“Ask the Doctor” Column: Multifocal Motor Neuropathy

This is a column in which leading clinicians answer your questions about neuropathy – usually one question per issue, and more when we are able. Please send your questions to “Ask the Doctor” c/o Neuropathy News, The Neuropathy Association, 60 East 42nd Street, Suite 942, New York, NY 10165.

DEAR DOCTOR: I am 54-year old business executive and I’ve had a right “drooping foot” for three years. I had surgery on my right foot five months ago, but it did not help. My symptoms have been gradually worsening with cramping of the leg muscles. My doctors did a nerve conduction study which showed multifocal conduction blocks in the motor nerves, but not in sensory nerves. He also explained that my lab studies showed high titers of serum antibodies to the ganglioside M1 (GM1) and to the asialo-GM1, which are markers of myelin (the insulation of the nerve). I was diagnosed with multifocal motor neuropathy and treated with intravenous immunoglobulin (IVIG) which improved my symptoms dramatically. I’d like to get a better understanding for my diagnosis. What is multifocal motor neuropathy? - Anonymous

DR. JIN LOU ANSWERS: Multifocal motor neuropathy (MMN) is a rare, but treatable, autoimmune form of neuropathy affecting one in every 100,000 people, men more than women. It results when the body’s immune system inadvertently mistakes markers on the body’s own nerve cells as foreign and then the immune system attacks the nerves and/or the myelin sheath that insulates the nerves. Because the myelin sheath helps with the transmission of messages along a nerve quickly, injury to the sheath or to the nerve itself results in slowed or faulty conduction of nerve signals/impulses, manifesting as muscle weakness.

Symptoms of multifocal motor neuropathy usually begin with slowly progressive weakness of the hands. Leg and foot weakness may follow, as well as decreased muscle volume (also known as muscle wasting), muscle cramps, and involuntary twitching and cramping of muscles. The weakness is asymmetric, i.e. a muscle group on only one side of the body may be affected. Over time, numbness or tingling of affected areas may occur in some patients, but sensation is not lost.

MMN is a neurological disorder affecting the peripheral motor neurons; but it is treatable. It differs from other motor neuron diseases such as amyotrophic lateral sclerosis (ALS), often referred to as Lou Gehrig’s Disease, and hence it is important to get to the right diagnosis. Your doctor will confirm your diagnosis based on your history of symptoms, a complete neurological examination, blood tests, and a neurophysiologic study that includes an electromyogram (EMG) and a nerve conduction study (NCS). Blood tests will disclose the presence of antibodies directed against ganglioside M1 (GM1), a component of nerve cells, and support the diagnosis of MMN.

Because MMN is an immune-mediated disorder, its therapies are directed at modulating the immune system. IVIG infusions, a blood product of pooled IgG immunoglobulins, are the mainstay of MMN treatment. You will notice your muscle strength gradually improve within several days to two weeks of the initiation of IVIG treatment, with the effect of the treatment peaking in a few weeks and lasting from several weeks to several months. Depending on your response to IVIG and your symptoms, you may need infusions at regular intervals. Over many years, however, you may experience decreasing response to IVIG and continued, slow progression of muscle weakness. Sometimes, IVIG may not be the therapy of choice; as an alternative, your doctor may prescribe an immunosuppressive drug like cyclophosphamide.

Jin J. Luo, M.D., Ph.D., F.A.A.N.E.M. is assistant professor of Neurology and Pharmacology and director of EMG/Neuromuscular Medicine in the Department of Neurology at Temple University.