Alteration of Proximal Conduction Velocity at Distal Nerve Injury in Carpal Tunnel Syndrome: Demyelinating Versus Axonal Change

Abstract

The objective of this study was to determine the cause of median forearm motor conduction velocity (FMCV) slowing in patients with carpal tunnel syndrome, due to either focal conduction abnormality over wrist or retrograde conduction slowing, and to decide whether the slowing is related to severity of compression or not. Fifty carpal tunnel syndrome patients confirmed by conventional nerve conduction study with abnormal electromyography of the abductor pollicis brevis muscle were group 1, and 100 with normal electromyography, group 2. One hundred volunteers served as controls. In addition to conventional nerve conduction study of median and ulnar nerves, palmar stimulations for median mixed and motor nerves were also performed to calculate wrist-palm mixed nerve conduction time and motor conduction velocity (W-P MCV). For group 1, group 2, and control subjects, respectively, W-P MCV were 19.73+/-7.65 (mean+/-SD), 32.7+/-6.83, and 52.75+/-6.4 m/s, whereas median FMCV were 48.63+/-8.32, 54.42+/-2.11, and 57.86+/-4.24 m/s. There was a significant reduction in the W-P MCV (62.6%, P<0.00001) and a decrease in the median FMCV (15.95%, P<0.00001) in group 1, and 38% reduction in W-P MCV (P<0.00001) and 5.9% decrease in median FMCV (P<0.00001) in group 2 when compared with controls, but ulnar FMCV and sensory nerve conduction study results did not, suggesting the reduction of median W-P MCV is not parallel with that of median FMCV in both patients groups. Furthermore, there is a poor correlation of median FMCV and W-P MCV in patient groups, implying conduction blockage of the large myelinating fibers at the wrist, leaving only slower axons to be measured, is not the likely cause of reduction of FMCV. In addition, the reduction of compound muscle action potential amplitude of abductor pollicis brevis muscle, conduction block at wrist and weak correlation of median FMCV and compound muscle action potential amplitude of abductor pollicis brevis exclusively occurred in group 1. Therefore, the retrograde conduction slowing really occurs among patients with carpal tunnel syndrome-markedly in those with abnormal electromyography and mildly in those with only demyelination. This finding counters conventional wisdom that nerve function changes only in segments distal to injured sites.

The Real Role of Forearm Mixed Nerve Conduction Velocity in the Assessment of Proximal Forearm Conduction Slowing in Carpal Tunnel Syndrome

“…However, in further studies, we used distal palm stimulation which demonstrated that W-P MCV were markedly slow with relatively mild slowing in forearm median motor CV in CTS and there was no patients demonstrating conduction block at wrist, suggesting that a conduction block of the large myelinating fibers is not a likely cause of proximal slowing in CTS (Chang et al, 2002a). If the conduction block or conduction slowing of the large myelinating fibers was the primary cause of proximal slowing in CTS, the decrease in W-P MCV and forearm median motor CV should be parallel or proportional because the acrosswrist segment is the common pathway for W-P MCV and forearm median motor CV (Chang et al, 2000(Chang et al, , 2002a(Chang et al, , 2008. However, the reduced forearm median motor CV does not parallel or be proportional to the decrease in W-P MCV in current investigation.…”

“…Occasionally, some patients also have a slowing of forearm median motor CV (Buchthal et al, 1974;Chang et al, 1993Chang et al, , 2000Chang et al, , 2002aChang et al, , 2003aChang et al, , 2004Chang et al, , 2008Melvin et al, 1973;Pease et al, 1990;Stoehr et al, 1978;Uchida and Sugioka, 1993;Thomas et al, 1967). This slowing is commonly ascribed to two possible mechanisms by electromyographers.…”

“…The first is a conduction block of the fastest myelinating fibers in the carpal tunnel, measuring only the CV of the slower axons (Chang et al, 1993;Melvin et al, 1973;Stevens, 1997;Thomas and Fullerton, 1963;Wilson, 1998). The other presumed mechanism is retrograde axonal atrophy or retrograde conduction slowing (RCS), which may result in an increase in electrical resistance and a decrease in CV (Anderson et al, 1970;Chang et al, 2000Chang et al, , 2002aChang et al, , 2003aChang et al, , 2004Chang et al, , 2008Dyck et al, 1981;Devor and Govrin-Lippmarn, 1986). A direct measurement of the forearm (wrist-elbow) median mixed nerve CV (forearm-mix CV), with recording at the elbow and stimulation at the wrist, may reflect the real CV over the forearm and pinpoint the actual mechanism (Chang et al, 1993;Pease et al, 1990;Uchida and Sugioka, 1993;Wilson, 1998).…”

Low sensitivity of F-wave in the electrodiagnosis of carpal tunnel syndrome

“…Several electrophysiological studies reported reduction of the median forearm MCV not related to distal nerve conduction (Fox and Bangash, 1996;Chang et al, 2000Chang et al, , 2008. This retrograde slowing of MCV is likely to be related to axonal degeneration rather than demyelinating changes at wrist (Chang et al, 2008) and therefore it might be responsible of F-INV.…”

“…Several electrophysiological studies reported reduction of the median forearm MCV not related to distal nerve conduction (Fox and Bangash, 1996;Chang et al, 2000Chang et al, , 2008. This retrograde slowing of MCV is likely to be related to axonal degeneration rather than demyelinating changes at wrist (Chang et al, 2008) and therefore it might be responsible of F-INV. Nevertheless the logistic regressions demonstrated that F-INV could not be predicted by the reduction of elbow-wrist MCV and CMAP amplitude The stages of progressive electrophysiological severity scale are: stage 0, normal SCV and DML, including PALM-diff and M4-U4 SCV difference; stage 1, normal digit-wrist segment (M3 and M4 SCV) and abnormal PALM-diff and/or abnormal U4-M4 SCV difference; stage 2, slowing of median digit-wrist segment SCV and normal DML; stage 3, slowing of digit-wrist segment SCV and DML delay; stage 4, absence of sensory nerve action potential (SNAP) in digit-wrist segment (at least M4) and DML delay; stage 5, absence of SNAP and CMAP (Padua et al, 1997).…”