Multifocal Motor Neuropathy With Conduction Blocks Workup

Updated: Nov 12, 2018
  • Author: Sasa Zivkovic, MD, PhD; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Workup

Laboratory Studies

Anti-GM1 antibodies

Most studies report elevated titers of anti-GM1 antibodies in 50% of patients with multifocal motor neuropathy (MMN), but values and sensitivity depend on the methodology. Very high titers of anti-GM1 antibodies (>1:6400) have 80% specificity for MMN, but only 20-30% of patients with MMN have titers of 1:1800 and higher. Lower titers (1:400-800) are less specific and can be found with other neuropathies and amyotrophic lateral sclerosis (ALS).

Clinical features of MMN patients with high titers of anti-GM1 antibodies are typically indistinguishable from patients with negative titers.

The variable sensitivity of different methods of measuring anti-GM1 antibodies is well described. The highest yields and sensitivity of up to approximately 90% have been reported with covalent enzyme-linked immunosorbent assay (ELISA) methodology, while some commercially available assays for anti-GM1 antibodies may have sensitivity that is as low as 20-30%.

The sensitivity of testing may be further increased by adding anti-GM1/galactocerebroside antibodies, although this test may still not be available through commercial laboratories. [17]

Other autoantibodies

Various studies showed elevated titers of other antibodies in MMN including NS6S, neurofascin-186, and gliomedin antibodies. [18, 19] Clinical significance of these antibodies is still not well understood, and these assays are not widely comercially available. 

Creatine kinase (CK)

CK is frequently elevated (< 3 times the upper limit of reference range).

Cerebrospinal fluid (CSF) analysis

Findings are usually normal or reveal a mildly elevated protein content (not as much as in chronic inflammatory demyelinating polyradiculoneuropathy [CIDP]; less than 1 g/L). Cell count is normal.

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Imaging Studies

Neuroimaging studies are not routinely performed in patients with suspected MMN.

Magnetic resonance imaging (MRI) of the brachial plexus may show an increased signal intensity on the T2-weighted images and nerve thickening of the nerve roots and proximal nerves of the arm, usually without contrast enhancement. The differential diagnosis of MRI findings includes radiation-related nerve injury and trauma, while tumors are usually associated with contrast enhancement.

Similar findings were reported with cranial nerve involvement.

In patients with MMN, neuromuscular ultrasound frequently shows enlargement of multiple nerves including cervical spinal roots and proximal arm nerves (rarely in proximal legs). [20]  Certain features, such as increased regional variance and asymmetric enlargement may help distinguish MMN from other chronic immune-related neuropathies. Similarly, the degree of spinal root and distal upper extremity nerve enlargement may help to distinguish MMN from ALS.

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Other Tests

Nerve conduction study (NCS) with needle electromyography (EMG) is essential in demonstrating the presence of multifocal motor involvement without significant sensory component. [14]  When MMN is defined clinically, some patients may not have demonstrable conduction block on conventional NCS.

NCS of motor nerves shows multifocal conduction block. Other signs of demyelination may be present, including decreased velocities, prolonged terminal latencies, temporal dispersion, and delayed (or absent) F waves. Sensory NCS findings are normal, even across the same segments with demonstrated motor conduction block. Additionally, electrodiagnostic evidence of axonal degeneration has been demonstrated in at least one nerve from as many as 50% of patients.

Conduction block (see image below) is indicative of focal demyelination and has been variably defined as a 15-50% reduction of the compound muscle action potential (CMAP) at proximal compared to distal sites of stimulation. Testing of multiple segments in several nerves may be required to demonstrate conduction block, and spinal root needle stimulation may be helpful to demonstrate proximal conduction block. The site of the conduction block should not be at a common nerve entrapment site. See the image below.

Nerve conduction studies demonstrating conduction Nerve conduction studies demonstrating conduction block with temporal dispersion after proximal stimulation.

Unlike ALS, needle EMG in MMN does not reveal the presence of widespread fibrillations, even though fasciculations and myokymia may be observed. Recruitment may be decreased as a result of conduction block, without significant changes in motor unit potential morphology.

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Histologic Findings

Nerve biopsy is not routinely performed in the evaluation of patients with suspected MMN.

Sural nerve biopsy findings may be normal, but findings may also show mild demyelination and poor remyelination in the absence of significant inflammation. Evidence of axonal injury with regeneration may also be present.

The relevance of morphologic abnormalities in sensory nerves in a predominantly motor neuropathy such as MMN is uncertain.

Biopsy of motor nerves is not routinely performed in clinical practice, but several reported cases document demyelination and remyelination in MMN.

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