Fosfomycin Safety in Myasthenia Gravis
Fosfomycin appears to be safe for use in patients with myasthenia gravis (MG), as it is not listed among the antibiotics known to exacerbate neuromuscular transmission or trigger myasthenic crisis.
Evidence-Based Safety Profile
The available evidence does not identify fosfomycin as a problematic antibiotic in MG patients:
No documented neuromuscular blockade: Unlike fluoroquinolones, which exhibit neuromuscular blockade in animal and in vitro models and have caused myasthenic crises requiring ventilatory support in 30% of cases, fosfomycin lacks this mechanism of action 1
Absence from high-risk antibiotic lists: Published case series and reviews of MG exacerbations identify fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) and even beta-lactams (amoxicillin) as triggers, but fosfomycin is notably absent from these reports 1, 2, 3, 4
Clinical Considerations for Use
When prescribing fosfomycin to MG patients, standard contraindications still apply:
Avoid in patients with hypernatremia, cardiac insufficiency, or renal insufficiency due to the high sodium content of intravenous formulations 5, 6
Monitor serum potassium levels during IV fosfomycin therapy, as hypokalemia occurs in approximately 6% of ICU patients and could theoretically worsen muscle weakness 6
Perform susceptibility testing before initiating therapy, particularly for resistant organisms (fosfomycin susceptibility ranges from 39-99% for carbapenem-resistant organisms) 5, 7
Practical Algorithm for Antibiotic Selection in MG
When treating bacterial infections in MG patients:
Absolutely avoid: Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) - these carry 30% risk of myasthenic crisis and 5% mortality 1, 2
Use with caution and close monitoring: Aminoglycosides (interfere with neuromuscular transmission), macrolides, and beta-lactams including amoxicillin (documented MG exacerbations within days of administration) 3, 4
Generally safe options: Fosfomycin, tigecycline (successfully used for community-acquired pneumonia in MG patients), and cephalosporins like cefuroxime 6, 2
For combination therapy: Fosfomycin can be safely combined with tigecycline, polymyxins, or carbapenems for resistant infections without adverse pharmacodynamic interactions 5, 6
Common Pitfalls to Avoid
Failing to recognize infection as a trigger: Respiratory infections are the most frequent precipitant of MG exacerbations, making prompt antibiotic treatment essential despite concerns about drug selection 8, 4
Overlooking rapid onset of antibiotic-induced exacerbations: Fluoroquinolone-related MG deterioration occurs within a median of 1 day, requiring immediate recognition and drug discontinuation 1
Not monitoring for clinical worsening: Even with "safer" antibiotics, MG patients require close observation during infections, as the infection itself (not just the antibiotic) can precipitate crisis 4
Ignoring previous adverse reactions: Positive rechallenge occurs in 16% of fluoroquinolone exposures, so careful medication history is essential 1