A patient with myasthenia gravis has antibodies against nicotinic acetylcholine receptors at the neuromuscular junction. Which electromyographic (EMG) finding is characteristic on repetitive nerve stimulation at low frequency (3 Hz)?
- A Incremental (increasing) CMAP amplitude with successive stimuli
- B Fibrillations and positive sharp waves at rest
- C Decremental (decreasing) CMAP amplitude (>10% decrement by 4th–5th stimulus) ✓
- D Myotonic discharges ('dive bomber' pattern)
Explanation
In myasthenia gravis, reduced functional nicotinic AChR at the NMJ means each successive nerve impulse (at 3 Hz repetitive stimulation) releases a normal quantum of ACh but activates fewer receptors because the safety margin for neuromuscular transmission is diminished. The compound muscle action potential (CMAP) decrements by >10% from the 1st to 4th or 5th stimulus — the characteristic decremental response. Post-exercise facilitation (brief improvement after 10–15 s exercise) followed by post-exercise exhaustion (worsening 2–4 min later) is also characteristic. Incremental response (Lambert-Eaton syndrome) and fibrillations (denervation) are different conditions.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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Written and medically reviewed by the StethoPrep medical team.