Neurology

Multifocal Motor Neuropathy

What is MMN?

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. 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.
 

Who is impacted?

The prevalence of this very rare disease is estimated to be 0.6 cases in every 100,000 people, which makes it even more rare than Guillain-Barre Syndrome, a spontaneously self-limiting disorder in which 1-2/100,000 cases occur each year in North America and Europe.
 

How is MMN diagnosed?

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

• Has a purely motor disorder affecting individual nerves
• Does not have UMN (upper motor neuron) signs such as brisk reflexes at the knees or ankles or spasticity in the limbs
• Has no difficulty speaking or swallowing
• Has no sensory deficits
• Has evidence of focal areas of nerve in which electrical impulses are slowed or blocked (conduction block) which can be detected on electrophysiology tests

How is MMN treated?

It is now established that intravenous immunoglobulin (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 retreatment every 2-5 weeks and over time may need increased doses of IVIG.