Multifocal Motor Neuropathy

Multifocal motor neuropathy is a rare condition where the body’s immune system mistakenly attacks nerves that control muscles, causing slowly worsening weakness mainly in the hands and arms, though treatment can help manage symptoms effectively.

Table of contents

• What is multifocal motor neuropathy?
• What causes this condition?
• Signs and symptoms
• How doctors diagnose MMN
• Treatment options
• What to expect with MMN
• Living with MMN

What is multifocal motor neuropathy?

Multifocal motor neuropathy, often called MMN or MMNCB (multifocal motor neuropathy with conduction block), is a rare disease that affects the nerves controlling your muscles^[1]. The condition was first described in medical literature in 1986, making it a fairly recent discovery^[1].

MMN is rare, affecting fewer than 1 in 100,000 people worldwide^[4]. In the United States, it’s estimated that between 3,000 and 10,000 people have this condition^[6]. The disease is more common in men, who are diagnosed almost twice as often as women^[5]. Most people are diagnosed between the ages of 35 and 70, with diagnosis most commonly occurring between ages 40 and 50^[2][5].

Unlike some severe neurological conditions, MMN is not fatal and does not shorten life expectancy^[5][7]. The condition does not affect the brain or muscles needed for breathing or swallowing^[6]. While the disease cannot be cured, it is treatable, and many people can maintain a good quality of life with proper management^[2].

What causes this condition?

MMN is considered an autoimmune disease, meaning the body’s immune system mistakenly attacks its own tissues^[2][4]. In this case, the immune system attacks multiple motor nerves—the nerves that carry signals from your brain to your muscles to control movement^[4].

A key feature of MMN is something called a conduction block. This happens when a nerve signal doesn’t travel all the way down the nerve to reach the muscle^[4]. The nerve can’t properly transmit electrical signals that make your muscles move^[2].

Many people with MMN have specific proteins in their blood called anti-GM1 antibodies. These are found in about one-third to over 80% of patients with MMN, depending on the testing method used^[1][5][7]. These antibodies—proteins normally made by the immune system to fight infections—attack GM1 gangliosides, a fatty material in the nerves that helps keep them healthy^[4]. The abundance of GM1 in motor nerves compared to sensory nerves may explain why MMN affects only movement, not sensation^[1].

Researchers believe these antibodies cause problems with sodium and potassium channels at or around the node of Ranvier, a part of nerve cells important for transmitting signals. This is why some doctors use the term “nodo-paranodopathy” to describe MMN^[1]. However, because not everyone with MMN has these antibodies, scientists still have more to learn about what causes this condition^[4].

Signs and symptoms

The main symptom of MMN is muscle weakness that slowly worsens over time^[4]. The condition typically starts in one hand or foot, and the weakness is asymmetrical, meaning it affects different muscles on opposite sides of your body^[4][6]. For example, you might have weakness in your left arm and right leg.

Most commonly, the first symptoms appear in one hand or wrist. This may include^[2][4]:

• Difficulty extending or bending your wrist upward (called wrist drop)
• Trouble with fine motor skills, such as turning a doorknob or fastening buttons
• Dropping objects frequently
• Finger weakness
• Decreased grip strength

Sometimes the weakness starts in the legs instead. About one-third of people with MMN first notice foot drop, where the foot drags when you try to walk^[14]. This happens because the muscles that lift the foot become weak^[2][4].

Other symptoms of MMN include^[2][4][7]:

• Muscle cramping
• Involuntary muscle twitching or spasms (called fasciculations)
• Muscle wasting or reduced muscle tone (atrophy) over time

An important feature that helps doctors identify MMN is what symptoms are not present. Unlike many other nerve conditions, MMN typically does not cause^[2][4][7]:

• Pain
• Numbness or tingling
• Loss of sensation or feeling

The disease is progressive, meaning symptoms tend to worsen slowly over time with no periods of improvement^[14]. As weakness increases, you may experience severe fatigue in the affected muscles, which can impact your endurance and ability to perform physical activities^[4]. Over time, you may find it hard to do daily tasks like typing on a keyboard or getting dressed^[2].

