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Differentiating Multifocal Motor Neuropathy from Entrapment Neuropathy—A Diagnostic Challenge

US Neurology, 2016;12(1):17–21 DOI: http://doi.org/10.17925/USN.2016.12.01.17

Abstract:

Multifocal motor neuropathy (MMN) is a rare, treatable neuropathy, but good long-term outcomes are dependent on early treatment. Appropriate diagnostic criteria exist for MMN, but it has a heterogeneous clinical presentation, and, when confined to a small group of nerves, its clinical presentation can overlap with entrapment neuropathies. Electrodiagnosis is a useful diagnostic technique but occasionally MMN can be misdiagnosed as entrapment neuropathies. Misdiagnosis should rarely occur since the nerve involvement in MMN is usually not at sites of common nerve entrapment, and the impact of misdiagnosis can be substantial. The treatments of these conditions differ significantly: intravenous immunoglobulin is the standard therapy for MMN, while treatment options for entrapment neuropathies include conservative treatment, such as splinting, corticosteroids, or surgery. Such treatment in MMN may worsen symptoms, potentially leading to progressive motor symptoms, including muscle weakness, atrophy, and significant motor disability. Two exemplary cases are presented, which highlight the importance of differential diagnosis of these conditions. Emerging technologies, such as high-resolution sonography and magnetic resonance neurography, will aid in defining future diagnostic criteria of MMN.
Keywords: Multifocal motor neuropathy, entrapment neuropathy, carpal tunnel syndrome, conduction block, anti-ganglioside-monosialic acid 1 antibodies, intravenous immunoglobulin
Disclosure: Richard A Rison, MD, FAAN, FAANEM, FANA, has no conflicts of interest to disclose. Said R Beydoun, MD, FAAN, FAANEM, is a consultant, speaker, and served on the advisory boards for Baxter and Grifols. He has received research grants from Alexion, CSL, GSK and Pfizer.
Acknowledgments: Editorial assistance was provided by Katrina Mountfort at Touch Medical Media, London, and sponsored by Baxter.
Open Access: Richard A Rison, MD, FAAN, FAANEM, FANA, has no conflicts of interest to disclose. Said R Beydoun, MD, FAAN, FAANEM, is a consultant, speaker, and served on the advisory boards for Baxter and Grifols. He has received research grants from Alexion, CSL, GSK and Pfizer.
Received: June 01, 2015 Accepted: July 15, 2015
Correspondence: Said R Beydoun, MD, FAAN, FAANEM, Professor of Neurology, Department of Neurology, Keck Medical Center of USC, University of Southern California, Los Angeles, CA, US. E: sbeydoun@usc.edu
Support: The publication of this article was supported by Baxter. The views and opinions expressed are those of the authors and do not necessarily reflect those of Baxter.

Multifocal motor neuropathy (MMN) is a rare, immune-mediated neuromuscular condition characterized by impairment of the peripheral motor nerves, leading to muscle weakness, affecting the arms more than the legs. The disease was formerly known as MMN with conduction block (MMNCB), since a predominant feature is CB at multiple sites along the motor nerves, although at early stages the axons are not affected.1–3 Entrapment neuropathy (EN) occurs when single nerves in various body locations become chronically compressed or mechanically injured as a result of various factors, including anatomical structures, fibro-osseous tunnels, ligaments, local edema, tumors, diabetes, medications, arthritis, hypothyroidism, obesity, or repetitive movements. Nerve compression causes sensory symptoms, such as paresthesias, numbness, dysesthesias, muscle weakness, and atrophy (if untreated), in the affected anatomical area.

The involvement of a single or more than one nerve can resemble some of the motor symptoms of MMN, and consequently, in its early stages, it is possible to confuse the two conditions, which can have serious clinical consequences.4

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Keywords: Multifocal motor neuropathy, entrapment neuropathy, carpal tunnel syndrome, conduction block, anti-ganglioside-monosialic acid 1 antibodies, intravenous immunoglobulin