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Multifocal Motor Neuropathy Practical Electrophysiology for the Diagnosis of Multifocal Motor Neuropathy Hessel Franssen 1 and Leonard H van den Berg 2 1. Associate Professor; 2. Professor, Section of Neuromuscular Disorders, Department of Neurology, Rudolf Magnus Institute for Neuroscience, University Medical Centre Utrecht, The Netherlands Abstract Nerve conduction studies (NCS) are necessary to distinguish multifocal motor neuropathy (MMN) from other disorders with a similar clinical picture. In MMN, NCS may show a unique combination of conduction block (CB) or conduction slowing consistent with demyelination, whereas sensory conduction in the same nerve is normal. This contribution discusses a relatively simple and practical electrophysiological approach for the diagnosis of MMN that can be used by any neurologist who has had training in NCS. When diagnosing MMN, the most important practical points are: careful stimulation technique, investigation according to a standardised protocol that includes at least five nerves per arm with stimulation up to Erb’s point, understanding of and adherence to criteria for conduction block and demyelinative slowing and exclusion of nerves with marked axon loss. Keywords Multifocal motor neuropathy, electrophysiology, nerve conduction studies, diagnosis, neuropathy Disclosure: Hessel Franssen and Leonard H van den Berg have received travel grants from Baxter. Acknowledgements: The authors acknowledge the contribution of Sue Lyon, Medical Writer, A Writer’s Touch Medical Communications, London, UK, in editing the manuscript for English style. Received: 5 March 2012 Accepted: 8 June 2012 Citation: European Neurological Review, 2012;7(2):118–23 Correspondence: Hessel Franssen, Section of Neuromuscular Disorders, Department of Neurology, Rudolf Magnus Institute for Neuroscience, F02.230, University Medical Centre Utrecht, PO Box 85500, 3508 GA Utrecht, The Netherlands. E: Support: The publication of this article was funded by Baxter Innovations GmbH. The views and opinions expressed are those of the authors and not necessarily those of Baxter Innovations GmbH. Multifocal motor neuropathy (MMN) presents clinically as a disorder of lower motor neurones with asymmetrical distribution and predominance in distal upper limbs. Electrophysiological investigation is considerably more sensitive and specific for MMN than magnetic resonance imaging of the brachial plexus. 1,2 Nerve conduction studies (NCS) are therefore necessary to distinguish MMN from other disorders with a similar clinical picture, such as progressive spinal muscular atrophy, Hirayama disease, plexopathy and radiculopathy. NCS may show a combination of findings unique to MMN, comprising motor conduction block (CB), slowing of motor conduction consistent with demyelination and, in the nerves with motor abnormalities, normal sensory conduction. There may also be evidence of motor axon loss, such as decreased distally evoked compound muscle action potentials (CMAP) and marked signs of denervation and re-innervation on needle electromyography. 3 It has not been resolved whether motor CB and slowing represent paranodal demyelination, segmental demyelination, or ion channel dysfunction at the node of Ranvier. As discussed below, there is some debate concerning precise electrophysiological diagnostic criteria for MMN, but it is nevertheless possible to outline relatively simple electrophysiological techniques that can be used to diagnose MMN by neurologists who have been trained in NCS. Advanced techniques, such as the single fibre 118 electromyography test for detection of conduction block in single axons, inching and the triple-collision technique, fall outside the scope of this paper. Stimulation NCS performed in the diagnosis of MMN are usually extensive and may require strong stimuli. It is therefore essential to use a careful technique to stimulate each site of a nerve. This reduces patient discomfort and technical errors arising from unwanted co-stimulation. The stimulator must have soft ends with felt pads and a large distance between cathode and anode. Stimulators with sharp metal ends will cause unnecessary pain, since it may be necessary to apply pressure on the skin with the stimulus electrodes in order to achieve supramaximal stimulation. A large distance between cathode and anode also makes it more likely that the stimulus reaches the nerve; this is particularly essential when stimulating at Erb’s point. A large cathode-anode distance can be achieved by a fixed stimulator block with an inter-electrode distance of 4 cm. Alternatively, for stimulation at Erb’s point, the cathode can be a monopolar bar and the anode a pad above the scapula. Proper stimulation entails that the largest response is evoked by the least possible current. This avoids unnecessary pain and errors © TOUCH BRIEFINGS 2012