Myasthenic Syndrome

Key Features

• There is defective release of acetylcholine in response to a nerve impulse, leading to weakness, especially of the proximal muscles of the limbs

• May be associated with small-cell carcinoma, sometimes developing before the tumor is diagnosed, and occasionally occurs with certain autoimmune diseases (eTable 39–1)

Clinical Findings

• Variable weakness, typically improving with activity

• Dysautonomic symptoms may also be present

• A history of malignant disease may be obtained

• Unlike myasthenia gravis, power steadily increases with sustained contraction

Diagnosis

• Electrophysiologic diagnosis: the muscle response to stimulation of its motor nerve increases remarkably after exercise or if ...