CMAPs

CMAPs, or compound muscle action potentials, are the summed electrical responses of a muscle population following stimulation of its innervating motor nerve. They reflect the integrity of the peripheral motor pathway and are a central element of nerve conduction studies and electromyography.

Measurement typically involves stimulating the motor nerve at a proximal site with surface or needle electrodes and recording the muscle response with surface electrodes over the target muscle. The stimulus is usually supramaximal to recruit all motor units. The resulting waveform is analyzed for several parameters, including amplitude (the height of the response), onset latency (time from stimulus to response start), and sometimes duration. Temperature, electrode placement, and the muscle examined influence the measurements.

Interpretation relies on patterns rather than single values. A reduced CMAP amplitude often indicates loss of functioning motor units due to axonal damage or severe myopathy. Prolonged onset latency or decreased conduction velocity suggests demyelination or impaired nerve conduction. The CMAP is complemented by late responses such as F-waves and by needle EMG to distinguish neuropathies from myopathies. In clinical practice, CMAPs help evaluate neuropathies, radiculopathies, and various myopathies, and they are used to monitor disease progression or response to therapy.

Limitations exist: CMAP size can be affected by skin impedance, temperature, and adiposity, making small or deep muscles difficult to assess. In severe axon loss, CMAPs may disappear, while early denervation can yield normal amplitudes.