Inflammatory neuropathies such as chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and multifocal motor neuropathy (MMN) are treatable neuropathies. Focal spinal muscular atrophy (fSMA) may have a similar manifestation as MMN with focal motor deficits, but is untreatable. Diagnosis of CIDP and MMN currently relies on the clinical presentation and extensive nerve conduction studies (NCS)^1,2. Making a distinction between MMN and fSMA is particularly challenging, as NCS cannot be evaluated reliably in the more proximal parts of the brachial plexus³. Suboptimal sensitivity of NCS to provide evidence of demyelination probably results in under diagnosing MMN and suboptimal treatment. One of the supportive diagnostic criteria for CIDP and MMN is hyperintensity and swelling of the nerves on fat-suppressed T2-weighted MRI of the brachial plexus^1,2, albeit that T2-weighted MRI is often inconclusive⁴. We hypothesized that changes in water diffusion, measured with Diffusion Tensor Imaging (DTI) of the brachial plexus may improve diagnosis. To that aim, we performed DTI of the brachial plexus of cohorts encompassing patients with CIDP, MMN, fSMA and healthy controls and assessed quantitative differences in DTI derived parameters.

After local IRB approval was obtained and written informed consent was given, 6 CIDP, 3 MMN, 9 fSMA patients and 18 healthy controls were scanned with a Philips Ingenia 3.0T scanner. All CIDP and MMN patients fulfilled EFNS criteria^1,2. NCS results were not older than 6 months. Patients were treatment naive or had no treatment for the previous 3 months.

The brachial plexus was covered with 16 coil elements by combining 8 elements of a 16-element body-array coil, 4 elements of the posterior part of a 16 channel head array coil and 4 elements of a spine array coil. For anatomical reference a fat-suppressed diffusion-prepared T2-weighted neurography sequence was used⁵. To reduce artefacts a susceptibility matching pillow was used⁵. The following imaging parameters were used. DTI: axial SE-EPI, FOV: 320x200mm², TE/TR: 77/7943ms, matrix size 128x80, slices: 43, voxel size: 3x3x3mm³, NSA: 4, gradient directions: 16, b-value: 800s/mm², fat suppression: SPIR and Slice Selective Gradient Reversal, scan duration: 9min39s. Neurography: fat-suppressed diffusion prepared 3D-Vista, FOV: 220x320x150mm³, TEeff/TR 61/2500ms, TSE factor: 100, echo spacing: 4.0ms, voxel size: 1.1×1.1×1.1mm³, fat suppression: SPAIR, scan duration: 4min12s.

DTI data was processed using DTItools for Mathematica 10.3 ⁶ and tractography was performed using the VIST/e software⁷. Tractography was seeded from 1 ROI per root per side, placed just outside the myelum in the sagittal plane. Mean diffusivity (MD), fractional anisotropy (FA) as well as axonal diffusivity (AD) and radial diffusivity (RD) values per nerve were computed⁸. Significant differences between groups were assessed using an ANOVA with Bonferroni posthoc testing.

One fSMA patient was excluded because of an alternative diagnosis, being ulnaropathy. Figure 1 shows typical DTI tractographies -colorcoded for FA- overlayed on maximum intensity projections (MIP) of the T₂-weighed MRI of CIDP-, MMN-, fSMA- patient and a healthy control. DTI data was successfully acquired in all but 1 fSMA patient due to motion. Tractography resulted in insufficient reconstructed fibertracts from root T1 and C5 as compared to C6 to C8, therefore C5 and T1 were excluded from further analysis. Average DTI parameters per nerve segment (combined left and right root and trunk) are presented in table 1. Within the subject groups, there were no significant differences between the nerve segments. Therefore values were averaged per subject and used for group analysis (figure 2A and B). FA was lower for CIDP patients compared to fSMA (P<0.01) and healthy controls (P<0.05). MD was higher for MMN compared to fSMA (P<0.05) and CIDP (P<0.05). AD was higher for MMN compared to fSMA (P<0.01) and CIDP (P<0.001). AD was lower for fSMA (P<0.05) and CIDP (P<0.05) than healthy controls. RD was higher for MMN compared to fSMA (P<0.05).The significant lower FA for CIDP patients compared to controls is in agreement with previous findings by Kakuda et al.⁹. However, the most clinically relevant finding is the significant difference in MD, AD, and RD between MMN and fSMA patients, since the higher AD seems to be specific for MMN. So far only one preliminary study has used DTI for optimizing diagnosis in MMN patients¹⁰. In contrast to our results they found lower AD values for MMN patients. However, both studies differ substantially with regard to patient cohorts and site of involvement. In conclusion these preliminary results indicate that DTI of the brachial plexus might be a valuable tool in the diagnostic process of chronic inflammatory neuropathies, and more importantly, to differentiate between MMN and fSMA.