Myasthenia Gravis - MG

Myasthenia gravis (MG) is an autoimmune disease that results in the failure of neuromuscular transmission.

Epidemiology

• Incidence – 200-400/1,000,000
• Gender – bimodal distribution for age of onset
  • Women – 20-30 years
  • Men – 55-60 years

Pathophysiology

• AChR antibodies are formed which induce:
  • Binding and activation of  complement at the neuromuscular junction
  • Accelerated degradation of AChR molecules crosslinked by antibody  
  • Functional AChR block

Clinical Presentation

• Characteristic muscle weakness that worsens after use of affected muscles
• Swallowing difficulties
• Diplopia
• Extraocular muscle (EOM) weakness
  • Symptoms are present in 2/3 of patients
  • If symptoms remain limited to EOM (10% of patients) then MG is ocular MG
• Death in severe cases
• Commonly associated with thymoma

Diagnosis

• Indications for ordering
  • Initiate testing based on classic symptoms
• Laboratory testing
  • Serum antibody testing
    • Anti-AChR measurement – positive in 85% of patients
      • However, negative AChR does not exclude disease
    • Anti-MuSK – detectable in 30-40% of anti-AChR negative patients
    • Anti-striational protein – detectable in 80% of thymomatous MG and 30% of nonthymomatous MG patients
• Clinical testing
  • Consider anticholinesterase testing (Tensilon test using Edrophonium) in AChR-negative patients
  • Consider repetitive stimulation or single fiber electromyogram (EMG)
    • Positive in 90% of MG patients

Differential Diagnosis

• Guillain-Barré
• Amyotrophic lateral sclerosis (ALS) – Lou Gehrig disease
• Eaton Lambert syndrome
• Botulism

Treatment

• Anticholinesterase drugs, thymectomy, immunosuppression and plasmapheresis

Indications for Ordering

• Tests generally appear in the order most useful for common clinical situations
• Click on number for test-specific information in the ARUP Laboratory Test Directory

• Click on number for test-specific information in the ARUP Laboratory Test Directory