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Diagnostic Tests. Electromyography (EMG) Characteristic of MG: a rapid reduction in the amplitude of compound muscle action potentials in response to repetitive nerve stimulations Motor nerve conduction velocity is normal Anti-AchE medication is stopped 6 – 24 hrs. before testing

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diagnostic tests
Diagnostic Tests
  • Electromyography (EMG)
    • Characteristic of MG: a rapid reduction in the amplitude of compound muscle action potentials in response to repetitive nerve stimulations
    • Motor nerve conduction velocity is normal
    • Anti-AchE medication is stopped 6 – 24 hrs. before testing
    • Best to test weak muscles or proximal muscle groups
    • Electric shocks are delivered at a rate of 2-3 sec to the appropriate nerve
diagnostic tests1
Diagnostic Tests
  • AChR or MuSK receptor antibodies
    • Detection of anti-AChR antibodies provides a reasonably sensitive and highly specific test for the diagnosis of MG
    • juvenile myasthenics who do not have anti-ACh antibodies have instead MuSK R (tyrosine kinase receptor) antibodies, which also is localized at the NMJ
    • Measured levels of anti-AChR does not correspond well to the severity of MG
    • Negative test does not exclude the disease
diagnostic tests2
Diagnostic Tests
  • Anticholinesterase Test
    • Anticholinesterase inhibitors rolong and exaggerate the effects of Ach in the synapse and thereby provide an increment in muscle power
    • Edrophonium chloride (5mg initial dose) is commonly used for diagnostic testing because of its rapid onset (30s) and short duration (5min) of its effect
    • Reserved for patients with clinical findings suggestive of MG but negative anti-AChR and EMG
    • False-positives occur in patients with other neurologic disorders (e.g. amyotrophic lateral sclerosis)
diagnostic tests3
Diagnostic Tests
  • Anticholinesterase test (recommendations for infants <2yrs old)
    • A specific fatigable weakness should be measurable (e.g. ptosis of the eyelids)
    • Edrophonium is not recommended for use in infants because its effect is too brief for objective assessment and an increased incidence of acute cardiac arrhythmias is reported in infants
    • IM Neostigmine (0.5-1.5mg) is instead used
      • Dosage: 0.04 mg/kg; if the result is (-), another dose of 0.04 mg/kg may be administered after 4hrs). The peak effect is seen in 20–40 min
      • Should not be given IV because of risk of cardiac arrythmias
reference
Reference
  • Nelson Textbook of Pediatrics, 18th ed.
  • 2013 block A trans