Updated: 3/22/2018

Myasthenia Gravis

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Introduction
  • Clinical definition
    • autoimmune disorder of the neuromuscular junction
  • 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
  • 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
  • Prognosis
    • most patients with ocular involvement progress to generalized myasthenia gravis
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
 

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