Multifocal motor neuropathy: review of a treatable immune mediated disorder

Initially described only about 20 years ago, multifocal motor neuropathy (MMN) is a predominantly distal, mainly upper limb, asymmetrical, pure motor neuropathy. MMN is immune mediated, and frequently associated with antiganglioside antibodies to GM1 of IgM type. The hallmark of MMN is the presence of conduction block (CB) on motor nerve conduction studies. There are, however, forms of MMN where CBs are not present or cannot be detected with routinely available diagnostic techniques. MMN is a treatable disorder, with about 80% of patients responding to intravenous immunoglobulins. For that reason, it needs to be distinguished from motor neuron disease, which is much more common but remains untreatable. MMN also needs to be distinguished from other immune mediated neuropathies as it is unresponsive to plasma exchanges and steroids, which may both cause it to deteriorate. As treatment is less effective later in the course of MMN, early recognition is vital. Long term management is, however, frequently complicated by progressive axonal degeneration resulting in functional decline despite treatment.