EMG/nerve conduction studies in MG typically show what pattern?

• A. Incremental response on repetitive nerve stimulation
• B. Decremental response on repetitive nerve stimulation
• C. No change in response
• D. Increased nerve conduction velocity

Answer: (B)

Myasthenia gravis causes fatigable failure of neuromuscular transmission because autoantibodies reduce the number or function of postsynaptic acetylcholine receptors. When nerves are stimulated repeatedly at a low rate during EMG studies, the muscle response (the CMAP) tends to get smaller with each successive stimulus. This decrement reflects the reduced safety margin: as stimulation continues, fewer receptors are available to respond, so the end-plate potential may fail to trigger a muscle action potential and the CMAP amplitude declines. The drop is more noticeable in affected muscles and can lessen with rest or after brief recovery.

This pattern helps distinguish MG from other conditions. An increment seen with repetitive stimulation points to Lambert-Eaton myasthenic syndrome, not MG. No change in response wouldn’t fit MG, and increased nerve conduction velocity isn’t a feature of a neuromuscular transmission disorder like MG.