Drugs to avoid in myasthenics:

Myasthenia Gravis for the Internist
Neurologic disorder characterized by muscle weakness worsened by muscled use and improved w/ rest Epidemiology: women in 20s-30s, men >60 Pathogenesis: auto-Ab’s vs. acetylcholine receptor (80-90% of cases), muscle specific receptor tyrosine kinase Ab’s Cardinal feature: degree of weakness worsens w/ exercise of affected muscles, improves w/ rest Eye muscles: ptosis, diplopia (extraocular eye muscle involvement) Bulbar muscles: facial muscle weakness (smiling), weakness of palate and tongue, difficulty chewing, difficulty swallowing, choking Limb and trunk muscles: usually proximal muscles i. Inducible ptosis by sustained upward gaze ii. Fatigue of upper extremities after outstretched arms iii. Breath count – deep inspiration followed by counting out quickly during expiration (<20 ≈ FVC<1) iv. Weakness w/ neck flexion (and extension): 4/5 or less is concerning i. IV injection of Edrophonium chloride (acetylcholinesterase inhibitor) EMG studies, repetitive nerve stimulation Patient history - # intubations, bipap, common myasthenia triggers (ex infection, medication, noncompliance), myasthenia manifestations specific for that particular patient Breath count (<20 is concerning) – compare to baseline Low FVC (<=1 or compared to baseline) Æ measure ABG if you don’t think they need intubation in next 20 min. Intubate if necessary (FVC much lower than baseline, esp if <1, use of accessory muscles, concerning ABG) *Note: low FVC (or breath count) can be result of poor effort – always look at patient, assess overall clinical status, and remember that FVCs may vary according to how tired the patient is (time of day) and when they got their last dose of Ach-i Pyridostigmine (mestinon) – acetylcholinesterase inhibitor i. Muscarinic side effects – abdominal cramping, diarrhea, fasciculations, weakness (distinguish from weakness precautions, consider nocturnal BIPAP (for tenuous patients) i. Azathioprine, Cyclosporine, Mycophenolate (cellcept), Plasmapheresis, IVIg ii. Steroids may initially worsen myasthenia, so start in hospital Elective intubation for respiratory depression (esp w/ FVC <1) –> admit to NCCU (or MICU, if NCCU full) Drugs to avoid in myasthenia
Drugs that impair N-M transmission and may increase weakness
Antibiotics
Aminoglycosides: tobramycin, gentamycin, netilmicin, neomycin, streptomycin, kanamycin
Fluoroquinolones: ciprofloxacin, norfloxacin, ofloxacin, gatifloxacin
Tetracyclines
clindamycin
Sulfonamides
Penicillins – considered safe, though anecdotes of ampicillin causing resp depression
Macrolides: azithromycin, clarithromycin
Ritonavir
-avoid magnesium containing drugs in renal failure Drugs implicated as potentially harmful in myasthenia gravis patients based on either anecdotal case reports or
in-vitro microelectrode studies
Beta blockers
propanolol, oxprenolol, timolol, practolol, atenolol, labetalol, metoprolol, nadolol lithium, phenothiazines, amitriptyline, imipramine, amphetamines, haldol

Source: http://www.silkview.com/refs/myasthenia.pdf