- Multi-Focal Motor Neuropathy (MMN) is a condition warranting consideration in patients suffering from progressive muscle weakness.
- Delays in diagnosis often occur because of unfamiliarity with the disorder, its ability to mimic other diseases causing weakness, and the lack of consistent clinical, laboratory, and electrodiagnostic findings.
- Recent studies have shown how nerve ultrasound, a non-invasive imaging modality, not only helps to differentiate this motor neuropathy from the more serious motor neuronopathies (such as ALS) but also aids in discriminating between hereditary and acquired peripheral neuropathies.
Multi-focal motor neuropathy is an acquired immune-mediated neuropathy characterized by a sub-acute onset, slowly progressive and usually asymmetrical weakness, and minimal to no pain or other sensory involvement. The weakness is typically distal, affecting the upper > than lower extremities, following the pattern of a known peripheral nerve (such as Radial, Ulnar, Median, Peroneal, etc.) or multiple nerves. Fasciculations and cramping are common, but not invariably associated with the disease process.
Diagnostic Findings and Clinical Challenges:
The presence of normal sensory NCV studies and abnormal motor NCV studies (showing a conduction block, temporal dispersion, or significant NCV slowing) in nerve regions not susceptible to compression (such as the forearm segment of the Median and/or Ulnar nerves) provide strong support for this diagnosis.
However, not all patients with MMN have evidence of a motor conduction block as the abnormal demyelinated segment may reside at regions not accessible to surface NCV procedures. (such as within the Brachial Plexus). Diagnosis can also be made more difficult when the needle EMG shows evidence of active denervation (similar to what could be found in motor neuron disease) or when the CK levels are elevated (usually no > than 3x the upper limit of normal, but still somewhat suggestive of a myopathic process).
MRI of the brachial plexus may show increased T2 signal (without enhancement on contrast studies) but this is an inconsistent finding and plexus imaging is often not even considered. Significantly elevated titers (>1/6400) of anti-GM1 autoantibodies carries an 80% specificity for the disease but such high levels are only seen in 20-30% of patients with MMN. In fact, low titers (1/400-800) can also be seen in diseases such as ALS and other neuropathies that are often being considered as differentials.
The Role of Nerve Ultrasound Imaging:
High resolution sonography of the peripheral nervous system allows nerves to be readily visualized and to assess their morphology.
For the last several years, neural sonography has focused primarily on focal entrapments. However, recent research on diffuse peripheral neuropathies has shown not only the presence of prominent pathologic changes, but also that the patterns of these changes vary depending on the type of neuropathy encountered. (1) In other words, the ultrasonic findings may be able to discriminate between demyelinating and axonal pathology as well as hereditary and immune-mediated disease processes.
Beekman and colleagues (2) in one of the earliest reports on MMN found multiple sites of neural enlargement within the brachial plexus, median, ulnar, and radial nerves in a majority of the 21 patients they studied. Additionally, sonography also disclosed nerve enlargement in nerves showing no electrodiagnostic abnormalities. Other authors found similar nerve enlargements in a MMN patient without electrodiagnostic evidence of conduction block but with evidence of denervation on needle EMG. (3)
At times, electrodiagnostic data is unable to provide a clear distinction between hereditary and immune mediated neuropathies. Zaidman and colleagues found that nerve enlargement was commonly diffuse and generally more than twice the normal size in hereditary neuropathies such as CMT-1. This differed greatly from immune-mediated neuropathies showing either no or mild enlargement and regional rather than diffuse involvement. (4)
Most recently, Grim et al investigated the ability of ultrasound to differentiate between MMN and ALS. (5) They found (as did another noted above) that focal enlargements in MMN patients were often not co-localized with regions of conduction block on NCV studies and furthermore that such nerve enlargement is not found in ALS patients.
- Nerve ultrasound is being found to be a valuable adjunct in the accurate diagnosis of various categories of diffuse peripheral neuropathy.
- It can be helpful in cases where electrodiagnostic testing is equivocal and also demonstrate abnormalities in regions either inaccessible to or found to be normal on EDX assessment.
- Of possibly even greater importance is ultrasound’s ability to discriminate between more serious diseases such as ALS and as well as to differentiate hereditary from immune-mediated processes.
For further reading:
- Ultrasound of Inherited vs. Acquired Demyelinating Polyneuropathies
- mmn1 (Multi-focal motor neuropathy with and without conduction block)
- mmndiagnosis (Multi-focal motor neuropathy diagnosis)
- Goedee HS et al. High resolution sonography in the evaluation of the peripheral nervous system in polyneuropathy-a review of the literature. Eur J Neurol. 2013 Oct 20(10)
- Beekman et al. Ultrasonography shows extensive nerve enlargements in multifocal motor neuropathy. Neurology 2005 Jul 26;65(2)
- Pitarokoili K et al. Nerve ultrasound in a case of multifocal motor neuropathy without conduction block. Muscle Nerve. 2015 Jan 23. (Epub ahead of print)
- Zaidman CM et al. Ultrasound of inherited vs. acquired demyelinating polyneuropathies. J Neurol. 2013 Dec;260(12)
- Grimm A. et al. Nerve ultrasound for differentiation between amyotrophic lateral sclerosis and multifocal motor neuropathy. J Neurol. 2015 April;262(4).