Myasthenia Gravis

Why Lambert-Eaton myasthenic syndrome is correct

• Lambert-Eaton myasthenic syndrome (LEMS) is a paraneoplastic disorder most often associated with small cell lung cancer, caused by autoantibodies against presynaptic voltage-gated calcium channels.
• Reduced acetylcholine release produces proximal weakness and hyporeflexia that characteristically IMPROVE with brief repetitive activity (post-exercise facilitation) — the opposite of myasthenia gravis.
• Positive anti-VGCC antibodies and a negative edrophonium response point to a presynaptic disorder rather than a postsynaptic acetylcholine-receptor disease.
• Management is treatment of the underlying malignancy plus amifampridine (3,4-diaminopyridine).

Why the others are wrong

• Amyotrophic lateral sclerosis — causes mixed upper and lower motor neuron signs without an autoantibody or post-exercise facilitation (buzzword-matching weakness to a motor neuron disease).
• Generalized myasthenia gravis — a postsynaptic acetylcholine-receptor disease in which weakness WORSENS with sustained or repetitive activity and the edrophonium test is typically positive; its tumor association is thymoma, not small cell lung cancer, so neither the facilitation pattern nor the antibody here fits; chosen by anchoring on ptosis.
• Paraneoplastic dermatomyositis — proximal weakness accompanied by a heliotrope rash, Gottron papules, and elevated creatine kinase, not antibody-mediated neuromuscular-junction failure with facilitation (premature closure on paraneoplastic proximal weakness).
• Cushing syndrome from ectopic ACTH — small cell lung cancer is the classic source of ectopic ACTH, and the resulting hypercortisolism causes a proximal myopathy, but steroid myopathy spares the eyelids, does not improve with repeated effort, and is not antibody-mediated; it typically comes with hypokalemia, hypertension, and hyperglycemia rather than ptosis and anti-VGCC antibodies (buzzword-matching small cell lung cancer to its endocrine syndrome).

Why Ciprofloxacin is correct

• Fluoroquinolones impair neuromuscular transmission, reducing presynaptic acetylcholine release and blocking postsynaptic receptors, so a stable myasthenic patient can decompensate within days of the first doses.
• All systemic fluoroquinolones carry an FDA boxed warning for exacerbation of muscle weakness in myasthenia gravis, and the labeling advises avoiding them in patients with a known history of the disease.
• The timeline fits a drug-induced exacerbation rather than an infection-triggered one: the weakness began the day after the antibiotic was started, the cystitis has resolved, and she is afebrile.
• Management is to stop the offending drug, substitute a neuromuscular-junction-safe agent if the infection still needs treatment, and monitor respiratory and bulbar function (single-breath count, forced vital capacity, negative inspiratory force), escalating to IVIG or plasma exchange if crisis develops.

Why the others are wrong

• Cephalexin - cephalosporins do not appear on myasthenia gravis cautionary-drug lists and are among the preferred antibiotics when a patient with myasthenia gravis needs treatment; note that this is a property of the cephalosporins specifically, not of beta-lactams as a class (the trap is treating every antibiotic as equally risky rather than learning the specific agents that are).
• Fosfomycin - single-dose therapy for uncomplicated cystitis that inhibits bacterial cell wall synthesis at MurA; it has no activity at the neuromuscular junction and is considered safe in myasthenia gravis.
• Nitrofurantoin - a standard first-line cystitis agent whose characteristic toxicities are pulmonary (an acute hypersensitivity pneumonitis that can begin within the first days of therapy, or chronic interstitial disease after months), hepatic, and a peripheral neuropathy with prolonged use or renal impairment; none of these acts at the neuromuscular junction or produces fatigable weakness (the trap is equating any neurologic or pulmonary adverse effect with a myasthenic exacerbation).
• Trimethoprim-sulfamethoxazole - a first-line agent for uncomplicated cystitis where local resistance allows; sulfonamides appear on some broad myasthenia gravis caution lists on the basis of rare reports, and trimethoprim can raise potassium, but hyperkalemic weakness is not fatigable and typically spares the eyelids and bulbar muscles, and unlike the fluoroquinolones this drug carries no FDA warning for myasthenia gravis and is generally considered safe to use (the trap is mistaking a metabolic cause of weakness for a neuromuscular-junction effect).

Additional high-yield points

• Antibiotics on myasthenia gravis cautionary-drug lists: fluoroquinolones, aminoglycosides, macrolides, telithromycin (labeled as contraindicated in myasthenia gravis, though no longer marketed in the United States), and, among the beta-lactams, ampicillin, high-dose intravenous penicillins, and imipenem; cephalosporins are not listed, which is why they are the usual substitute.
• Non-antibiotic drug precipitants include intravenous magnesium (including magnesium given for preeclampsia), beta-blockers, penicillamine, quinine and procainamide, immune checkpoint inhibitors, and neuromuscular blocking agents.
• Non-drug precipitants of exacerbation include infection, surgery, pregnancy and the postpartum period, tapering of immunosuppression, heat, and untreated thyroid disease.
• Pyridostigmine remains the initial symptomatic therapy for myasthenia gravis, but it does not protect against drug-induced decompensation, so reviewing the medication list is part of every visit.
• Myasthenic crisis (respiratory muscle weakness requiring ventilatory support) is treated with IVIG or plasma exchange plus immunosuppression, not by escalating pyridostigmine, which risks cholinergic crisis with bradycardia, secretions, and worsening weakness.