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Multifocal motor neuropathy

Orpha number ORPHA641
Synonym(s) MMN
MMNCB
Multifocal motor neuropathy with conduction block
Prevalence 1-9 / 100 000
Inheritance Unknown
Age of onset Adult
ICD-10
  • G61.8
ICD-O -
OMIM -
UMLS -
MeSH -
MedDRA -

Summary

Multifocal motor neuropathy with persistent conduction blocks (MMNCB) is a dysimmune neuropathy characterised by pure, asymmetrical and multifocal motor deficiency, which begins in and predominantly affects the upper limbs, and follows a chronic evolution. The prevalence is estimated at 1-2 cases in 100 000, but it is likely to be underestimated. In the majority of cases (80%) onset occurs between 20 and 50 years of age. Males are more frequently affected than females, with a male to female sex ratio of 2.6: 1. Clinical signs usually appear in one of the upper limbs. The course of the disorder is unpredictable as it may be limited to one or two motor nerves, or it may progress to other motor nerves in the contralateral upper limb, and potentially to the lower limbs. The motor deficiency is always asymmetrical and multitroncular, and is generally associated with cramps, fasciculations, and eventually amyotrophy. Osteotendinous reflexes are usually reduced or absent in the affected areas. There is no sensory deficiency, although some patients present with paresthesia and exceptionally with cranial nerve involvement (especially of cranial nerve XII). Significant titres of seric IgM antibodies directed against ganglioside GM1 and less often against gangliosides asialo-GM1, GM2 and GD1a are found in 30 to 40% of the cases. Although their role in the physiopathology of MMNCB remains to be established, their presence, together with an often positive response to intravenous immunoglobulins (IVIg) further support the role of a dysimmune mechanism in the genesis of the neuropathy. Diagnosis is based on the observation of conduction blocks (CBs) on electrophysiological examination, and incidentally on the presence of seric antibodies against ganglioside GM1. Multifocal partial CBs, restricted to motor nerves, are associated with the focal slowing of motor conduction velocity and the presence of denervation signs on the detection electromyography. Sensory potentials are normal. MMNCB should be distinguished from other peripheral motor diseases, and in particular from amyotrophic lateral sclerosis. When clinical, electrophysiological and immunological investigations do not allow conclusive evidence to be reached on the basis of published criteria, a trial treatment with the infusion of 2g/kg IVIg can be proposed. MMNCB does not usually respond to treatment with corticosteroids and plasmapheresis, which may worsen the condition of some patients. In contrast, IVIg (2g/kg infused over 2-5 days) are given as a first-line therapy, and in 70% of the cases this treatment leads to fast and spectacular but only transient results. Patients therefore require repeated infusions. However, studies of the long-term administration of intravenous immunoglobulins in multifocal motor neuropathy have yielded controversial results. Two retrospective studies have shown progressive motor deterioration in affected regions, correlating with electrophysiological signs indicative of axonal degeneration, whereas a third study, dealing with significantly higher levels of IVIg found signs of sustained long-term improvement. In cases where IVIg administration is inadequately effective or the disease worsens, another immunomodulatory treatment may be prescribed. Cyclophosphamide, cyclosporine, azathioprine, beta 1a interferon or rituximab are potential candidates, but their efficacy has not been demonstrated through randomised controlled trials. Due to its toxicity, cyclophosphamide is a less suitable option. The functional deficiency linked to the disease is variable, but it can lead to a moderate to major impairment of daily activities, including walking abilities in case of lower limb involvement.

Expert reviewer(s)

  • Dr Jean-Marc LEGER

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