Drugs That Can Aggravate Myasthenia Gravis

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Antibiotics: tetracyclines, aminoglycosides, polymy-xins, gyrase inhibitors, penicillins

Psychoactive drugs: benzodiazepines, barbiturates, tricyclic antidepressants, chlorpromazine, haloperidol, droperidol, lithium

Anticonvulsants: phenytoin, ethosuximide, barbiturates

Cardiovascular agents: Quinidine, ajmaline, procainamide, lidocaine, ganglioplegics, nifedipine, ^-block-

Cephalosporins, chloramphenicol

Promethazine, thioridazine. Chlordiazepoxide, ma-protiline, mianserin or carbamazepine can be used at low doses and with careful monitoring


Digitalis, reserpine, methyldopa, tocainide, verapamil (low-dose)

Miscellaneous: ACTH, corticosteroids2, D-penicil-lamine, morphine and derivatives, magnesium, general anesthesia (muscle relaxants)

1 Mask symptoms of myasthenia. 2 High starting dose.

Aspirin, gold, indometacin, acetaminophen, diclofenac, local/regional anesthesia, spinal anesthesia, inhalant anesthetics/deeper general anesthesia

(Selected drugs from McNamara and Guay, 1997)

Table 73 Myasthenia-related crises (p. 342)


Symptoms and Signs

Precipitating Factors

Myasthenic crisis

Restlessness, anxiety, confusion, respiratory weakness, weak cough, dysphagia, dysarthria, mydriasis, ptosis, tachycardia, pallor

Infectious diseases, surgical interventions, anesthesia, drugs, psychosocial stress, impaired drug uptake (vomiting, diarrhea), disease progression, previously undetected myasthenia (and previously mentioned factors)

Cholinergic crisis

Restless, anxiety, confusion, respiratory weakness, weak cough, dysphagia, dysarthria, miosis, bradycardia, skin reddening, muscle fasciculation/spasms, salivation, tenesmus, diarrhea

Overdosage (relative) of AChE inhibitors; acetylcholine poisoning

Table 74 Ancillary tests in myasthenia gravis (p. 342)

Table 74 Ancillary tests in myasthenia gravis (p. 342)



Interpretation of Results

Edrophonium chloride test1 (Ten-silon, Camsilon)

Increase in muscle strength (with improvement of ptosis, eye movements, speech, and swallowing)

Marked improvement (beginning 30 seconds after administration and lasting roughly 5 minutes) o unequivocal response. Sensitivity for OMG2: ca. 86%, for GMG3: ca. 95%

Electromyography (EMG)4

Documentation of impaired neu-romuscular conduction (decrement in amplitude seen with serial stimulation; jitter may be observed in single-fiber EMG)

A decrement of 10% or more is pathological. Sensitivity of serial stimulation in OMG: ca. 34%; in GMG: up to 77%. Prior muscle exercise o more pronounced decrement. Sensitivity of single-fiber EMG: ca. 92%

Serum acetylcholine receptor antibody titer

Documentation of presence of acetylcholine receptor antibodies

Sensitivity: 50% in OMG, ca. 90% in GMG. False-positive results may occur in Lambert-Eaton syndrome, rarely in amyotrophic lateral sclerosis

Diagnostic imaging5

Measurement of thymus

Thymic enlargement due to thy-moma or hyperplasia

(Phillips and Melnick, 1990)

(Phillips and Melnick, 1990)

1 Short-term inhibition of cholinesterase, given intravenously for diagnostic purposes. 2 Ocular myasthenia gravis. 3 Generalized myasthenia gravis. 4 Example: Repeated stimulation of accessory nerve (3/sec for 3 seconds) and recording of activity in trapezius muscle. 5 CT (contrast-enhanced) or MRI (younger patients, better differentiation of thymic hyperplasia).

Table 75 Toxic myopathies (p. 347)


Substances (selected)

Muscle weakness with or without pain; rhabdomyo-lysis may occur

Alcohol, chloroquine, cimetidine, clofibrate, cocaine, colchicine, ciclosporin, disulfiram, emetine, er-gotamine, gemfibrozil, induced hypokalemia (diuretics, licorice), imipramine, isoniazide, lithium, lovastatin, meprobamate, niacin, pentazocine, thyroid hormones, vincristine, zidovudine


Alcohol, allopurinol, cimetidine, clofibrate, clonidine, dihydroergotamine, ergotamine, methyldopa, suc-cinylcholine, vincristine, zidovudine

Polymyositis, pseudo-lupus erythematosus

Bezafibrate, chlorpromazine, cimetidine, clofibrate, d-penicillamine, etofibrate, etofyllin clofibrate, fenofi-brate, gold, hydralazine, isoniazide, l-tryptophan, penicillin, phenytoin, procainamide, tetracyclines, zidovudine


Ciclosporin, 20,25-diazocholesterol, diuretics, d-peni-cillamine, fenoterol, pindolol, propranolol

Local muscle lesions (pain, swelling, local muscular atrophy)

Heroin, insulin, meperidine, pentazocine


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