Waning
Progressive decline in compound motor action potential amplitude during repetitive nerve stimulation. Characteristic of postsynaptic neuromuscular junction disorders like myasthenia gravis.
Full Definition
Waning describes the progressive decrease in compound motor action potential (CMAP) amplitude and area observed during low-frequency repetitive nerve stimulation, typically at 2-5 Hz. This phenomenon occurs in postsynaptic neuromuscular junction disorders when the number of available acetylcholine receptors is reduced due to antibody-mediated blockade or destruction. As repetitive stimulation depletes the readily releasable pool of acetylcholine, fewer receptors are activated with each stimulus, leading to progressively smaller muscle responses. A decrement of greater than 10% between the first and fourth responses is considered abnormal. Waning is the classic finding in myasthenia gravis and distinguishes postsynaptic from presynaptic neuromuscular transmission disorders.
Usage
Usage note: Specify the percentage decrement when documenting waning responses.
In Context
- "Significant waning was observed with 25% decrement in the trapezius muscle." — Repetitive stimulation study
- "The absence of waning at rest helped exclude myasthenia gravis." — Electrodiagnostic evaluation