Preserved cutaneous silent periods in severe entrapment neuropathies

Abstract

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.

The cutaneous silent period is preserved in cervical radiculopathy: significance for the diagnosis of cervical myelopathy

“…Similarly, Svilpauskaite et al [14] showed that only transection of a peripheral nerve, but not severe entrapment, abolished the CSP. In agreement with previous reports that demonstrated relative preservation of CSP responses in other peripheral nerve syndromes [2,5,7,14], we now provide evidence that the CSP is not affected by cervical radiculopathies. Our findings are also congruous with the observation that many patients seeking medical treatment for cervical radiculopathy do not manifest features of overt axonal loss.…”

“…Aurora et al [2] reported normal CSP onset latencies in mild to moderate carpal tunnel syndrome, and concluded that small fiber dysfunction occurred late in the course of this compressive neuropathy. Kofler et al [5] found that preserved CSPs may serve to document residual nerve continuity in severe entrapment neuropathies when fast-conducting fibers were so compromised that their continuity could not be detected by standard electrodiagnostic techniques. Similarly, Svilpauskaite et al [14] showed that only transection of a peripheral nerve, but not severe entrapment, abolished the CSP.…”

Dissociation between cutaneous silent period and laser evoked potentials in assessing neuropathic pain

“…This hypothesis is also in line with several clinical observations showing that the CSP is preserved in severe entrapment neuropathies, whereas routine nerve conduction studies have failed to demonstrate residual afferent nerve fiber continuity. 9 Although we found it unsuitable for assessing nociceptive pathway function in patients with neuropathic pain, the CSP has proven useful for both experimental and clinical purposes. The CSP has been used to measure descending influences on spinal excitability of inhibitory pathways in patients with basal ganglia and pyramidal disorders.…”

“…2,20 Peripheral neuropathy usually manifests with sensory disturbances including hypesthesia, paresthesia, and neuropathic pain associated with nociceptive pathway damage. 10 Although some studies have used the CSP to investigate nociceptive pathway function in patients with peripheral neuropathy, 9,12,18 others have questioned whether the CSP is a true nociceptive response. 7,16 Hence, the diagnostic value of the CSP for assessing nociceptive pathway function in patients with neuropathic pain is still debated.…”

“…3,12,21 The CSP has provided useful information on the excitability of spinal inhibitory pathways in patients with pyramidal syndrome, 5 in movement disorders, 6,14,17 and in clinical studies of nociceptive afferents (A␦ fibers) in peripheral neuropathies. 9,12,18 Laser-generated radiant heat pulses selectively excite free nerve endings in the superficial skin layers. They activate A␦ and C mechanothermal nociceptors and they evoke scalp potentials related to A␦ fibers generated by the operculoinsular cortex and cingulate gyrus (laser evoked potentials, or LEPs).…”