EMG changes in myasthenia gravis:
Correct Answer: Decreased amplitude of contraction on repeated stimulation
Description: Ans. d (Decreased amplitude of contraction on repeated stimulation). (Ref. Harrison, Medicine, 18th/Ch. 386)EMG in myasthenia gravis is characterized by increased amplitude evoked response on repeated stimulation. In myasthenia gravis there is defective release of acetylcholine at the neuromuscular junction, the compound muscle action potential elicited by a single stimulus is generally very small.Laboratory testing in MG:# Anti-AChR radioimmunoassay: 85% positive in generalized MG; 50% in ocular MG; definite diagnosis if positive; negative result does not exclude MG 40% of AChR antibody negative patients with generalized MG have anti-MuSK antibodies.# Repetitive nerve stimulation: decrement of >15% at 3 Hz: highly probable# Single-fiber electromyography: blocking and jitter, with nor fiber density;confirmatory,but not specific# Edrophonium chloride (Tensilon) 2 mg + 8 mg IV; highly probable diagnosis if unequivocally positive.# For ocular or cranial MG: exclude intracranial lesions by CT or MRI
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