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.
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The Experts' Responses
1) How would you describe the presentation of a "typical" MMN patient? "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."
"Asymmetrical hand weakness, slowly progressive and indolent, with minimal atrophy."
"MMN most commonly begins with weakness in the hands, although virtually any muscle may be affected. In mild cases, one can usually recognize weakness in the distribution of individual named peripheral nerves. In more severe cases the distributions may overlap, causing more widespread weakness. In some cases, the demyelinating nature of the condition is evidenced by the degree of weakness being disproportionate to the degree of atrophy."
2) What diseases do you include in the differential diagnosis of MMN? "Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), mononeuritis multiplex, and multiple entrapments."
"The differential would include motor neuron disease, vasculitic neuropathies, hereditary neuropathy with liability to pressure palsy (HNPP), and other neuropathic disorders that can present with asymmetric weakness initially."
"Motor neuron diseases including monomelic amyotrophy and amyotrophic lateral sclerosis, and the multifocal motor variant of CIDP."
"ALS, CIDP, Syrinx, entrapments."
"CIDP, ALS, and various other forms of neuropathies (due to diabetes, lymphoproliferative disorders, infections, vasculitis, cryoglobulinemia, HNPP, and entrapment) can resemble MMN clinically."
3) Why differentiate MMN from other disorders? "MMN is treatable, and while CIDP is also treatable, therapies most likely to benefit the patient will be different."
"Some autoimmune conditions that can resemble MMN respond to different forms of immunomodulatory therapy. Other non-autoimmune conditions in the differential diagnosis, such as HNPP or other multiple entrapments, must be approached differently."
4) What can electrophysiology and/or nerve biopsy reveal about MMN? "Electrophysiology reveals features of a demyelinating polyneuropathy, limited to motor neurons. Conduction block is often present, although other features of demyelination such as temporal dispersion of waveforms and conduction slowing are also helpful. Nerve biopsy is not useful in this disorder."
"[EMGs] reveal motor conduction blocks at sites not prone to compression."
"Diagnosis still may be clinical, but EMG is extremely helpful if it shows block. Nerve biopsy is not helpful."
"We do not perform nerve biopsies on patients when we are confident about the diagnosis on the basis of other testing. Electrophysiology is essential to look for conduction block and/or demyelinating features, which we encounter in nearly all patients. Sensory nerve conduction studies are unaffected."
"Nerve conduction studies should show conduction block, but may also show other features of demyelination, including prolonged F-waves and slowed conduction velocities."
5) Do you use autoantibody testing in diagnosing MMN? If so, how? "Anti-GM1 antibodies are present in between 50-85% of patients, depending on the methodology."
"In straightforward cases with conduction block, no [I do not use autoantibody testing]. In cases where I cannot identify conduction block, I routinely order anti-GM1 antibodies. From my perspective, high titers of these antibodies are a valuable marker for potentially treatable, motor-predominant neuropathies."
"Finding evidence for an autoimmune etiology provides support for using immunosuppressive therapy. Response to immunosuppressive treatment can also be monitored with autoantibody testing."
"GM-1 autoantibodies are not sensitive, although they are specific and therefore are quite useful for confirming a suspected diagnosis."
6) What therapies have you found successful in treating MMN? "IVIg is the mainstay of therapy. Most patients respond."
"Cyclophosphamide and human immune globulin are helpful in 50-75% of patients."