Musculoskeletal System >
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