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
2.binding to postsynaptic region
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
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
Commonly prescribed agents & possible association with exacerbation of MG
Less commonly prescribed agents & possible association with exacerbation of MG
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.