Sonography of Carpal Tunnel Syndrome According to Pathophysiologic Type: Conduction Block Versus Axonal Degeneration

Objectives: The purpose of this study was to investigate sonographic findings according to the pathophysiologic type in patients with carpal tunnel syndrome. Methods: We retrospectively reviewed the records of 80 patients (148 hands) with carpal tunnel syndrome. Patients were classified into 3 groups according to electrophysiologic findings: (1) conduction block and conduction delay; (2) axonal degeneration; and (3) mixed. We used sonographic evaluations to assess the cross-sectional area at the distal wrist crease and the distal forearm and the wrist-to-forearm ratio of the median nerve. Results: Patients with axonal degeneration had significantly larger cross-sectional areas and wrist-to-forearm ratios than those with a conduction block (P <.05). The increased wrist-to-forearm ratio correlated with a reduced amplitude of the sensory nerve action potential, which reflects the degree of axonal degeneration. Conclusions: The cross-sectional area and wrist-to-forearm ratio were associated with the pathophysiologic type of carpal tunnel syndrome, with larger nerve swellings seen in patients with axonal degeneration compared with those with demyelinating lesions. In addition to helping in the localization of the nerve lesion, sonography may indicate the type of nerve lesion.