eMedicine Specialties > Neurology > Neuromuscular Diseases

Multifocal Motor Neuropathy With Conduction Blocks: Differential Diagnoses & Workup

Author: Sasa Zivkovic, MD, MSc, Assistant Professor, Department of Neurology, Division of Neuromuscular Diseases, University of Pittsburgh and VA Pittsburgh Healthcare System
Contributor Information and Disclosures

Updated: Jan 25, 2008

Differential Diagnoses

Amyotrophic Lateral Sclerosis
Chronic Inflammatory Demyelinating Polyradiculoneuropathy

Other Problems to Be Considered

Lewis-Sumner syndrome (MADSAM)
Mononeuritis multiplex
Hereditary motor sensory neuropathy type 2
Hereditary neuropathy with liability to pressure palsies (HNPP)
Lead neuropathy
Porphyric neuropathy
Progressive muscular atrophy
Spinal muscular atrophy (adult onset)

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).
    • 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]). Cell count is normal.

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.

Other Tests

  • Nerve conduction study (NCS) with needle electromyography (EMG) is essential in demonstrating the presence of multifocal motor involvement without significant sensory component. 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 Media file 1) 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.
    • 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.

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.

More on Multifocal Motor Neuropathy With Conduction Blocks

Overview: Multifocal Motor Neuropathy With Conduction Blocks
Differential Diagnoses & Workup: Multifocal Motor Neuropathy With Conduction Blocks
Treatment & Medication: Multifocal Motor Neuropathy With Conduction Blocks
Follow-up: Multifocal Motor Neuropathy With Conduction Blocks
Multimedia: Multifocal Motor Neuropathy With Conduction Blocks
References
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References

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Further Reading

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Keywords

MMN, multifocal motor neuropathy with conduction block, acquired immune-mediated demyelinating neuropathy, amyotrophic lateral sclerosis, ALS, demyelinating injury, axonal injury, anti-GM1 antibodies, intravenous immunoglobulin, IVIG, cyclophosphamide, muscle atrophy, autoimmune peripheral neuropathy, autoimmune peripheral neuropathy, nerve conduction study, NCS, multifocal motor involvement, axonal degeneration

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Contributor Information and Disclosures

Author

Sasa Zivkovic, MD, MSc, Assistant Professor, Department of Neurology, Division of Neuromuscular Diseases, University of Pittsburgh and VA Pittsburgh Healthcare System
Sasa Zivkovic, MD, MSc is a member of the following medical societies: American Academy of Neurology and American Association of Neuromuscular and Electrodiagnostic Medicine
Disclosure: Nothing to disclose.

Medical Editor

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

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: Nothing to disclose.

Managing Editor

Glenn Lopate, MD, Associate Professor, Department of Neurology, Division of Neuromuscular Diseases, Washington University School of Medicine; Chief of Neurology, St Louis ConnectCare, Consulting Staff, 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: Nothing to disclose.

CME Editor

Selim R Benbadis, MD, Professor, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, University of South Florida School of Medicine, Tampa General Hospital
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: Nothing to disclose.

Chief Editor

Nicholas Y Lorenzo, MD, Chief Editor, eMedicine Neurology; Consulting Staff, Neurology Specialists and Consultants
Nicholas Y Lorenzo, MD is a member of the following medical societies: Alpha Omega Alpha and American Academy of Neurology
Disclosure: Nothing to disclose.

 
 
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