Myasthenia Gravis 07/13/2017

  • Definition
    1. Two clinical forms of myasthenia
      1. Ocular
        • Limited to the eyelids and exraocular muscles
      2. Generalized
        • Affects the ocular muscles and variable combinations of bulbar, limb, and respiratory muscles
  • Pathogenesis
    1. Auto-antibodies directed against acetylcholine receptors (AChR)
      • Autoantibodies are present in 90% of patients
  • Epidemiology
    1. Can happen at any age, but there is a Bimodal distribution
      1. Early peak –> 20-30s
        • Female predominance
      2. Late peak –> 60-80s
        • Male predominance
  • Presenting symptoms
    1. Occular symptoms (Ptosis and/or diplopia) –> 50% of patients
    2. Bulbar Sx (dysarthria, dysphagia, fatigable chewing –> 15% of patients
    3. Muscle weakness that worsens throughout the day
    4. Respiratory muscles
      1. Most feared symptom –> Can lead to respiratory failure (“myasthenic crisis”)
  • Diagnosis
    1. Bedside tests
      1. Ice pack test
        • Bag of ice is placed on the eyelid for 2 minutes then removed. –> Improvement in ptosis is measured
        • 80% sensitivity
      2. Tensilon (Edrophonium) test
        • 2mg doses of edrophonium are given –> improvement in symptoms measured
    2. Lab tests
      1. Two Auto-antibodies:
        • Acetylcholine receptor antibody (AChR-Ab)
          • Present in 85% of patients with generalized disease
        • Muscle specific receptor tyrosine kinase (MuSK)
          • Present in up to 50% of patients who are AChR-Ab negative.
    3. Electrophysiological studies
      1. Repetitive nerve stimulation (RNS)
        • 75% sensitive
        • Most frequently used due to wide availability
      2. Single-fiber EMG
        • Used if RNS is negative because it is 95% sensitive
        • Less widely available
  • Associated conditions
    1. Thymic tumors
      1. 75% of patients with AChR-antibodies have thymic abnormalities (70% hyperplasia vs 10% thymoma)
      2. CT vs MRI to evaluate
    2. Autoimmune disorders
      1. Thyroid disease (5%)
      2. RA
      3. SLE
  • Treatment
    1. Acetylcholinesterase inhibitors
      • Pyridostigmine (mestinon)
        • Some patients only need this
        • Cholinergic side effects
    2. Immunomodulator drugs
      1. Chronic immunomodulators
        • Glucocorticoids
        • Azathioprine, mycophenolate, cyclosporine
      2. Rapid
        • Useful in myasthenic crisis or preoperatively before thymectomy à not useful long term
          • IVIG
          • Plasmapharesis
    3. Thymectomy

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