Enlarged median nerve in idiopathic carpal tunnel syndrome.

To delineate change in median nerve size in carpal tunnel syndrome (CTS), we compared ultrasonograms of nerve cross-sections from patients (201 wrists of 125 women with idiopathic CTS) and controls (200 dominant wrists of 200 women). Major and minor axes, cross-sectional area, and the flattening ratio (major axis/minor axis) were measured at four levels: 1, distal edge of the flexor retinaculum; 2, center of the hook of the hamate; 3, wrist crease; and 4, distal one-third level of the forearm. Axes and areas were greater in the patients at levels 1, 2, and 3. The flattening ratio was greater in the controls at levels 1 and 2, and in the patients at level 3. When the groups were combined, there were linear correlations between the cross-sectional nerve area and electrophysiologic severity at levels 1, 2, and 3. These findings indicate that idiopathic CTS is characterized by severity-correlated intracarpal enlargement of the median nerve and not by compressive deformation, such as a reduction in the caliber of the nerve.