Review of the dimensions of the median nerve and carpal tunnel using sonography in asymptomatic adults

Objective: The study aims to establish the normal range of all sonologic measurements of carpal tunnel (CT) structures in an asymptomatic population. Methods: Sonological evaluation of 150 wrists in 75 asymptomatic adults was performed. The cross-sectional area (CSA) of the median nerve at four levels, the flattening ratio (FR) at three levels, the antero-posterior (AP) diameter of the CT and the distance of the transverse carpal ligament (TCL) from the trapezium-hamate (TmH) line were measured. Results: The mean (standard deviation (SD)) CSA of the median nerve at the distal forearm, CT inlet, mid and outlet were 6.8 (1.3), 7.4 (1.1), 7.5 (1.0), 7.1 (1.0) mm², respectively. The mean (SD) FR at the CT inlet, mid and outlet were 2.66 (0.54), 2.55 (0.54), 3.69 (0.82), respectively. The mean (SD) AP diameter of the CT was 10.4 mm (1.1). Volar bowing of the retinaculum was seen in 7.3% of normal wrists. Conclusions: The normal range at two SDs of the CSA of the median nerve at the inlet was 5.2-9.6 mm². The upper limit of volar bowing of the flexor retinaculum was 0.8 mm. The FR overlaps with values obtained in other studies of patients with carpal tunnel syndrome. The mean AP diameter of the CT was 10.4 mm (SD 1.1). To the best of our knowledge, this is the largest study performed in an asymptomatic population assessing the different sonological parameters related to the CT.