Clinical Differential Diagnosis of Multifocal Motor Neuropathy

The term multifocal motor neuropathy (MMN) was first introduced over 20 years ago¹ 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, immunomodulatorytreatment 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 functionalimpairment.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 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 MMN¹⁵ 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

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