From the Guidelines
To safely replace magnesium in a patient with myasthenia gravis in crisis, avoid using IV magnesium due to its potential to worsen muscle weakness, as recommended by the American Society of Clinical Oncology clinical practice guideline 1. Instead, consider alternative methods to manage magnesium levels, such as oral supplementation or other routes of administration that do not involve IV magnesium. Key points for management:
- Monitor vital signs, respiratory function, and neuromuscular status closely.
- Have calcium gluconate (1 gram IV) readily available as an antidote, in case of worsening muscle weakness.
- Consider reducing the dose of other medications that may exacerbate muscle weakness, such as neuromuscular-blocking agents, in patients with myasthenia gravis, as recommended by the Critical Care Medicine guidelines 1.
- Check serum magnesium levels before and after replacement, and adjust the management plan accordingly. Magnesium replacement should be done cautiously in myasthenia gravis patients because magnesium can potentially worsen muscle weakness by inhibiting acetylcholine release at the neuromuscular junction. However, correcting hypomagnesemia is important as it can exacerbate symptoms and complicate management of the crisis, as noted in the Journal of Clinical Oncology guidelines 1. If the patient shows signs of worsening weakness, stop any potential exacerbating factors immediately and consider giving calcium gluconate to counteract the effects. In severe cases, temporary ventilatory support may be necessary, as recommended by the American Society of Clinical Oncology clinical practice guideline 1. Always balance the need for magnesium replacement against the risk of exacerbating myasthenic symptoms, and coordinate closely with the patient's neurologist throughout the process, as emphasized in the Critical Care Medicine guidelines 1.
From the FDA Drug Label
When repeated doses of the drug are given parenterally, knee jerk reflexes should be tested before each dose and if they are absent, no additional magnesium should be given until they return. Serum magnesium levels usually sufficient to control convulsions range from 3 to 6 mg/100 mL (2. 5 to 5 mEq/L). The strength of the deep tendon reflexes begins to diminish when magnesium levels exceed 4 mEq/L. Reflexes may be absent at 10 mEq magnesium/L, where respiratory paralysis is a potential hazard.
To safely replace magnesium in a patient with Myasthenia Gravis (MG) in crisis,
- Monitor serum magnesium levels and the patient’s clinical status closely to avoid the consequences of overdosage.
- Key indicators of a safe dosage regimen include the presence of the patellar reflex (knee jerk) and absence of respiratory depression.
- Target serum magnesium levels should be between 3 to 6 mg/100 mL (2.5 to 5 mEq/L) to control convulsions.
- Knee jerk reflexes should be tested before each dose, and if they are absent, no additional magnesium should be given until they return.
- Be aware of the potential for respiratory paralysis at high magnesium levels (10 mEq/L).
- Have an injectable calcium salt available to counteract potential hazards of magnesium intoxication. 2
From the Research
Magnesium Replacement in Myasthenia Gravis Crisis
- Magnesium replacement can worsen neuromuscular weakness in patients with myasthenia gravis (MG) 3
- A case study reported that a 62-year-old woman with MG experienced acute respiratory failure induced by magnesium replacement, requiring intubation and mechanical ventilation 3
- The effects of magnesium replacement on MG patients have not been well documented, and caution is advised when considering magnesium replacement in these patients 3
Management of Myasthenic Crisis
- Myasthenic crisis is a life-threatening complication of MG that requires prompt treatment, including intubation and mechanical ventilation 4, 5, 6
- Plasma exchange and intravenous immunoglobulins are conventional treatments for myasthenic crisis, with plasma exchange potentially being more effective in cases involving respiratory failure 4
- Immunotherapy, including steroids and immunosuppressants, is also crucial in managing myasthenic crisis 4, 6
- Newer therapeutic options, such as efgartigimod, have shown promise in treating refractory myasthenic crises 7