A 30-year-old woman with ptosis and fatigable weakness undergoes repetitive nerve stimulation of a hand muscle at 3 Hz. Which result supports a diagnosis of myasthenia gravis?

• A Incremental response greater than 100 percent between first and tenth stimuli
• B Normal compound muscle action potential amplitude with fibrillation potentials on needle EMG
• C Prolonged distal motor latency with reduced conduction velocity
• D Decremental response greater than 10 percent between the first and fourth or fifth stimulus ✓

Explanation

Myasthenia gravis reduces the safety factor of neuromuscular transmission, so successive end plate potentials fall below threshold and the evoked compound muscle action potential decrements by more than 10 percent at low-frequency stimulation. An incremental response exceeding 100 percent after brief exercise is the signature of Lambert-Eaton myasthenic syndrome, where calcium entry facilitates release with repeated activation. Slowed conduction indicates demyelinating neuropathy, not a junctional disorder.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.