Myasthenia Gravis

“Chronic autoimmune, type 2 hypersensitivity, inflammatory disease of the neuromuscular junction causing skeletal muscle weakness“

Risk Factors

• Modifiable:
  • Smoking
• Non-Modifiable:
  • Females under 40
  • Males over 60
  • Mother with myasthenia gravis(MG) in neonatal MG

Aetiology

• Antibody to component of neuromuscular junction causing fatigable muscle weakness

Pathophysiology

• B cell producing autoantibody to nAch Receptor:
  • ACh unable to bind => complement activation => inflammation and destruction of myocytes =>↓ACh receptor => ↓muscle contraction
• B cell producing muscle-specific receptor tyrosine kinase( MuSK) autoantibodies
  • Target intracellular muscle protein=> myocyte destruction =>↓muscle destruction
• Paraneoplastic syndrome (bronchogenic carcinoma, thymic neoplasm)
  • Cancer cells producing autoantibodies to a component of NMJ => myocyte destruction =>↓muscle contraction

Clinical Presentation

• Muscle weakness that worsens with activity and improves with rest.
• Extraocular muscle weakness (ocular myasthenia)
• Ptosis
• Diplopia
• Change in facial expression
• Difficulty swallowing
• Dyspnoea
• Dysarthria
• Limb weakness

Investigations

• Bloods (anti-AChR, anti-MuSK or anti-LRP4 and thyroid function)
• Edrophonium test
• Single fibre electromyography
• Chest CT or MRI(thymoma)
• Brain MRI

Management

• Anticholinesterase (neostigmine, pyridostigmine)
• Immunosuppressant (prednisone, azathioprine, cyclosporin, mycophenolate mofetil, tacrolimus)
• Thymectomy

Complications

• Myasthenic crisis (weakness of respiratory muscles)
• Aspiration pneumonia