Decremental CMAP
Progressive reduction in compound muscle action potential amplitude during repetitive nerve stimulation, indicating neuromuscular junction transmission defect.
Full Definition
A decremental compound muscle action potential (CMAP) refers to the progressive decline in amplitude of muscle responses during repetitive nerve stimulation testing, typically performed at 2-3 Hz stimulation frequency. This electrodiagnostic finding indicates impaired neuromuscular junction transmission and is characteristic of disorders such as myasthenia gravis and Lambert-Eaton myasthenic syndrome. A reduction of >10% between the first and fourth response is considered abnormal, while >15% is definitely pathological. The pattern and degree of decrement can help differentiate between presynaptic and postsynaptic neuromuscular junction disorders. Post-exercise facilitation may be observed in presynaptic conditions.
Usage
Usage note: Specify the percentage of decrement and muscles tested. Note any post-exercise changes.
In Context
- "Repetitive stimulation revealed a 25% decremental CMAP in the trapezius muscle, consistent with myasthenia gravis." — Electrodiagnostic report
- "The decremental CMAP improved to 8% following brief exercise, suggesting presynaptic neuromuscular junction dysfunction." — NCS findings