History
Typically, multifocal motor neuropathy (MMN) manifests with a slowly progressive, asymmetric, predominantly distal weakness developing over years. Weakness usually starts in a distribution of a single peripheral nerve with unilateral wrist drop, foot drop, or grip weakness. Initial involvement of the distal upper extremities is most common. Rarely, MMN may manifest with initial phrenic or cranial nerve involvement. Cramps and twitching are common, but muscle atrophy is minimal if present at all. Sensory symptoms are minimal or absent. Transient exacerbation may occur during pregnancy. As with other neuromuscular disorders, patients also commonly complain of fatigue.
Electrodiagnostic evaluation may document the presence of asymptomatic conduction blocks in other clinically unaffected nerves, and it may document more extensive involvement in patients with relatively few symptoms. Positive serology for anti-GM1 antibodies is supportive of the diagnosis of MMN, particularly higher titers.
Clinical and electrodiagnostic criteria
Definite MMN, as follows: [3, 14]
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Weakness without objective sensory loss in the distribution of 2 or more nerves is present for more than 1 month
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Definite motor conduction block is present in at least one motor nerve with normal sensory nerve conductions
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Exclusion criteria not present
Probable MMN, as follows:
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Weakness without objective sensory loss in the distribution of 2 or more nerves
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The presence of probable motor conduction block in 2 or more motor nerve segments with normal sensory nerve conduction studies
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Exclusion criteria not present
OR:
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Weakness without objective sensory loss in the distribution of one nerve
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The presence of probable motor conduction block in one motor nerve segments with normal sensory nerve conduction studies
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Exclusion criteria are not present.
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At least 2 supportive criteria met
Supportive criteria, as follows:
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Elevated titers of serum IgM anti-GM1 antibodies
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Increased cerebrospinal fluid (CSF) protein (< 1 g/L)
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MRI shows increased signal on T2-weighted imaging of brachial plexus with diffuse nerve swelling
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Objective clinical improvement following intravenous immunoglobulin (IVIG) treatment
Exclusion criteria, as follows:
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Upper motor neuron signs, such as spasticity, clonus, extensor plantar response
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Marked bulbar involvement
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Sensory involvement more marked than minor vibration loss in the lower limbs
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Diffuse symmetric weakness during initial weeks
Physical
On physical examination, the most remarkable finding is asymmetric weakness in the distribution of individual peripheral nerves that is out of proportion to muscle atrophy. Fasciculations may be present.
Cranial nerves
Cranial nerves are rarely affected, but this may be an uncommon initial manifestation of MMN. Cranial nerve involvement may be limited to the twelfth cranial nerve.
Speech is normal.
Deep tendon reflexes
Deep tendon reflexes are typically absent (particularly in affected limbs) or normal. However, brisk tendon reflexes were found in 9% of patients, even in the weakened limbs.
Motor strength
Asymmetric weakness may occur in a nonmyotomal pattern, usually in the distribution of individual nerves. The upper limbs, particularly the hands, are more commonly involved than the lower limbs. Weakness of respiratory muscles is very rare. [15] Weakness frequently worsens with exposure to cold. [16]
Muscles innervated by motor nerves with persistent conduction block are usually weak.
Muscle atrophy
Atrophy may be present in weak muscles, but it is usually fairly mild. Late in the disease course atrophy may be more prevalent.
Upper motor neuron signs
These signs are absent.
Muscle tone
Tone is decreased or normal. No clonus, extensor plantar response, or pseudobulbar palsy is present. Pathologic reflexes (eg, Babinski, Hoffman) are not present.
Sensory examination
Sensory examination should be normal, and the sensory loss may be suggestive of Lewis-Sumner syndrome (multifocal acquired demyelinating sensory and motor neuropathy [MADSAM]).
Coordination
Coordination is normal.
Gait
Gait is usually normal, unless more prominent involvement of lower extremity muscles occurs.
Fasciculations and cramping
These are common and may occur outside of the distribution of clinically affected nerves.
Other
No rash or gynecomastia is present.
Causes
MMN is an autoimmune peripheral neuropathy without a known cause. Rarely, MMN may develop following treatment with tumor necrosis factor (TNF) – α antagonists. [4]
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Nerve conduction studies demonstrating conduction block with temporal dispersion after proximal stimulation.
