Page 171 - Critical Care Notes

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Myasthenia Gravis (MG)
MG is a neuromuscular autoimmune disease causing muscle weakness and
fatigability of skeletal muscles.
Pathophysiology
Circulating antibodies block acetylcholine receptors at the postsynaptic neuro-
muscular junction → inhibits acetylcholine → inhibits depolarization of muscles
→↑ nerve impulse transmission → diminishes muscle contraction, including
diaphragmatic muscle →↓ vital capacity and respiratory failure.
Causes include thymic hyperplasia and tumor of the thymus gland.
Clinical Presentation
■ Abnormal muscle weakness that increases during activity and improves
after rest; eye muscle weakness; possible ptosis, diplopia, and inability to
maintain upward gaze
■ Weakness of limb, axial, bulbar, and/or respiratory muscles, especially
those related to chewing, talking, swallowing (dysphagia), breathing, and
neck and limb movements; inability to close mouth and inability to raise
chin off chest
■ Slurred speech, neck muscle weakness with head bobbing
■ Diaphragmatic and intercostal weakness → dyspnea, difficulty coughing
■ Crises that may be triggered by fever, infection, trauma, extreme tempera-
tures, adverse reaction to medication or emotional stress → respiratory dis-
tress, dysphagia, dysarthria (difficulty speaking), eyelid ptosis, diplopia,
prominent muscle weakness, neurological changes, absence of sweating,
hyperthermia → respiratory failure
■ Myasthenia crisis manifested by respiratory paralysis
Diagnostic Tests
■ Muscle fatigability test
■ Antibodies against acetylcholine receptor (AChR Ab) ≤0.03 nmol/L or
negative
■ Edrophonium chloride (Tensilon) test: Available as 10 mg/mL. Administer
2 mg IV over 15–20 sec; resolution of facial muscle weakness and ptosis
and improved muscle strength should be seen in 30 sec; if no response in
45 sec, inject the remaining 8 mg. May repeat the test after 30 min; have
atropine, ECG monitoring, and advanced life-saving equipment available.

NEURO