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

Abstract

Background: Carpal tunnel syndrome (CTS) leads to distortion of axonal architecture, demyelination and brosis 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, bre 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 signi cantly lower FA than controls (mean difference 0•12 [95% CI 0•09, 0•16]). Similarly, the median nerve of patients with CTS had a signi cantly 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.

Meta-analysis of the normal diffusion tensor imaging values of the peripheral nerves in the upper limb

“…Our work corroborates prior dMRI studies in peripheral nerves 16 , 17 (and the brain 56 ) which demonstrate that nerves exhibit more isotropic diffusion with advancing age. This is expected because aging axons lose their integrity, axoplasmic transport is slowed and the myelin sheath deteriorates which gives way to segmental demyelination and axonal loss without remyeliation 57 .…”

“…Studies in the brain have shown that dMRI outputs are dependent on scanner hardware, acquisition settings, preprocessing techniques, reconstruction algorithms and extraction methods 2 , 10 – 15 . Recent work has shown similar dependence in the brachial plexus 16 and median nerve in the hand 17 . Therefore, before researchers and clinicians can reliably use dMRI to assess the ‘health’ of the major nerves of the upper limb, there is a need to define the range of “normal” values and how they vary with experimental conditions.…”

Comparison of distortion correction preprocessing pipelines for DTI in the upper limb

“…This study suggests that software used for preprocessing DTI improves the geometric accuracy of dMRI data from upper limb with respect to the anatomical ground truth, but this comes at the expense of introducing large differences in the diffusion metrics extracted from peripheral nerves. This is clinically important because the magnitude of variation in DTI parameters between preprocessing software is of the magnitude that might be attributable to disease or injury 52,53 . Consequently, in the absence of standardization of methods used by pipelines, readers should be aware of the potential for preprocessing to bias the output.…”

“…This is clinically important because the magnitude of variation in DTI parameters between preprocessing software is of the magnitude that might be attributable to disease or injury. 52 , 53 Consequently, in the absence of standardization of methods used by pipelines, readers should be aware of the potential for preprocessing to bias the output.…”