Proximal and Distal Nerve Conduction Studies in Carpal Tunnel Syndrome

Abstract

Introduction/Aims: The exact reason for the decrease in median nerve motor conduction velocity (MMCV) in the forearm in carpal tunnel syndrome (CTS) is debatable. However, some studies support the hypothesis of blockage of large, myelinated fibers in the carpal tunnel, and/or retrograde axonal degeneration. We aimed to determine the decrease in the MMCV of the second lumbrical (FMMCVL) and abductor pollicis brevis (APB) muscles recorded in the forearm (FMMCVAPB) in patients with CTS compared with a control group and also to compare the MMCV among CTS subgroups with different severity. Methods: The study included data on the hands of 51 patients and 20 volunteer controls. Patients were divided into four groups according to the severity of the involvement, minimal, mild, moderate, and severe. Routine median, ulnar, and comparative conduction studies were performed. By stimulating the median nerve at the palm, wrist, and elbow, responses were recorded over the APB and second lumbrical muscles. MMCV was assessed in the transcarpal segment (TCMMCV) and forearm. Results: The mean FMMCVAPB and FMMCVL in patients were 53.8 ± 4.2 m/s and 54.3 ± 5.6 m/s, respectively, and were significantly slower than in the controls (P < 0.001 and P = 0.012, respectively). The FMMCVL was significantly decreased in the severe group compared with the other three groups (P < 0.001). FMMCVL and FMMCVAPB relatively decreased as CTS severity increased. Conclusion: In this study, FMMCVAPB and FMMCVL were found significantly slower. Slowing of FMMCVL in the forearm is related to the severity of CTS. The decrease in MMCV in the forearm did not parallel the decrease in TCMMCV. This suggests that retrograde axonal degeneration may be the contributing factor to forearm slowing. There is also an association between axonal degeneration and disease severity.