Myasthenia gravis

Myasthenia gravis is a rare autoimmune disorder which causes fatigable muscle weakness.

Symptoms of myasthenia gravis (MG) vary from day to day but tend to be worse in the evening.

The most common muscles affected, in decreasing frequency, are:
  • Levator palpebrae superioris
  • Extraocular muscles
  • Proximal limb muscles
  • Muscles of facial expression
  • Neck extensors

Myasthenic crisis is worsening of myasthenia resulting in paralysis of respiratory muscles and the requirement for respiratory support. This is usually precipitated by infections or insufficient treatment.

  • Electrodiagnostic – repetitive stimulation leads to decreased evoked response
  • Antibodies
    • acetylcholine receptor antibodies – present in 80%-90% of patients with generalised MG
    • MuSK antibodies - present in around 30%
  • the tensilon test = administration of edrophonium chloride causes an unequivocal improvement in strength

Note reflexes are normal or brisk in MG – in contrast, they are absent in Lambert Eaton syndrome,

  • Acetylcholinesterase inhibitors
  • Prednisolone
  • Plasmaphoresis if crisis
  • IV IG if crisis
  • Thymectomy – up to 80% experience improvement

Drugs which should be avoided as they worsen MG include:
  • Penicillamine
  • Some antibiotics (aminoglycosides, macrolides, fluoroquinolones)
  • Beta blockers
  • Calcium channel blockers
  • Quinine
  • Phenytoin

Other factors which can worsen MG include:
  • Hot temperature
  • Emotion
  • Hyperthyroidism (note around 10% of patients with MG have hyperthyroidism)
  • Pregnancy
  • Hypokalaemia

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