Marieke van Rosmalen

Quantitative MRI characteristics of chronic inflammatory neuropathies 93 6 INTRODUCTION Inflammation of peripheral nerves is the underlying disease mechanism that causes muscle weakness and sensory deficits in chronic inflammatory polyneuropathies, including multifocal motor neuropathy (MMN) and chronic inflammatory demyelinating polyneuropathy (CIDP). MMN is characterized by asymmetric weakness without sensory deficits that dominates in the arms, while CIDP may cause pure motor, pure sensory, or mixed deficits that are most pronounced in the legs. Nerve conduction studies may show conduction blocks and nerve imaging studies have revealed multifocal thickening of nerves in both CIDP and MMN. 1,2 The immunological pathophysiologies of CIDP and MMN differ considerably, but have not been elucidated completely. Autopsy studies, sural nerve biopsy, immunostaining with patient sera in vitro and animal models have provided insight in underlying immunological mechanims. 3–9 There is an obvious need for additional tools to study the condition of peripheral nerves in vivo to further dissect the underlying pathophysiological mechanisms. Quantitative magnetic resonance imaging (MRI) may bridge this gap: diffusion tensor imaging (DTI) and measurements of T2 relaxation times and fat fraction may indicate specific pathophysiological changes in the myelin sheath and axon in patients with CIDP and MMN. DTI is an MRI technique that provides quantitative parameters as fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD). These parameters give insight in the microstructural integrity of (nervous) tissue and seem to correlate with histological findings. 10–13 Previous DTI studies evaluated peripheral nerves such as the tibial, sciatic and median nerve of patients with CIDP or MMN and healthy controls. 14–19 The brachial plexus was analyzed in a recent exploratory pilot study in a small cohort of patients and showed different FAvalues between patients with CIDP and MMN. 20 Studies using other quantitative MRI techniques, e.g. T2 mapping or fat fraction analysis, documented an increase in T2 relaxation time in the brachial and lumbosacral plexus and in the tibial nerve in small cohorts of patients with CIDP. 21–23 Complete and systematic studies of the brachial plexus are lacking. We therefore performed a detailed and systematic quantitative MRI study in a large cohort of patients with CIDP, MMN, motor neuron disease (MND) and healthy controls, to compare diffusion parameters, T2 relaxation times and fat fraction of the brachial plexus. The aim of this study was to interpret these results in light of underlying pathophysiological mechanisms of CIDP and MMN.