Temporal dispersion normally is more prominent at proximal stimulation sites because the slower fibers progressively lag behind the faster fibers ( Fig. Temporal dispersion occurs as these individual nerve fibers fire at slightly different times (i.e., larger, faster fibers depolarize before smaller, and slower ones). Likewise, there is a normal variation in the size of individual sensory fiber action potentials, with larger fibers generally having larger amplitudes. In general, the larger fibers depolarize before the smaller ones. Within any sensory nerve, there are large, medium, and smaller myelinated fibers, which depolarize and conduct at slightly different velocities. In the case of sensory studies, many individual sensory fibers depolarize and summate to create the SNAP. These changes are normal findings that result from a combination of temporal dispersion and phase cancellation.įor both sensory and motor studies, the recorded potential (SNAP and CMAP) is a compound potential. The duration of the proximal potential is markedly increased, and the amplitude and area are greatly reduced compared to the distal potential ( Fig. ![]() If the same proximal and distal stimulation sites are used for sensory studies, however, the proximal SNAP varies greatly from the distal one in terms of duration, area, and amplitude. If measured carefully, the proximal CMAP duration may increase slightly, and both the area and amplitude may fall slightly. Preston MD, in Electromyography and Neuromuscular Disorders, 2021 Proximal Stimulation: Normal Temporal Dispersion and Phase Cancellationĭuring routine motor conduction studies, the CMAPs recorded by proximal and distal stimulations are nearly identical in configuration ( Fig.
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