What antibiotics are contraindicated in a patient with myasthenia gravis?

Antibiotics Contraindicated in Myasthenia Gravis

Aminoglycosides (streptomycin, gentamicin, tobramycin, neomycin, kanamycin, amikacin) and fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) are absolutely contraindicated in myasthenia gravis patients, as they can precipitate life-threatening myasthenic crisis and respiratory failure. 1, 2, 3

High-Risk Antibiotics to Avoid

Aminoglycosides - Absolute Contraindication

• Streptomycin is explicitly contraindicated in myasthenia gravis due to its neuromuscular blocking properties and ability to potentiate weakness 1
• All aminoglycosides (gentamicin, tobramycin, neomycin, kanamycin, amikacin) act as neuromuscular blocking agents by reducing presynaptic calcium uptake and acetylcholine release 1
• Neuromuscular blocking potency is highest with neomycin, followed sequentially by gentamicin, streptomycin, kanamycin, amikacin, and tobramycin 1
• Even topical aminoglycosides (tobramycin eye drops) can unmask or exacerbate myasthenia gravis after as little as 3 days of use 4

Fluoroquinolones - Avoid Use

• The FDA explicitly warns that levofloxacin should be avoided in patients with known myasthenia gravis due to neuromuscular blocking activity and postmarketing reports of deaths and ventilatory support requirements 3
• Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) worsen neuromuscular blockade and can precipitate myasthenic crisis with respiratory failure 2, 3
• Myasthenia gravis exacerbations develop a median of 1 day following fluoroquinolone exposure, with 30% requiring ventilatory support and 5% mortality 5
• These antibiotics progressively decrease miniature endplate potential amplitude, directly blocking neuromuscular transmission 6

Macrolides - Use Only After Careful Consideration

• Macrolide antibiotics (azithromycin, erythromycin, clarithromycin) should only be administered after careful consideration and with appropriate monitoring 2
• The FDA label for azithromycin specifically warns of exacerbation of myasthenia gravis symptoms and new onset of myasthenic syndrome 7
• Patients receiving macrolides require close monitoring for worsening muscle weakness, particularly proximal limb and ocular muscles 2

Antibiotics Requiring Extreme Caution

Tetracyclines

• Should only be used after careful consideration and with appropriate monitoring 1
• Theoretical concerns exist regarding tetracycline chelation of calcium, which could worsen neuromuscular transmission 1

Clindamycin

• Concerns exist about clindamycin's ability to block acetylcholine release, potentially working synergistically with aminoglycosides 1
• Should only be administered after careful consideration and with appropriate monitoring 1

Penicillins - Emerging Concern

• While traditionally considered safe, amoxicillin has been reported to cause acute myasthenia gravis exacerbations within days of administration 8
• Six documented cases showed worsening of MGFA clinical classification requiring therapeutic intervention 8
• Patients receiving amoxicillin should be closely monitored for possible acute relapse, though full recovery typically occurs within 1-2 months 8

Safer Antibiotic Options

• Topical bacitracin appears safe for ophthalmic use in myasthenia gravis patients 2
• Tigecycline has been successfully used to treat community-acquired pneumonia in myasthenia gravis patients who cannot tolerate fluoroquinolones 9
• When antibiotics are necessary, consultation with neurology for antibiotic selection is recommended 2

Critical Monitoring Parameters

When potentially problematic antibiotics must be used:

• Monitor for worsening muscle weakness, particularly proximal limb and ocular muscles 2
• Assess respiratory function with vital capacity and negative inspiratory force 2
• Watch for bulbar symptoms (speech, swallowing difficulties), diplopia, and ptosis 2
• Observe for clinical deterioration that could be related to the antibiotic 1

Common Pitfalls to Avoid

• Do not assume topical aminoglycosides are safe - even eye drops can trigger exacerbations 4
• Do not rechallenge with fluoroquinolones - 16% of patients experience recurrent myasthenic crisis with reintroduction 5
• Do not rely solely on traditional teaching that penicillins are universally safe - emerging evidence suggests amoxicillin can cause exacerbations 8
• Immediately discontinue any suspected antibiotic trigger at the first sign of worsening weakness 2