Multifocal motor neuropathy, also known as MMN, is a rare nerve disorder which mainly affects the lower arms and hands, and in some cases, the muscles of the legs. The cause of MMN is not exactly clear, but the current thinking is that the disease is caused by an abnormal response of the patient’s immune system which damages the nerves.
Let’s look a little closer at the term ‘multifocal motor neuropathy’ so you can understand what this means. Neuropathy is a medical term for disease or malfunction of the nerves. So a motor neuropathy is a malfunction of the motor nerves, or the nerves which are responsible for movement of the limbs. The word ‘multifocal’ means having more than one focus or more than one area of the nerves involved. If we put this all together, multifocal motor neuropathy simply means a malfunction of the motor nerves of the upper limbs caused by damage to the nerves in multiple areas.
Like Lewis-Sumner syndrome, the disease does not seem to have a genetic component and there is no current evidence that it can be inherited. Fortunately the disease is not life threatening and many patients work and stay active for many years after their diagnosis. The disease is not painful, because it only affects the nerves which control movement and not those involved in sensation.
Some physicians may mistake MMN for ALS (Lou Gehrig’s disease) as many of the symptoms are similar. However, MMN is treatable while ALS is not. Your physician will conduct a careful history and physical exam and will order blood tests to rule out other conditions and to look for elevated levels of an antibody called GM1. Nerve conduction tests will be ordered as well. This study makes use of mild electrical stimulation to let the physician see how well the nerves are working.
Multifocal Motor Neuropathy Symptoms
- Weakness in hands
- Weakness in lower arms
- Cramping in hands and lower arms
- Hands become smaller
The treatment of MMN is similar to Lewis-Sumner syndrome, with the patient receiving intravenous injections of antibodies known as immunoglobulins. Many patients who get this treatment have a rapid improvement in their symptoms, but because this improvement rarely lasts beyond six weeks, the injections need to be repeated every two to six weeks. There are also some reports that a drug called cyclophosphamide can help, at least partially.
Multifocal motor neuropathy has been reported after a number of vaccinations including flu vaccine, MMR, and tetanus. See The spectrum of post-vaccination inflammatory CNS demyelinating syndromes.