Neurology Book

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Myasthenia Gravis

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  1. Pathophysiology
    1. Neuromuscular autoimmune disease
    2. Antibodies form to nicotinic acetylcholine receptors
    3. Results in progressive weakness and fatigability
  2. Symptoms: Muscle Weakness provoked by exertion
    1. Proximal, Asymmetric Limb Muscle Weakness (85%)
    2. Cranial Muscle Weakness
      1. Lid lag
      2. Diplopia
      3. Facial Muscle Weakness
      4. Slurred speech
      5. Dysphagia
  3. Signs
    1. Muscle Weakness as above
    2. Deep Tendon Reflexes normal
  4. Labs
    1. Anticholinesterase (edrophonium) Test
      1. Results in improved muscle strength
    2. Electromyogram (EMG)
      1. Decremental response to repetitive nerve stimulation
    3. CT neck or MRI neck
      1. Thymoma evaluation
    4. Thyroid Function Test
      1. Hyperthyroidism (3-8%)
    5. Rheumatoid Factor (RF)
    6. Antinuclear Antibody (ANA)
  5. Differential Diagnosis
    1. Lambert-Eaton Syndrome
    2. Botulism
    3. Drug-induced Myasthenia
      1. Penicillamine
      2. Polymyxin
      3. Tetracycline
      4. Aminoglycosides
      5. Procainamide
      6. Propranolol
      7. Phenothiazine
      8. Lithium
    4. Neurasthenia
    5. Hyperthyroidism
    6. Intracranial mass lesion with extraocular affect
  6. Complications
    1. Respiratory compromise
    2. Aspiration Pneumonia
    3. Cholinergic crisis
  7. Management: Medication
    1. Anticholinesterase (Cholinergic)
      1. Mestinon (Neostigmine and Pyridostigmine)
    2. Immunosuppressive therapy
      1. Prednisone
        1. Start at 20 mg qd
        2. Increase gradually by 5 mg every 3 days to 60mg
        3. Continue for 3 months OR
          1. Until clinical improvement stops or declines
        4. Taper gradually to every other day
      2. Azathioprine (Imuran)
        1. Dosing
          1. 2 mg/kg/day
        2. Efficacy
          1. Effective when given with prednisone
          2. Effect not seen for 6 months or more
        3. Monitoring
          1. Complete Blood Count (CBC)
          2. Liver Function Tests (LFT)
    3. Plasmapheresis (Plasma Exchange) and IVIg
      1. Indicated for emergent worsening/crisis
      2. Response rate: 70%
  8. Management: Thymectomy
    1. Indications
      1. Age <60 years
      2. Inadequately controlled on Mestinon
      3. Thymoma discovered
    2. Effect
      1. Clinical improvement after thymectomy in 80%
      2. Benefits may not be seen for 6 months
      3. Transcervical thymectomy may be preferred
    3. References
      1. Calhoun (1999) Ann Surg 230:555

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