Marieke van Rosmalen

Chapter 1 12 CLINICAL BACKGROUND Chronic inflammatory neuropathies Chronic inflammatory demyelinating polyneuropathy (CIDP) and multifocal motor neuropathy (MMN) are both rare polyneuropathies with an inflammatory cause. CIDP is characterized by slowly progressive (mostly) symmetric pure motor, pure sensory, or mixed deficits that are most pronounced in the legs, while MMN is marked by asymmetric weakness without sensory deficits that dominates in the arms. Both polyneuropathies respond to treatment. 1,2 Early treatment can improve muscle strength or sensory symptoms, and prevents progression of symptoms and permanent axonal damage which underlines the importance of a timely diagnosis. 1,2 Patients with CIDP and MMN both respond to treatment with immunoglobulins. Patients with CIDP, but not with MMN, also respond to treatment with corticosteroids or plasmapheresis. Another important difference between these disorders is that 26% of patients with CIDP may experience remission that allows discontinuation of treatment, while this is uncommon in MMN. 3 Diagnosis of CIDP and MMN is based on diagnostic consensus criteria that use a combination of clinical phenotype, nerve conduction study results and ancillary investigations. 4,5 The latter play an important role when nerve conduction studies do not meet the required electrodiagnostic criteria. 4–7 They include laboratory findings and imaging abnormalities of the peripheral nerves, in particular MRI of the brachial plexus. TECHNICAL BACKGROUND Principles of MRI physics Magnetic resonance imaging is an imaging technique that is able to visualize pathology of the nervous system. It is widely used in clinical practice for examination of the brain, spinal cord, muscle and more recently also the peripheral nervous system. Physics of MRI is complicated but some knowledge on its principles is essential to correctly assess and interpret the images. In short, all protons in body tissue spin on their own axes ( Figure 1.1A ). After placing the patient in a static magnetic field, i.e. the MRI scanner, the resulting magnetization of all protons inside the patients’ tissue align parallel to the magnetic field ( Figure 1.1B ). These protons rotate around the long axis of the primary magnetic field (B 0 ), which is called precession. Precession rate is termed as the Larmor frequency . The average of many protons produces the net magnetization. Then, a radiofrequency pulse is emitted from the scanner which creates a magnetic field perpendicular to B 0 ( Figure 1.1C ). When the radiofrequency pulse is at resonance, it creates a phase coherence in the precession of all the proton spins. The net magnetization of all protons rotating in Larmor frequency generates an electric current in the receiving coil, i.e. an electrical conductor, that is placed in the vicinity of the tissue of interest. This current is the nuclear magnetic resonance (NMR) signal . The NMR