Magnesium is Absolutely Contraindicated in Myasthenia Gravis Crisis
Magnesium replacement must be avoided in patients with myasthenia gravis crisis, as it can precipitate or worsen respiratory failure by inhibiting acetylcholine release at the neuromuscular junction and desensitizing the postsynaptic membrane. 1, 2, 3
Mechanism of Harm
Magnesium acts at the neuromuscular junction through two dangerous mechanisms in myasthenia gravis patients:
- Inhibits presynaptic acetylcholine release, further compromising already impaired neuromuscular transmission 4
- Desensitizes the postsynaptic membrane, worsening the existing receptor dysfunction 4
- Can convert stable myasthenia into respiratory crisis requiring intubation and mechanical ventilation 4
Clinical Evidence of Harm
A documented case demonstrates the severe consequences of magnesium administration:
- A 62-year-old woman with myasthenia gravis in crisis received standard intravenous magnesium replacement for hypomagnesemia 4
- Neuromuscular weakness dramatically worsened immediately following magnesium administration 4
- Respiratory failure was exacerbated, necessitating intubation, mechanical ventilation, and extended ICU stay 4
Guideline Recommendations
Intravenous magnesium is explicitly listed among medications that must be strictly avoided in myasthenia gravis patients: 1, 2, 3
- β-blockers 1, 2, 3
- IV magnesium (absolutely contraindicated) 3
- Fluoroquinolone antibiotics 1, 2, 3
- Aminoglycoside antibiotics 1, 2, 3
- Macrolide antibiotics 1, 2, 3
Management of Hypomagnesemia in Myasthenic Crisis
If severe hypomagnesemia requires treatment in a patient with myasthenia gravis, mandatory neurology consultation must occur before any magnesium administration. 3
Alternative Approach:
- Prioritize treating the myasthenic crisis first with plasmapheresis (5 sessions over 5 days) or IVIG (2 g/kg total dose over 5 days at 0.4 g/kg/day) 3
- Administer high-dose corticosteroids: methylprednisolone 1-2 mg/kg/day IV or prednisone 1-1.5 mg/kg/day 2, 3
- Ensure ICU-level monitoring with frequent pulmonary function assessments (NIF and vital capacity) 2, 3
- Address the underlying cause of hypomagnesemia (diarrhea, malabsorption, diuretics) rather than replacing magnesium directly 5
Critical Monitoring Parameters
During myasthenic crisis management, monitor for respiratory failure using the "20/30/40 rule": 2, 3
- Vital capacity <20 ml/kg
- Maximum inspiratory pressure <30 cmH₂O
- Maximum expiratory pressure <40 cmH₂O
Any of these thresholds indicate imminent need for intubation and should prompt immediate immunotherapy initiation. 2
Common Pitfall to Avoid
The most dangerous error is treating routine electrolyte abnormalities in myasthenia gravis patients without considering the neuromuscular effects of replacement therapy. Standard protocols for magnesium replacement that are safe in other ICU patients can be catastrophic in myasthenia gravis. Always verify the patient's neuromuscular diagnosis before administering magnesium in any form. 4