Diffusion Tensor Imaging of the Median Nerve: A Systematic Review and Meta-Analysis of Normal Values in Asymptomatic Adults and How They Change in Carpal Tunnel Syndrome 

Background: Carpal tunnel syndrome (CTS) leads to distortion of axonal architecture, demyelination and fibrosis within the median nerve. Diffusion tensor imaging (DTI) characterises tissue microstructure and generates reproducible proxy measures of nerve ‘health’ which are sensitive to myelination, axon diameter, fibre density and organisation. This meta-analysis summarises the normal DTI values of the median nerve, and how they change in CTS. Methods: : This systematic review included studies reporting DTI of the median nerve at the level of the wrist in adults. The primary outcome was to determine the normal fractional anisotropy (FA) and mean diffusivity (MD) of the median nerve. Secondarily, we show how the FA and MD differ between asymptomatic adults and patients with CTS, and how these differences are independent of the acquisition methods. Results: : 32 studies of 2643 wrists, belonging to 1575 asymptomatic adults and 1068 patients with CTS were included. The normal FA was 0·58 (95% CI 0·56, 0·59) and the normal MD was 1·138 x10 -3 mm 2 /s (95% CI 1·101, 1·174). Patients with CTS had a significantly lower FA than controls (mean difference 0·12 [95% CI 0·09, 0·16]). Similarly, the median nerve of patients with CTS had a significantly higher mean diffusivity (mean difference 0·16 mm 2 /s x10 -3 [95% CI 0·05, 0·27]). The differences were independent of experimental factors. Conclusion: We provide summary estimates of the normal FA and MD of the median nerve in asymptomatic adults. Furthermore, we show that diffusion throughout the length of the median nerve becomes more isotropic in patients with CTS.