Multifocal motor neuropathy

Last revised by Rohit Sharma on 19 Mar 2022

Multifocal motor neuropathy (MMN), also known as multifocal motor neuropathy with conduction block, is a rare autoimmune peripheral neuropathy, which primarily affects the upper limbs, with insidious deterioration over decades. 

Multifocal motor neuropathy typically affects young to middle-aged men with a prevalence of less than 1 in 100,000 1

Multifocal motor neuropathy presents with asymmetric weakness, with patchy peripheral nerve involvement, usually beginning in the distal upper limb 4. Lower motor neuron signs are always seen, there is generally an absence of muscle atrophy early in the disease, and there are is generally no sensory involvement 4. The condition is slowly progressive, and in later stages the condition can appear more confluent affecting all four limbs and can demonstrate muscle atrophy 4.

On motor nerve conduction studies, the condition classically demonstrates conduction block 4. Sensory nerve conduction studies are normal 4.

In multifocal motor neuropathy, focal inflammatory demyelination of single motor nerves is present 1. This may be mediated by anti-GM1 ganglioside IgM antibody, although there is conjecture as to whether this antibody is pathogenic or an epiphenomenon of the condition 1,4. Rarely, multifocal motor neuropathy may be caused by TNF-alpha inhibitors (e.g. infliximab) 6.

Asymmetrical focal or diffuse signal hyperintensity and enlargement of the brachial plexus is classical. This differentiates multifocal motor neuropathy from other causes of lower motor neuron syndromes, such as cervical entrapment neuropathies, spinal muscular atrophies, nerve sheath tumors, and cervical ribs. Imaging findings demonstrate poor correlation with disease severity 2,4.

Although less accessible than MRI of the brachial plexus and cervical nerve roots, MR neurography is particularly useful in multifocal motor neuropathy, showing prominent fascicular enlargements of peripheral nerves 5. This is associated with only mild muscle denervation edema 5. These findings differ greatly from those of motor neuron disease 5.

Approximately 80% of patients with multifocal motor neuropathy respond well to intravenous immunoglobulin (IV Ig) 3. Treatment is given regularly, with IV Ig being the only recognized therapy 4. Of note, multifocal motor neuropathy may worsen with steroid treatment.

  • motor neuron disease (MND): can present with a lower motor neuron-predominant pattern of weakness, however can be differentiated by the concurrent presence of upper motor neuron and/or bulbar signs, different findings with nerve conduction studies, and different findings on MRI and MR neurography 4,5

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