Preserved cutaneous silent periods in severe entrapment neuropathies

Noxious digital nerve stimulation during isometric contraction of hand muscles leads to transient suppression of the electromyographic activity, the so-called cutaneous silent period (CSP), which is mostly due to a spinal reflex mediated by A-delta fibers. We investigated two patients with carpal tunnel syndrome (CTS) and two patients with ulnar entrapment at the elbow (UNE), in whom routine sensory conduction studies failed to document afferent fiber continuity across the lesion site. In three patients, motor nerve conduction studies and electromyography failed to demonstrate intact efferent fibers. Noxious stimulation of digit II elicited distinct CSPs in ulnar-innervated hand muscles in both patients with CTS, and stimulation of digit V evoked CSPs in median-innervated hand muscles in both UNE patients. The presence of a CSP can only be explained by preserved A-delta fibers crossing the respective lesion site. Thus, preserved CSPs may serve to document residual nerve continuity in severe entrapment neuropathies when fast-conducting fibers are so compromised that their continuity cannot be detected by standard electrodiagnostic techniques.