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Multifocal Motor Neuropathy Clinical Differential Diagnosis of Multifocal Motor Neuropathy Jean-Marc Léger 1 and Eugen Gavriliuc 2 1. Professor; 2. Fellow, Centre National de Réference Maladies Neuromusculaires Rares, CHU Pitié-Salpêtrière and University Pierre et Marie Curie (Paris VI), Paris, France Abstract Multifocal motor neuropathy (MMN) is a rare, clinically well-defined condition within the spectrum of chronic, immune-mediated neuropathies. A typical patient history involves slowly or stepwise progressive, predominantly distal, asymmetrical limb weakness and muscle wasting, most frequently in the arm, that may have developed over a period of years. As a rare condition, MMN may present a diagnostic challenge for non-specialists and some patients may wait years for a correct diagnosis. Timely and accurate diagnosis is essential for patients with MMN. Unlike some motor neuropathies, MMN is treatable with intravenous immunoglobulin and untreated patients are likely to experience progressive muscle weakness that may result in serious functional impairment and impaired quality of life. The aim of this article is therefore to provide a guide for non-specialist neurologists to the clinical recognition and differential diagnosis of MMN. Keywords Multifocal motor neuropathy, diagnosis, symptoms, signs Disclosure: Jean-Marc Léger was the chief investigator of a trial published in multifocal motor neuropathy (MMN) with Endobulin ® and of a retrospective study in MMN with Tégéline ®. Eugem Gavriliuc has no conflicts of interest to declare. Received: 5 March 2012 Accepted: 1 June 2012 Citation: European Neurological Review, 2012;7(2):124–7 Correspondence: Jean-Marc Léger, Centre National de Réference Maladies Neuromusculaires Rares, CHU Pitié-Salpêtrière and University Pierre et Marie Curie (Paris VI), 47 boulevard de l'Hôpital, 75651 Paris cedex 13, France. E: jean-marc.leger@psl.ap-hop-paris.fr 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. The term multifocal motor neuropathy (MMN) was first introduced over 20 years ago 1 and it is now recognised as a clinically well-defined condition within the spectrum of chronic, immune-mediated neuropathies. 2–4 MMN is a rare disease, with an estimated prevalence of no more than one or two per 100,000. 5 However, results from French and Dutch studies suggest that MMN is under-diagnosed and that it can take several years for patients to achieve an accurate diagnosis after they first present. 6 Accurate and timely recognition of MMN is important because it is a treatable disorder. In randomised studies, immunomodulatory treatment with intravenous immunoglobulin (IVIg) results in a significant improvement in patient functioning and muscle strength 7–10 and the drug is now recognised as the gold-standard treatment of MMN. 11 If left untreated, MMN results in progressive muscle weakness that for some patients ultimately results in serious functional impairment. 12 Delayed diagnosis may also have implications for patients’ response to IVIg, since early initiation of treatment may help to postpone axonal degeneration and permanent deficits. 6,13 MMN is under-recognised because it is a rare disease and non-specialists report difficulty in its clinical and electrophysiological diagnosis. 14 As a result, the disorder may be confused with other presentations, some of which are life threatening and do not respond to immunomodulatory treatment. Since physicians report that increased dissemination of diagnostic criteria would raise awareness of the 124 possibility of MMN, 14 the aim of this article is to provide a guide for non-specialist neurologists to the recognition and differential diagnosis of MMN. Clinical Signs and Symptoms MMN is a purely motor deficit that affects individual nerves. It predominantly occurs in younger people, with a median age of onset of 40 years. 6 The disorder is more frequently seen in men than in women in a ratio of 2.7:1, and age of onset is usually younger in men. 6 A typical patient history involves slowly or stepwise progressive, predominantly distal, asymmetrical limb weakness and muscle wasting that may have developed over a period of years. In a study of 88 Dutch patients with confirmed MMN, onset of muscle weakness occurred most frequently in the distal arm – most often in the dominant hand – or rarely the distal leg. 6 Symptoms follow a relapsing course and worsen with exposure to cold. Muscle atrophy is mild in early MMN 15 but may develop with longer duration of disease. 16,17 Patients may describe loss of strength in the affected limb, within an anatomical distribution of individual motor nerves, so that they have difficulty in gripping objects and experience muscle cramps, involuntary muscle contractions and fatigue. The degree of disability generally correlates with the duration of the disease. 6 Consensus clinical criteria for the diagnosis of MMN are shown in Table 1. The lack of objective sensory abnormalities is a core criterion © TOUCH BRIEFINGS 2012