Myasthenia Gravis

Autoimmune disorder of neuromuscular junction transmission, characterized by weakness and fatiguability of proximal limb, bulbar and ocular muscles

Aetiology

  • Autoimmune - antibodies to ACh receptors (AChR) are found in 80-90% of patients
  • 75% of patients have thymic hyperplasia/thymoma
  • Can occur at any age but there are two peaks of incidence: females in 3rd decade, males in 6th or 7th decade (larger)

Pathophysiology

  • Autoimmune origin in many cases - autoantibodies against nAChr in the endplate results in a reduction in the number of functional channels → amplitude of endplate potential decreases
  • Even with normal amounts of ACh transmission becomes inefficient
  • Leads to muscle weakness and fatiguability - symptoms start when ACh receptors reduced to 30% of normal

Clinical presentation

  • Fatiguable weakness of skeletal muscle
  • Most common presentation with extraocular weakness (60%), facial and bulbar weakness
  • In a small number may remain only ocular
  • Usually generalised
  • Limb weakness typically proximal

Investigations

  • Serum AChR antibodies
  • Single fibre EMG
  • CT chest for thymoma

Management

Acute

  • ACh inhibitor - pyridostigmine
  • IV immunoglobin or plasma exchange
  • Thymectomy

Long term - immunomodulating

  • Steroids
  • Steroid sparing agents - azathioprine, mycophenolate
  • Emergency treatment with plasma exchange or immunoglobulin
  • Avoid certain drugs - anaethesia, gentamicin