On examination the key feature is fatigable weakness, with normal sensation, reflexes, and coordination. Ptosis and diplopia can be elicited with sustained upward gaze for 30-60 seconds. Repeated testing of proximal muscle strength also may bring out weakness. The clinical course of MG is highly varible between patients. Ocular MG rarely becomes systemic after 2 years.
Respiratory failure requiring ventilation can occur in severe cases.
Pathophysiology Myasthenia gravis is a disorder of neuromuscular transmission. In order to understand what causes MG, we must first describe what constitutes normal neuromuscular transmission. ACh is then stored in vesicles, forming a quantum, containing about 10,000 molecules of ACh. Quanta of ACh are released into the nerve terminal by calcium dependent exocytosis - the fusing of the vesicles with the outer membrane wall. Spacial and temporal summation of the EPP s are usually sufficient to reach threshold and cause a muscle membrane action potential.