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Updated: Jun 18 2021

Myasthenia Gravis

  • summary
    • Myasthenia Gravis is a common autoimmune disorder caused by autoantibodies directed against the acetlycholine receptor of the neuromuscular junction. Patients present with fluctuating proximal muscle weakness, ocular symptoms, and dysphagia.
    • Diagnosis is made with serologic testing for anti-AChR autoantibodies.
    • Treatment is medical management with acetylcholinesterase inhibitors.
  • Epidemiology
    • Incidence
      • has a bimodal distribution
        • more common in younger women (< 40 years of age) and older men (> 50 years of age)
    • Risk factors
      • HLA-B8
      • medications
        • penicillamine
        • aminoglycosides
  • Etiology
    • Pathogenesis
      • autoantibodies directed against a protein of the neuromuscular junction
        • autoantibodies can be directed against
          • nicotinic acetlycholine receptor (AChR)
            • more common
          • muscle-specific receptor tyrosine kinase (MuSK)
        • categorized as a type II hypersensitivity reaction
      • T-cells play a role as well
        • thought to stimulate B-cell antibody production
    • Associated conditions
      • thymoma
      • thymic hyperplasia
  • Presentation
    • Symptoms
      • general feature
        • fluctuating muscle weakness
          • most common weakness is worse with continued use
            • e.g., worse at the end of the day
        • true muscle fatigue
          • secondary to decreasing contractile muscle force
      • ocular symptoms
        • most common presenting symptoms
          • ptosis
          • diplopia
      • bulbar symptoms
        • dysphagia
        • dysarthria
        • fatigable chewing
      • proximal muscle weakness
    • Physical exam
      • ice-pack test
        • place ice on the patient's ptosis → ptosis improves
          • low temperatures change the kinetics of acetylcholinesterase, decreasing its activity
            • this increases the amount of acetylcholine in the synaptic cleft
      • edrophonium chloride (Tensilon test)
        • only used in patients with ptosis or ophthalmoparesis
          • this allows for improvement in muscle strength to be observed
        • edrophonium is an acetylcholinesterase inhibitor that has a short duration of action
          • this increases the amount of acetylcholine in the synaptic cleft
  • Imaging
    • Computerized tomography (CT) scan
      • indication
        • to rule out a thymoma
      • view
        • chest
  • Studies
    • Labs
      • serologic testing for autoantibodies
        • anti-AChR
          • this is the initial laboratory test to confirm the diagnosis
        • anti-MuSK
    • Electromyogram (EMG)
      • decremental decrease in the compound muscle action potential (CMAP)
  • Differential
    • Lambert-Eaton myasthenic syndrome (LEMS)
      • differentiating factor
        • muscle weakness that improves with use
        • autonomic manifestations
    • Botulism
      • differentiating factor
        • poor or impaired pupillary response to light
    • Thyroid ophthalmopathy
  • Treatment
    • Medical
      • corticosteroids
        • indication
          • a chronic immunotherapy agent
        • drugs
          • oral prednisone
      • acetylcholinesterase inhibitors
        • indication
          • considered first-line for symptomatic management
        • drugs
          • pyridostigmine
        • adverse effects
          • abdominal cramping and diarrhea
          • bradycardia
          • sweating
          • bronchial secretion
    • Procedural
      • intravenous immunoglobulins (IVIg) or plasmapharesis
        • indication
          • rapid immunotherapy for myasthenic crisis
    • Surgical
      • thymectomy
        • indication
          • in patients with a thymoma, irrespective if the patient has myasthenia gravis or not
  • Complications
    • Myasthenic crisis
      • respiratory weakness secondary to myasthenia gravis
  • Prognosis
    • Most patients with ocular involvement progress to generalized myasthenia gravis
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