Objective: The aim of this study was to investigate the frequency of the anatomic variation of a bifid median nerve in patients with carpal tunnel syndrome (CTS) and to determine the size criteria for sonography.
Methods: On axial sonograms of 320 hands of 170 patients with CTS and 240 hands of 120 unaffected individuals, the median nerve was evaluated morphologically for a bifid median nerve, and the cross-sectional area was measured at 3 levels (radial-ulnar junction, pisiform, and hook of the hamate). Electrophysiologic studies were performed in addition to clinical and sonographic evaluations in all patients, controls with a bifid median nerve, and controls with a cross-sectional area of greater than 0.09 cm(2).
Results: A bifid median nerve was seen in 32 (19%) of 170 patients and 11 (9%) of 120 controls. It occurred relatively frequently in patients with CTS (P < .01). The cross-sectional area of the bifid median nerve was relatively higher than that of the nonbifid median nerve in controls at 2 of the 3 levels (P < .001; P = .226; P < .01). The cutoff values for the cross-sectional area at the level of the pisiform were 0.11 cm(2) (sensitivity, 90%; specificity, 99%; P < .001) for patients with a bifid median nerve and 0.10 cm(2) (sensitivity, 98%; specificity, 81%; P < .001) for patients with a nonbifid median nerve.
Conclusions: A bifid median nerve occurs relatively frequently in patients with CTS. It may facilitate compression of the median nerve in the carpal tunnel because of its relatively higher cross-sectional area compared with a nonbifid median nerve. The size criterion for CTS in patients with a bifid median nerve is slightly higher than in those with a nonbifid median nerve.