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Evaluation of Restricted Motion Area of the Median Nerve in Patients with Carpal Tunnel Syndrome: A New Measurement Method Using an Ultrasonographic Video Image.

Background: The precise etiology of carpal tunnel syndrome (CTS) remains unclear. One of the accepted factors for CTS is the restriction of the median nerve. Previous reports using ultrasound had only observed and measured the movement of parts of the median nerve. In this study, we aimed to elucidate the difference in the movement of the entire median nerve in patients with CTS (before and after surgery) and healthy volunteers using a new measurement method. Methods: We expressed the amount of movement of the entire nerve by a new method creating the motion area of the median nerve (MAMn) from an ultrasonographic video image on the computer. We compared the MAMn, the real MAMn (RMMn) (the value obtained by subtracting the nerve cross-sectional area from the MAMn), and mobile ratio (MR) (the value obtained from dividing the MAMn by the nerve cross-sectional area) between six wrists of six cases of CTS (before and at an average of 3.5 months after surgery) and six wrists of six healthy volunteers. Results: During passive wrist flexion, the average MAMn, RMMn, and MR of healthy cases were 23.1 mm2, 16.4 mm2, and 3.52, respectively. The average MAMn, RMMn, and MR of cases of CTS were respectively 11.8 mm2, 5.4 mm2, and 1.86 preoperatively; and 16.2 mm2, 7.3 mm2, and 1.87, postoperatively. The MAMn, RMMn, and MR decreased more significantly in patients with CTS than in healthy volunteers (p < 0.01). The MAMn and RMMn increased postoperatively (p < 0.05), but the MR remained low. Conclusions: The new measurement method revealed that the mobility of the entire median nerve was significantly restricted in cases of CTS compared to healthy participants. However, after surgery, nerve restriction was not restored despite improvements in symptoms, suggesting that decreases in nerve mobility contribute to CTS but are not a direct cause of symptoms.