What is Multifocal Motor Neuropathy (MMN)?

Multifocal Motor Neuropathy is a rare disorder in which focal areas of multiple motor nerves are attacked by one’s own immune system.

Typically, MMN is slowly progressive, resulting in asymmetrical weakness of a patient’s limbs. Patients frequently develop weakness in their hand(s), resulting in dropping of objects or sometimes inability to turn a key in a lock. There is essentially no numbness, tingling, or pain. Patients with MMN can have other symptoms, including twitching, or small random dimpling of the muscle under the skin which neurologists call fasciculations.

MMN has many features similar to CIDP in that its onset is progressive over time, causing increased disability that reflects the greater number of nerve sites involved. However, unlike CIDP, MMN is asymmetric and affects the right and left side of the body differently.

MMN is chronically progressive without remission.

What causes MMN?

MMN is associated with increased levels of specific antibodies or sugar-containing lipid found in peripheral nerves.

Antibodies normally protect individuals from viruses and bacteria but may, under certain circumstances, bind to and facilitate an immune attack on the peripheral nerve.

Studies show that, even if these antibodies do not cause the nerve damage, they may be an important marker for the disease and facilitate diagnosis.

How is MMN diagnosed?

The diagnosis of MMN depends on demonstrating that the patient:

• has a purely motor disorder affecting individual nerves, and

• there are no signs such as brisk reflexes at the knees or ankles or spasticity in the limbs

• there is no difficulty speaking or swallowing

• there are no sensory deficits

• there is evidence of focal areas of nerve in which electrical impulses are slowed or blocked which can be detected on electrophysiology tests.

These criteria are designed to differentiate MMN from several other GBS and CIDP variants.

Tests include:

• an electrodiagnostic study, which includes nerve conduction studies (NCS) and needle electromyography (EMG)

• laboratory testing for specific antibodies.

How is MMN treated?

It is now established that intravenous immunoglobin (IVIg), a preparation of antibodies obtained from healthy volunteers, can be readily given through an arm vein and provides benefit to patients with MMN. The treatment usually does not completely reverse all of the symptoms, and those patients who do respond will require repeated treatments to maintain their improvement. Patients usually require retreatment every 2-5 weeks and over time may need increased doses of IVIg. IVIg is not a cure for MMN but currently no other therapy has proven to be widely effective.

Other immunosuppressive treatments such as corticosteroids and plasma exchange are ineffective and can actually make the disease worse.