What is Multifocal Motor Neuropathy?

Multifocal motor neuropathy, or MMN, is a rare disorder in which multiple motor nerves are attacked by one’s own immune system, causing muscle weakness that can worsen over time.

MMN is a slowly progressive disease that commonly affects one side of the body (asymmetric) and commonly begins in the finger, thumb, wrist, or lower limb. It affects the strength in the lower parts of the arms and hands more than the legs without a loss of sensation.

It is thought that with MMN, the immune system abnormalities affect a protective layer around nerve cells that control muscle function. The protective layer (myelin sheath) normally allows messages to be sent properly and quickly along the peripheral nervous system. When the protective layer breaks down (a process called demyelination), nerve damage can occur. When messages are interrupted, muscle weakness can occur.

Why is MMN sometimes misdiagnosed?

MMN is a rare nerve disorder with symptoms that are somewhat similar to those of other neuropathies that can impact the limbs.

For example, muscle weakness, spasms, and cramping often associated with MMN also are seen in individuals with amyotrophic lateral sclerosis (ALS). Unlike MMN, ALS is rapidly progressive, and causes respiratory muscle weakness.

In other people, MMN may be confused with nerve compression conditions including carpal tunnel syndrome.

An accurate diagnosis of MMN is important so that it can be treated appropriately. It is important to talk to your doctor about symptoms you may be experiencing.