High-Resolution MR Neurography of the Thenar Motor Branch of the Median Nerve in Carpal Tunnel Syndrome

Background: The anatomy of the median nerve (MN) and its branches in the carpal tunnel vary widely among population. Such knowledge, especially the course of the thenar motor branch (TMB), is of utmost importance in order to avoid iatrogenic injury during surgical procedures. Lanz classified 29 MN variations in the carpal tunnel into four groups. TMB anatomical variations were grouped into type I considering the course of the motor branch in relation to the transverse carpal ligament (extra-, sub-, supra- or transligamentous) and the site of branch off from the MN (ulnar, central or radial). The TMB anatomy has been well described in the literature based on intraoperative findings and anatomical reports. Non-invasive methods of TMB identification are scarce, however. Recently few studies demonstrated the reliability of high-resolution ultrasound in the identification of TMB variations and preoperative mapping of MN branching. To the best of our knowledge, TMB anatomy has never been studied by magnetic resonance (MR) imaging, especially by neurography.

Objective: To evaluate prospectively the possibility of identification of TMB anatomy and anatomical variations by using MR neurography.

Materials and Methods: Seven patients (9 hands) with clinical and electrophysiological diagnosis of carpal tunnel syndrome were scanned using a 3T MR imaging scanner with MR neurography sequences. TMB anatomical variations were classified according to the Lanz classification system and the maximum transverse diameter was measured.

Results: The TMB was clearly identified in all cases. The TMB arose from the radial or central aspect of the MN in all patients. Mean TMB diameter was 1.21 ± 0.05mm (range, 1.02–1.46). Eight hands had an extraligamentous TMB course (89%; group 0), and one hand had a supraligamentous course (11%; group 1D).

Conclusion: TMB anatomy and anatomical variations are easily identified on MR neurography, which may emerge as a useful tool for preoperative mapping of MN branches, for diagnosis of TMB pathologies and potentially for outcome correlation.