Multifocal Motor Neuropathy (MMN)

Multifocal motor neuropathy (MMN) is a rare condition that causes slowly worsening muscle weakness, usually in your arms or legs. MMN happens because your immune system attacks multiple motor (movement) nerves. But researchers don’t know why this happens.

MMN is rare — it affects fewer than 1 per 100,000 people worldwide. Another name for the condition is multifocal motor neuropathy with conduction block (MMNCB).

It’s natural to worry about how ongoing worsening muscle weakness could affect your life. The most important step is to see a healthcare provider. Finding the right diagnosis may take time, but your provider will listen, answer your questions and help guide you through the process.

Multifocal motor neuropathy symptoms

The main symptom of multifocal motor neuropathy is muscle weakness that slowly worsens over time. The weakness is asymmetrical, which means it affects different muscles on opposite sides of your body. For example, you may have left arm weakness and right leg weakness.

MMN most commonly affects one hand first. This may look like:

• Difficulty extending or bending your wrist upward (wrist drop)
• Difficulty doing fine motor skills with your hand, like turning a doorknob or fastening buttons
• Dropping objects frequently
• Finger weakness

Sometimes, muscle weakness can start in your legs. This may look like:

• Reduced toe movement
• Your foot drags when you walk (foot drop)

Other symptoms of MMN may include:

• Muscle cramps
• Muscle twitches
• Low muscle tone or atrophy

As the weakness gets worse, you may have severe fatigue in the affected muscles. This may impact your endurance and ability to do physical activities. Unlike other neurological conditions that affect your muscles, MMN doesn’t typically cause sensory issues, like pain, numbness or a pins and needles sensation (paresthesia). It also doesn’t typically affect the muscles that you need for breathing or swallowing.

Multifocal motor neuropathy causes

MMN is an autoimmune condition. It happens because your immune system attacks multiple motor (movement) nerves. These are the nerves that control your muscles with signals from your brain. More specifically, a conduction block affects how some of your motor nerves function. A conduction block happens when a nerve signal doesn’t travel all the way down the nerve to your muscle. Researchers don’t know why your immune system attacks your nerves in this way.

Many, but not all, people with MMN have specific autoantibodies: anti-GM1 antibodies. Autoantibodies are a malfunctioning type of antibody. Antibodies are proteins your immune system makes to identify and destroy invaders like germs, allergens or toxins in your blood. Autoantibodies attack your own bodily tissue instead of keeping it healthy.

Anti-GM1 antibodies attack GM1 gangliosides. This is a fatty material (lipid) in your peripheral nerves that helps keep them healthy. This is the main theory researchers have that explains what happens in MMN. But because not everyone with the condition has these antibodies, researchers have more to learn about MMN.

How doctors diagnose multifocal motor neuropathy

Multifocal motor neuropathy is a rare condition that progresses slowly, so it may take time to get a diagnosis. To start, your healthcare provider will ask about your symptoms and medical history. They’ll likely do a physical exam and a neurological exam, too.

You’ll likely also need to see a neurologist to get a formal diagnosis. They may recommend a variety of tests to confirm MMN or rule out other conditions. Tests may include:

• Nerve conduction study and electromyography (EMG): These tests can check for issues in the functioning of specific motor nerves. MMN most commonly affects your median and/or ulnar nerves. These tests can also determine if a conduction block is the reason for muscle weakness.
• Laboratory tests: Your provider may recommend various blood, urine (pee) and cerebrospinal fluid (CSF) tests. They’ll also test for anti-GM1 antibodies.
• Nerve ultrasound and magnetic resonance imaging (MRI): These imaging tests can help check the health of your nerves.

Your neurologist will use specific criteria to confirm MMN and rule out conditions that cause similar symptoms, like:

• Amyotrophic lateral sclerosis (ALS)
• Chronic inflammatory demyelinating polyneuropathy (CIDP)
• Lewis-Sumner syndrome
• Progressive muscular atrophy (PMA)

Multifocal motor neuropathy treatment

The main treatment for MMN is an intravenous immunoglobulin (IVIG) called Gammagard Liquid®.

IVIG is a product made up of antibodies from healthy donors that you receive through a vein with an IV. This treatment leads to improved muscle strength in most people with MMN. The improvement can range from minor to significant.

Unfortunately, the initial muscle strength improvement is typically short-term. Only about 20% of people stay in remission after a single dose of IVIG. Most people need repeated IVIG infusions to maintain their muscle strength. These treatments are usually given every two to eight weeks.

IVIG isn’t a cure for MMN, but no other therapies work as well as it does.

Multifocal motor neuropathy prognosis

While it can be frustrating to live with muscle weakness in your limbs, MMN generally isn’t harmful to your physical health. Because of this, the prognosis (outlook) is usually good. About 80% of people respond well to IVIG treatment and notice symptom improvements.

Even if IVIG doesn’t work for you, muscle weakness progresses slowly. Most people with MMN can do activities of daily living (like hygiene tasks, chores and preparing meals). MMN doesn’t typically affect life expectancy.

But living with a chronic (long-term) condition like MMN may affect your mental health. It’s important to see a mental health professional, like a psychologist, if MMN affects your emotional well-being.

What is the difference between MMN and ALS?

Amyotrophic lateral sclerosis (ALS) is a type of motor neuron disease that causes progressive muscle weakness throughout your body. Both ALS and MMN result in muscle weakness. But worsening muscle weakness happens more slowly in MMN than in ALS. Also, ALS typically affects muscles you need to breathe and swallow, whereas MMN usually doesn’t.