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Updated: Jun 2 2024

Myasthenia Gravis

Images junction.jpg
  • Snapshot
    • A 62-year-old man presents to his primary care physician for "seeing double." He reports that he has been experiencing these symptoms for a few months. His diplopia is worse after reading a book or watching television. He says that at times he notices his left eyelid "droops" at the end of the day and improves after waking up from sleep. He notes episodes of fluctuating weakness when chewing harder foods. On physical examination, there is left-sided ptosis. Pupillary reflex is intact. A plastic glove with ice is applied on the ptosis for approximately 2 minutes and the ptosis markedly improves. Serology is positive for anti-acetylcholine receptor antibodies and preparations are made for a computerized tomography scan of the chest.
  • Introduction
    • Clinical definition
      • autoimmune disorder of the neuromuscular junction
    • Associated conditions
      • thymoma
      • thymic hyperplasia
  • 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
  • Presentation
    • Symptoms
      • general feature
        • fluctuating muscle weakness
          • most commonly 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
        • may result in false positives or cause life threatening bradycardia; thus, this test is rarely performed
  • 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 plasmapheresis
        • indication
          • rapid immunotherapy for myasthenic crisis
    • Surgical
      • thymectomy
        • indication
          • in patients with a thymoma, irrespective if the patient has myasthenia gravis or not
          • associated with increased remission rates in patients with myasthenia gravis
  • Complications
    • Myasthenic crisis
      • respiratory weakness secondary to myasthenia gravis
  • Prognosis
    • Most patients with ocular involvement progress to generalized myasthenia gravis
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