Neuromuscular junction diseases, particularly myasthenia gravis, are high yield for neurology examinations because they command a deep understanding of pathology, diagnostic testing, and therapeutics. Test your knowledge with questions after completing this chapter packed with high-yield facts.

Author: Brian Hanrahan MD

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Myasthenia Gravis (MG)

  • MG is a postsynaptic autoimmune disease with antibodies directed toward acetylcholine receptors or receptor-associated proteins.
  • Patients usually present with weakness, diplopia, dysarthria, and/or dysphagia. Symptoms worsen with sustained or repetitive activity and improve with rest.
    • 15% of patients have only ocular deficits referred to as ocular myasthenia gravis.

Diagnostic tests

  • Clinical testing
    • The ice pack test (placing a bag of ice on a closed lid for one minute) can be performed to look for improved ptosis/diplopia as colder temperatures mediate neuromuscular transmission.
    • The Tensilon (edrophonium) test can also be done but is rarely performed anymore in clinical practice.
  • EMG testing:
    • Single fiber electromyography is the most sensitive test for MG (but not specific). If pathologic it will show variable depolarization of a single action potential described as “jitter”.
    • Repetitive nerve stimulation at 2 to 3 Hz would show a decrement in compound muscle action potentials (CMAPs) due to the progressive failure of neuromuscular transmission.

  • Antibody testing:
    • ~80% of cases have anti-Acetylcholine receptor (AChR) antibodies.
    • 40% of seronegative patients for anti-AChR (i.e. about 5-10% of all MG patients) have anti-muscle-specific receptor tyrosine kinase (MuSK) antibodies.
      • MuSK mediates the clustering of Ach receptors in the neuromuscular junction.
      • Anti-MuSK MG patients may have atypical presentations with prominent facial, bulbar, and neck involvement with sparing of ocular muscles.
    • LRP4 is involved in MuSK activation, and antibodies to LRP4 have more recently been implicated in MG. Approximately 50% of double-negative (i.e. MuSK and AChR negative) MG patients will have positive anti-LRP4.

Additional workup

  • CXR vs. CT chest to evaluate for thymoma.
    • Patients with myasthenia plus thymoma often have circulating serum striational antibodies (anti-titin, anti-ryanodine receptor (RyR), and anti-Kv1.4) in addition to anti-AChR antibodies.


 

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Table of Contents

Table of Contents