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. © 2000 John Wiley & Sons, Inc. Muscle Nerve 23: 1713–1718, 2000