Myasthenia & Drugs

05 November 2004

Neuromuscular transmission has large 400% safety factor due to abundance of Ach release and Ach receptors.

Clinical weakness only if 25% of Ach receptor or Ach production is remaining.

Mechanisms causing exacerbation of MG:

1.inhibiting Ach release

            chloroquine

            B blockers

            Gadolinium

2.binding to postsynaptic region

            imipenem

            tetracyclines

            procainamide

            pyrantel

            lithium

3.both mechanisms

            aminoglycoside

            dilantin

            chlorpromazine

            verapamil

Problems in literature 

Multiple case reports / series with numerous agents

Patients not always rechallenged with offending agent

Patients sick with other medical conditions eg sepsis

Predictable responses 

Reduced dose of muscle relaxant anesthesia 25% compared to normals; also found to be inversely related to Ach receptor antibody.

High dose steroids causing initial worsening before improvement (often).

Commonly prescribed agents & probable association with exacerbation of MG

            Aminoglycosides

            Ciprofloxacin

            Lithium

            Dilantin

            Procainamide

            Quinidine

            B blockers

Commonly prescribed agents & possible association with exacerbation of MG

            Ampicillin

            Anticholinergics

            Erythromycin

            Radiocontrast

            Verapamil

Less commonly prescribed agents & possible association with exacerbation of MG

            Chloroquine

Imipenem

Levonorgestrel

            Phenothiazine

Pyrantel

Transdermal nicotine

Current recommendations 

Most drugs, contraindication is only relative

Aggravate MG in a minority of situations but are the most suitable in certain situations eg antibiotics in sepsis.

Obtaining good MG control prior to initiation of potentially adverse drug.

Awareness that if MG control worsens, maybe due to newly introduced drug.