard" IV magnesium replacement without recognizing the myasthenia gravis diagnosis 5, 6. This has resulted in:

• Repeated episodes of respiratory failure when the connection wasn't recognized 5
• Unmasking of previously undiagnosed myasthenia gravis through life-threatening crisis 6, 7
• Prolonged mechanical ventilation and ICU stays 4, 6

All members of the healthcare team must be aware that IV magnesium can induce myasthenic crisis 5. This should be flagged prominently in the medical record and medication administration system.