Multifocal motor neuropathy

Multifocal motor neuropathy (MMN) is a disorder characterized by the presence of chronic asymmetric, predominantly distal, lower motor neuron weakness commonly involving the upper limbs. Sensory involvement is minimal or absent. The presence of conduction block is a characteristic electrophysiological feature although other features like temporal dispersion may be equally relevant. An autoimmune basis has been suggested and antiganglioside antibodies like IgM anti-GM1 may be raised. Treatment with intravenous immune globulin is effective in most patients but repeated infusions are needed. Cyclophosphamide is useful but toxic. Steroids or plasma exchange are usually ineffective and may even be harmful. Other treatments like beta-interferon and rituximab have recently been used. It is important to recognize this treatable disorder from motor neurone disease and other lower motor neurone syndromes.