Multifocal Motor Neuropathy With Conduction Blocks Workup

  • Author: Sasa Zivkovic, MD, PhD; Chief Editor: Nicholas Lorenzo, MD   more...
 
Updated: Oct 5, 2011
 

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).

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 most commercially available assays for anti-GM1 antibodies may have sensitivity that is as low as 20-30%.

Creatine kinase (CK)

CK is frequently elevated (< 3 times the 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, 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.

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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.[11] 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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Contributor Information and Disclosures
Author

Sasa Zivkovic, MD, PhD  Associate Professor, Department of Neurology, Division of Neuromuscular Diseases, University of Pittsburgh and VA Pittsburgh Healthcare System

Sasa Zivkovic, MD, PhD is a member of the following medical societies: American Academy of Neurology, American Association of Neuromuscular and Electrodiagnostic Medicine, and Peripheral Nerve Society

Disclosure: Nothing to disclose.

Specialty Editor Board

Paul E Barkhaus, MD  Professor, Department of Neurology, Medical College of Wisconsin; Director of Neuromuscular Diseases, Milwaukee Veterans Affairs Medical Center

Paul E Barkhaus, MD is a member of the following medical societies: American Academy of Neurology, American Association of Neuromuscular and Electrodiagnostic Medicine, and American Neurological Association

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD  Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Glenn Lopate, MD  Associate Professor, Department of Neurology, Division of Neuromuscular Diseases, Washington University School of Medicine; Director of Neurology Clinic, St Louis ConnectCare; Consulting Staff, Department of Neurology, Barnes-Jewish Hospital

Glenn Lopate, MD is a member of the following medical societies: American Academy of Neurology, American Association of Neuromuscular and Electrodiagnostic Medicine, and Phi Beta Kappa

Disclosure: Baxter Grant/research funds Other; Amgen Grant/research funds None

Selim R Benbadis, MD  Professor, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, Tampa General Hospital, University of South Florida College of Medicine

Selim R Benbadis, MD is a member of the following medical societies: American Academy of Neurology, American Academy of Sleep Medicine, American Clinical Neurophysiology Society, American Epilepsy Society, and American Medical Association

Disclosure: UCB Pharma Honoraria Speaking, consulting; Lundbeck Honoraria Speaking, consulting; Cyberonics Honoraria Speaking, consulting; Glaxo Smith Kline Honoraria Speaking, consulting; Pfizer Honoraria Speaking, consulting; Sleepmed/DigiTrace Honoraria Speaking, consulting

Chief Editor

Nicholas Lorenzo, MD  Consulting Staff, Neurology Specialists and Consultants

Nicholas Lorenzo, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Neurology, and American College of Physician Executives

Disclosure: Nothing to disclose.

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