What distinguishes multifocal motor neuropathy (MMN) with conduction block from CIDP and motor neuron disease?
MMN with conduction block is a presumed immune-mediated, chronic asymmetric motor neuropathy.
The presence of weakness, atrophy, and fasciculations with normal sensation and asymmetric hypoactive reflexes identifies it as a lower motor neuron syndrome.
Hyperreflexia does not typically occur, and there are no pathologic reflexes. Bulbar involvement is rare.
Unlike CIDP, motor deficits usually start and are most prominent in the distal upper limbs. There is also a distinct predilection for more restricted and multifocal involvement of motor nerves.
Motor conduction block in at least two nerves outside of the common entrapment sites with normal sensory nerve conduction study in the same segment defines MMN electrophysiologically.
Antibodies to the ganglioside GM1 have been reported in about 40% to 50% of affected patients.
Effective treatments include high-dose IVIG (first choice) and cyclophosphamide.
IVIG may result in fairly rapid, although temporary, improvement in association with partial resolution of conduction block. Corticosteroids and PE are not effective in treating MMN and can sometimes result in worsening of symptoms.
There is no correlation between the presence of conduction block and elevated anti-GM1 antibodies titer with responsiveness to IVIG.