Multifocal Motor Neuropathy (MMN) is a progressive muscle disorder characterized by muscle weakness in the arms and hands.
What is Multifocal Motor Neuropathy (MMN)?
MMN is a rare, chronic, progressive neuropathy characterized by muscle weakness in the distal limbs, but minimal or no sensory changes. It is usually asymmetric, with different muscles involved on one side of the body versus the other.
The cause of MMN is unknown, but some evidence points to it being an autoimmune disorder, in which the immune system abnormally attacks the motor nerves in various places.
MMN is also associated with increased levels of specific antibodies to GM1, a sugar-containing lipid found in peripheral nerves. These antibodies are not thought to actually cause the nerve damage, but serve as an important disease marker to facilitate diagnosis, since they have been detected in 30-80% of MMN patients.
MMN is very rare. The prevalence is estimated to be only 0.6 cases in every 100,000 people, which makes it even rarer than GBS.
Most cases of MMN occur in male patients between 50 and 60 years of age, although cases have been reported between ages 20 and 75. Men are affected about three times as often as women.
MMN is sometimes mistaken for amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease) but unlike ALS, MMN is treatable. A careful nerve conduction study can help lead to an accurate diagnosis.
What are the symptoms of MMN?
Symptoms of MMN, which usually develop slowly, progressively, and asymmetrically, include:
Weakness in one or both hands, resulting in dropping of objects or inability to turn a key in a lock
Weakness in the feet/ankles, resulting in foot drop
Small random dimpling of the muscle under the skin (fasciculations)
Reduced deep tendon reflexes in affected limbs
There is usually no numbness, tingling, or pain associated with the MMN symptoms; sensory nerve function remains normal. And in contrast to some patients with GBS, CIDP, or other motor neuropathies like ALS, swallowing difficulties and slurred speech do not occur in patients with MMN.
The clinical course of MMN is chronically progressive without remission.
How is MMN treated?
Treatment for MMN varies. Some individuals experience only mild, modest symptoms and require no treatment. But most cases will be treated with intravenous immunoglobulin (IVIG) as first-line therapy.
Improvement in muscle strength usually begins within hours or days after IVIG treatment is started, and lasts 3 to 6 weeks or more.
IVIG is the only treatment for MMN that is approved by the FDA and European regulatory agencies.
In a limited number of patients, cyclophosphamide (an immunosuppressant and cancer chemotherapy drug) is temporarily effective. Its use is limited, however, by the toxic side-effects and risks that accrue with chronic use.
Other immunosuppressive treatments, such as corticosteroids and plasma exchange, are ineffective and can actually make the disease worse.
IVIG can improve motor function in most patients with MMN, especially when treatment is initiated early (shortly after symptom onset). IVIG treatment usually does not completely eliminate the symptoms, so repeated treatments (about every 2-5 weeks) are often required to maintain the symptom improvement. Increased doses may also be needed over time.