ABSTRACT
Lower motor neuron disease is a poorly defined disease entity characterized by (slowly) progressive asymmetrical weakness of limbs without sensory loss1,2. Only since the last decade has a subset of patients been recognized with an immune-mediated disorder which is designated as multifocal motor neuropathy (MMN)3,4. MMN is more common in men than in women (8:1). The age at onset of weakness ranges from 18 to 65 years; in contrast with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), MMN does not occur in childhood or old (>65 years) age. As MMN is a potentially treatable disorder, its differentiation from lower motor neuron disease is important. The presence of motor conduction block outside entrapment sites, which also occurs in CIDP, and elevated serum antibodies to GM1 ganglioside, a potential autoantigen on the nodes of Ranvier and the surface of motor neurons, support an immunemediated pathogenesis of MMN. Anti-GM1 antibodies are not specific for MMN, as these antibodies are also found in the Guillain-Barré syndrome and motor neuron disease. Whether the presence of conduction block is necessary to identify an immunological (treatable) disorder is still a matter of debate, and may depend highly on the criteria for conduction block and the electrodiagnostic protocol5. We recently found that 3 5% of the nerves with conduction block innervate non-weakened muscles6. This implies that an extensive electrodiagnostic protocol including nerves that innervate non-weakened muscles improves the diagnostic yield of patients with MMN. Another diagnostic feature may be an abnormal (swollen nerves, increased signal intensity) magnetic resonance imaging scan of the brachial plexus, which is found in approximately half of patients with MMN (and CIDP). Cerebrospinal fluid (CSF) protein levels are below 1g/l in most (>95%) patients with MMN, which is in contrast to patients with CIDP7.
