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Recognition of new clinical phenotypes requires knowledge and clinical experience together with a new insight, often provided by an unexpected test result. Sometimes, as in multifocal motor neuropathy (MMN), the abnormal test result becomes the gold standard for diagnosis, superceding the clinical phenotype that initially defined the syndrome. MMN has a prevalence of 1 to 2 per 100,000. Despite its rarity, it is important because it is treatable, although not curable. It was initially recognized as a disorder that could be confused with ALS1 but is now well known as a distinct motor neuropathy. Clinically, MMN is characterized by slowly progressive asymmetric distal weakness, generally affecting the upper limbs. Initially, muscle bulk is relatively well preserved, and weakness is in the distribution of one or more peripheral nerves. Sensation is intact, although in 20% there may be patchy distal loss. Tendon reflexes are usually asymmetrically reduced. There are often local fasciculations and cramps, suggesting ALS as a possible diagnosis, but electrodiagnostic testing in MMN, and not in ALS, reveals partial motor conduction block (CB), defined as reduction in proximally stimulated muscle evoked response compared with the distally stimulated response, in one or more nerves at sites away from potential points of nerve compression. Patients with MMN improve …
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