Comparison of ultrasound and magnetic resonance imaging of the median nerve's recurrent motor branch and the value of its diameter in diagnosing carpal tunnel syndrome.

Background: Anatomical variations of the recurrent motor branch (RMB) are at risk of injury during carpal tunnel release procedures. Previous studies have visualized the RMB using ultrasound (US) and magnetic resonance imaging (MRI) but have not compared the imaging capabilities of the two. Previous investigations have overlooked two specific types of carpal tunnel syndrome (CTS): simultaneous compression of the median nerve and the RMB and isolated compression of the latter. This study aims to identify the best imaging method to prevent iatrogenic injury to the RMB by comparing US and MRI capabilities. It also aims to devise a new method for the comprehensive diagnosis of CTS by evaluating the initial diameter of the RMB (RMB-ID). Additionally, this study aims to gain insights into the distribution patterns of the different anatomic variations of the RMB in healthy individuals and patients through an analysis of these variations. A cross-sectional study was conducted.

Methods: Forty healthy adults subjected to bilateral US and MRI of the RMB were included in this study. The US and magnetic resonance images of each patient were subsequently compared. US imaging of the RMB was performed on 102 hands of healthy adults and 112 hands of patients with CTS. The cross-sectional area of the median nerve (MN-CSA) and RMB-ID were measured.

Results: US provided better visibility of the RMB than did MRI (P<0.05). No statistically significant difference was observed in the variation type composition of the RMB between the healthy and patient groups (P>0.05). The RMB-ID and the MN-CSA significantly differed between groups (P<0.001). The RMB-ID increased with the increase of the MN-CSA (R=0.842; P<0.001). The optimal cutoff point for diagnosing CTS of the RMB-ID was 0.85 mm, yielding a sensitivity of 83.0%, a specificity of 92.2%, and the area under the curve of 0.945. The MN-CSA was 0.115 cm, with a sensitivity of 73.2%, a specificity of 96.1%, and an area under the curve of 0.923 [95% confidence interval (CI): 0.887-0.958]. No statistically significant difference was observed in the area under the receiver operator characteristic curve between the two diagnostic methods (P>0.05). The interexaminer reliability for the RMB-ID and the MN-CSA measurements was 0.983 (95% CI: 0.978-0.987) and 0.966 (95% CI: 0.955-0.974), respectively.

Conclusions: US outperformed MRI in visualizing the anatomical variations of the RMB. The RMB-ID was an accurate and valid indicator for comprehensive diagnosis of CTS.