Influence of recording site on CMAP amplitude and on its variation over a length of nerve

Abstract

The distinctions between blocking, abnormal temporal dispersion, and normal conduction require delineation of the normal change in amplitude of the compound muscle action potential (CMAP) over a length of nerve. Effects of the recording site on CMAP amplitude and on its variation were studied in median and ulnar nerves of 13 healthy subjects. CMAPs were recorded from three sites: halfway along the muscles and 1 cm distal and proximal. Elbow-wrist amplitude percentages (CMAP%) were calculated. CMAP amplitudes varied considerably between sites and subjects. Amplitudes were maximal at the middle site in only 16 of 26 nerves. The site of maximal amplitude could not be identified on the basis of thumb anatomy. CMAP% was not related to CMAP amplitude, and differed by up to 32% between adjacent sites. CMAP formation involves spatial factors (electrode site, limb position, and limb anatomy), temporal factors (dispersion), and their interaction, explaining why CMAP% can exceed 100%. The site of the recording electrode affects CMAP amplitude and CMAP% to clinically relevant degrees. Standardization of the recording site may improve reliability of CMAP% studies.

CMAP amplitude cartography of muscles innervated by the median, ulnar, peroneal, and tibial nerves

“…This is due to interactions between spatial factors (limb position, muscle architecture, and recording site) and a temporal factor, namely, the dispersion of nerve fiber velocities. 17,18 Recording site and electrode size have important effects on CMAP characteristics. 2,17,18 Systematic differences in muscle anatomy and architecture, 3,4,6 i.e., muscle fiber orientations, lengths, and the positions of motor units and neuromuscular synapses, probably exist between the muscle groups of the median, ulnar, peroneal, and tibial nerves.…”

Compound muscle action potential cartography of an accessory peroneal nerve

“…6 The tortuous percentage map resembles one made earlier of the thenar eminence, and underlines that AMP% does not depend on amplitude. 7 The maps illustrated that the APN innervated only part of the EDB muscle. Percentages over 100% will be found only when the recording site overlies this part of the muscle.…”

“…A multielectrode with eight 1-mm pins at 5-mm intervals was placed along a grid line, allowing 8 CMAPs to be obtained simultaneously with a Nicolet (Madison, WI) Viking 3, using methods described earlier. 7 The multielectrode was held in place while supramaximal stimulation was performed, first at the volar aspect of the ankle, then behind the lateral malleolus, and finally at the fibular head, using stimulators taped in place. The multielectrode was then repositioned to obtain three new sets of 8 CMAPs.…”

Nerve conduction topography in geriatric hand assessment

“…The importance of the exact location of the E1 has been illustrated by several prior studies, which noted that subtle changes in position of the recording electrode showed significant differences in the recorded response [11][12][13][14][15]. Mapping the compound muscle action potential from 30 points on a 5 × 6 cm grid on hand and foot muscles, Swenson et al showed areas of prolonged latency, low amplitude, and reduced area, but with waveform morphology consistent with electrode placement over a motor point [16].…”

“…Blok et al used a 128-electrode array to record the spatial distribution of electrically evoked motor-unit action potentials in the thenar muscles [22]. Van Dijk et al [11] and Lateva et al [23] mapped the spatial distribution of CMAPs using a multielectrode with only eight or four recording surfaces, which had to be continuously repositioned. Van Dijk et al found that larger electrodes reduce the variability of CMAP parameters associated with slight displacements of electrode position [20,24].…”