How is MMN Diagnosed?
Electrodiagnostic Testing (EMG)
As with CIDP, the diagnosis of MMN rests on the nerve conduction studies. When affected nerves are tested, severe or even complete block of electrical impulses is seen. Conduction velocity is severely slowed through that same segment of nerve. However, conduction velocity is completely normal along the rest of the nerve. Sensory nerve conduction studies are normal. These are important features that distinguish MMN from CIDP. In CIDP, there is more diffuse slowly of motor conduction velocity and sensory nerve conduction studies are usually abnormal.
Cerebrospinal Fluid (CSF) Examination
The CSF is normal in MMN, and spinal tap is not indicated unless there are very atypical features. Nerve Biopsy Since the sensory nerves are not involved in MMN and since it is unwise to biopsy motor nerves, nerve biopsy is not indicated in MMN. Laboratory Testing About half of all patients with MMN have very high levels of specific antibodies in their blood called anti-GM1 antibodies. They are directed against a specific component of the myelin sheath, the glycolipid. These antibodies can be measured in the blood. They do not need to be routinely measured unless there are unusual features and particularly if the patient appears clinically to have MMN, but does not have the expected abnormalities with the nerve conduction testing. There are no other useful laboratory tests.
What Causes MMN?
Like GBS and CIDP, MMN is an autoimmune disease. It is extremely rare, but the exact prevalence is not known. It affects men more than women. It is not known why only the motor nerves are affected. There is no recognised trigger for the disease, and there are no known associated diseases. The disease is more common in young people between 20 and 40 years of age, although children and the elderly are occasionally affected. Occasional periods of more rapid progression can be seen, but it is not clear that these are precipitated by any particular event.
How is MMN treated?
The only proven effective treatment for MMN is IVIg. Several studies have established the effectiveness of this treatment. However, IVIg, does not cure the disease and long-term, probably life-long, treatment is necessary. The effect of the treatment usually lasts a few weeks to, at most, a few months. Most patients need to have the treatment repeated every 2 to 6 weeks. Cyclophosphamide may also help, but the risks usually outweigh the benefits. Patients may deteriorate more rapidly if treated with steroids or with PLEX (plasmapheresis).