Multifocal Motor Neuropathy

What is Multifocal Motor Neuropathy?

Multifocal Motor Neuropathy (MMN) 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. The upper extremities are often commonly affected.  Patients frequently develop weakness in their hand(s), resulting in dropping of objects or sometimes inability to turn a key in a lock. The weakness associated with MMN can be recognized as fitting a specific nerve territory. 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.

The prevalence of this very rare disease is estimated to be 0.6 cases in every 100,000 people. 

Who gets Multifocal Motor Neuropathy?

Men are more often affected with multifocal motor neuropathy than women. Symptoms usually appear before age 45.

How is Multifocal Motor Neuropathy diagnosed?

The diagnosis of MMN is a clinical one that depends on demonstrating that the patient:

• has a purely motor disorder affecting individual nerves
• there are no UMN (upper motor neuron) 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 (conduction block) which can be detected on electrophysiology tests

These criteria are designed to differentiate MMN from ALS, (Lou Gehrig’s disease), Lewis-Sumner Syndrome (a multifocal inflammatory nerve disease with motor and sensory symptoms), and vasculitis or inflammation of small blood vessels in the peripheral nerve.

A neurologist is usually needed to determine the diagnosis, which is based on the history and physical examination. Tests include:

• an electrodiagnostic study, which includes nerve conduction studies (NCS) and needle electromyography (EMG).
• Laboratory testing for IgM GM1 antibodies is also frequently done. The absence of these antibodies, however, does not exclude the diagnosis.
  

How is Multifocal Motor Neuropathy 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. It is the only treatment for this disorder that is approved by the Federal Drug Administration (FDA) and regulatory agencies in Europe and Canada. IVIg can lead to improved motor function in most patients with MMN, with the response varying from minimal to very large. Early treatment shortly after symptom onset is always more effective. 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 re-treatment every 2-5 weeks and over time may need increased doses of IVIg or increased frequency of treatment over time. Subcutaneous immunoglobulin is also an option.

IVIg is not a cure for MMN, but currently no other therapy has proven to be widely effective. In a limited number of patients a cancer chemotherapy drug, cyclophosphamide, is temporarily effective. However, its use is limited 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.

Video - https://youtu.be/dhv4haxM_1Q 

More Information

Foundation for Peripheral Neuropathy
American Chronic Pain Association
Johns Hopkins Department of Neurology