Multifocal Motor Neuropathy

05 November 2004

1982: chronic, asymmetric motor & sensory neuropathy more marked in upper than lower limbs with persistent multifocal partial conduction blocks in motor nerves but not sensory

1985: chronic, asymmetric pure motor neuropathy without sensory impairment showing persistent  multifocal partial motor conduction block.

CLINICAL

80% male

age 20 –50 (average 30)

Progressive, asymmetric limb weakness

Minimal sensory impairment

Distal > proximal

            Often related to distribution of individual nerves: radial, median, ulnar

Arm > leg

Atrophy at late stage

66% fasciculations & cramps

Occasionally myokymia

Occasionally cranial nerve or respiratory involvement

ELECTROPHYSIOLOGIC

Persistent multifocal partial motor conduction blocks outside usual sites of compression

Conduction block criteria varies:

            >20% reduction area / amp CMAP in proximal vs distal

            absence of marked temporal dispersion for focal vs diffuse demyelination

Normal sensory conduction studies

Normal / slightly reduced motor CV

Normal / slightly increased DML

Normal / slightly\y increased F wave

EMG: fib & fasciculations, MUAP of increased duration & amp ltd to nerves with conduction block.

OTHER INVESTIGATIONS

Paraprotein

anti GM1: dependent upon lab quality

MRI: swollen nerves & increased signal intensity on T2

Histopath: mild onion bulb formation; paraprotein deposits on nodes of Ranvier

TREATMENT

80-90% responsive to IVIG within 2 weeks

Some response to high dose iv cyclophosphamide

Does not respond to plasmapharesis, steroids(may worsen)