High Resolution 3-T Magnetic Resonance Tractography Cannot Localise Conduction Block in Multifocal Motor Neuropathy...

Definite diagnosis of multifocal motor neuropathy with conduction block (MMNCB) requires identification of electrophysiological motor conduction block (CB), which is sometimes challenging to demonstrate. Magnetic resonance (MR) neurography is recommended as a supportive diagnostic criterion. Brachial plexus short tau inversion recovery (STIR) hyperintensity is described in MMNCB but the utility of tractography (DTI) is less well understood. We sought evidence for the utility of DTI and STIR parameters of MMNCB forearm nerve segments with clear NCS-demonstrated CB, which could be applied to less accessible nerves.

Ten MMNCB individuals with definite CB and 10 healthy controls underwent dedicated forearm 3-T MRI with DTI and STIR. Axial, radial and mean diffusivity (AD, RD and MD) and fractional anisotropy (FA) were calculated. DTI profiles were visually inspected for possible abnormalities, and pooled values of individual diffusion parameters were analysed for group differences. STIR sequences were reviewed for areas of hyperintensity. MRI and defined NCS abnormalities were correlated.

Composite nerve segment profile inspection showed occasional patterns (elevated AD/MD/RD, reduced FA) in regions of CB, but indistinguishable changes were also seen in MMNCB nerves without CB and controls (sensitivity 28.6%-42.9%, positive predictive value 44.4%-54.5%). Pooled diffusion parameter data identified no statistically significant group differences. Some MMNCB nerves demonstrated focal STIR hyperintensity poorly correlating with CB.

Existing DTI and STIR techniques cannot identify useful characteristics of MMNCB pathology in forearm nerves. Whilst statistically significant group differences can be identified, inspection of individual images cannot identify consistently found abnormalities, and the differences are too small to be clinically useful.