Methods for assessing small peripheral nerve fiber function objectively are limited. The cutaneous silent period (CuSP), a transient suppression of electromyographic voluntary activity that follows painful stimuli, could serve as an objective functional measure of the A delta fibers.
Objectives: To establish normal values of CuSP, to compare these values to those in the literature and to discuss the yield of the CuSP in pathological conditions.
Material and methods: We investigated the CuSP of the upper and lower limbs of 40 normal subjects.
Results: We observed that the spinal circuitry mediating the CuSP is mainly unilateral and restricted to one limb, and that CuSP latency decreases and duration increases with increasing stimulus intensity, then CuSP stabilizes with strong stimuli. In an additional study, we observed that painful cutaneous stimuli are either inhibitory, causing a pause in an ongoing contraction (CuSP), or excitatory, inducing a contraction of a muscle at rest (RIII). Inhibition and excitation have similar timings.
Conclusion: The method for studying the CuSP is simple and well tolerated; it is useful to study A delta fibers in peripheral neuropathies and the central circuitry of this cutaneous nociceptive response in conditions affecting the spinal cord. Comparison of CuSP studies suggests the need for a standardization of the method.