Diagnosis of MMN

MMN ALS CIDP differences

by: Imran Khan, M.BBS |JUNE 29, 2020

Multifocal motor neuropathy (MMN) is a demyelinating peripheral neuropathy (PN). The condition is slowly progressive and usually presents with asymmetric distal weakness in the upper extremities. It affects men more often than women and, in most cases, will cause symptoms before age 45. The clinical signs of MMN may resemble a motor neuron disease, such as amyotrophic lateral sclerosis (ALS), or chronic inflammatory demyelinating polyneuropathy (CIDP), making diagnosis challenging. Because MMN should be treated differently from ALS or CIDP, an accurate diagnosis is important for proper patient management.

As described by these neuromuscular specialists, MMN is a treatable neurological disorder. First and foremost, MMN must be distinguished from degenerative motor neuron diseases, such as ALS, because of the dramatically different prognosis and therapy. MMN should also be differentiated from CIDP since patients with MMN will respond favorably to intravenous immunoglobulin (IVIg) or cyclophosphamide. In contrast, those with CIDP may benefit from corticosteroids, a medication which can actually exacerbate the weakness seen in MMN.

The clinical work up of a patient with MMN consists of various components, each of which can affect proper patient management. Categorizing the cause of the neuropathy, through routine lab analysis, electrophysiological studies, and antibody testing, can help diagnose the illness, determine the most appropriate treatment, and can increase the probability of a positive clinical outcome.

"Generally middle-aged patients, slightly more common in men, with slowly progressive, asymmetric distal weakness, more frequently in the arms than legs. Over time, other extremities may become involved, but not in all cases. Cramping and muscle twitching may be associated symptoms."

Table1:Differential Diagnosis Multifocal Motor Neuropathy

Lower motor neuron weakness Distal, asymmetrical Distal & proximal, symmetricalAsymmetrical
Upper motor neuron signs Absent or Present Absent or PresentPresent
Sensory loss Absent or Present Absent or PresentAbsent
Focal demyelinating on NCV 10% of cases FrequentRare
Sensory conduction Normal SNAP Low to absent SNAP Normal SNAP
Cerebrospinal fluid protein Normal Elevated protein or NormalNormal
Anti-GM1 antibodies 80-90%2 AbsentRare
Conduction Block in NCV test Present PresentRare

NCS = nerve conduction study
SNAP = sensory nerve action potential
Conduction Block = reduction in amplitude
Chart adapted from: Bosch, E.P. and Smith, B.E. Disorders of Peripheral Nerves. In Neurology in Clinical Practice, 2nd ed., ed. W.G. Bradley et al., 2091. 2000. Boston: Butterworth-Heinemann.

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