How doctors diagnose MMN

Diagnosing MMN can be challenging because it’s a rare condition that progresses slowly. Many people wait more than a year before getting a correct diagnosis^[17]. The condition is often mistaken for other diseases, particularly amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease, because the two conditions share similar symptoms^[2][7]. However, unlike ALS, MMN is treatable^[2].

You will likely need to see a neurologist, a doctor who specializes in problems with the brain and nervous system^[2]. The doctor will ask about your symptoms and medical history, and perform a physical exam. They may ask questions like^[2]:

• Which muscles are giving you trouble?
• Is it worse on one side of your body?
• How long have you been feeling this way?
• Do you have numbness or tingling?
• Does anything make your symptoms better or worse?

To confirm the diagnosis, doctors use several tests^[2][4]:

Nerve conduction study (NCS) measures how fast electrical signals travel through your nerves. The doctor places sensors on your skin over a nerve—one to send a small electric shock and another to record the activity^[2]. The key finding in MMN is a conduction block at sites where nerves are not usually compressed^[6][7].

Electromyography (EMG) uses small needles inserted into muscles to measure electrical activity. The doctor will ask you to slowly flex and relax your muscles while a machine records the activity^[2]. This test is usually done at the same time as the nerve conduction study.

Blood test looks for anti-GM1 antibodies, which are found in many people with MMN^[2]. However, not everyone with MMN has high levels of these antibodies, and having them doesn’t guarantee a diagnosis or predict how well treatment will work^[8].

The diagnosis is based on finding that you have a purely motor disorder affecting individual nerves, with no upper motor neuron signs, no sensory problems, and evidence of conduction block on testing^[7].

Treatment options

While MMN cannot be cured, it is a treatable condition, and most patients respond well to treatment^[1][2]. The main goal of treatment is to reduce symptoms, improve muscle strength, and slow disease progression.

Intravenous immunoglobulin (IVIG) is the primary and most effective treatment for MMN. It is the only treatment approved by the U.S. Food and Drug Administration for this condition^[8][11]. IVIG is a medicine made from antibodies collected from donated blood. It is given through a vein in your arm.

Most people—about 80 to 90%—improve with IVIG treatment^[1][8][11]. Treatment typically begins with an initial dose of 2 grams per kilogram of body weight given over 2 to 5 days^[8][11][12]. After this, most patients need regular maintenance infusions to prevent symptoms from returning. These are usually given every 4 to 8 weeks, and the exact dose and frequency depends on how well you respond to treatment^[8][11].

IVIG infusions can be performed in several settings: in a hospital, at an outpatient infusion center, in a doctor’s office, or at home^[11]. Long-term IVIG treatment can improve muscle strength and functional ability, though some people may find their response decreases over time^[11].

Subcutaneous immunoglobulin (SCIG) is an alternative way to receive immunoglobulin therapy. Instead of getting the medicine through a vein, it is injected under the skin^[11]. SCIG has similar effectiveness to IVIG but may have fewer side effects related to the infusion^[11]. It requires more frequent dosing, typically weekly, but can often be given at home by the patient or family members^[11].

If IVIG is not effective enough, doctors may consider other treatments^[11]:

• Cyclophosphamide, an immunosuppressive drug, has shown consistent effectiveness in treating MMN^[1][11][12]. It may be used alone or in combination with other treatments.
• Other medications like rituximab, cyclosporine, azathioprine, and interferon-beta have been used in some cases with varying success^[1][11].

Some treatments are known not to work for MMN and may even make symptoms worse. These include corticosteroids (steroids) and plasmapheresis (a procedure that filters blood) when used without cyclophosphamide^[11]. Mycophenolate has also been shown to be ineffective^[11].

In addition to medication, other therapies can help manage MMN^[2]:

• Physical therapy can help maintain muscle strength, flexibility, and range of motion
• Occupational therapy can teach you easier ways to perform daily tasks and recommend helpful equipment
• Orthotic devices, such as braces or splints, can support weakened muscles and improve mobility

What to expect with MMN

MMN is a chronic condition, meaning it lasts for a long time and typically persists indefinitely^[5]. With very rare exceptions, the condition does not shorten life^[5]. The disorder appears to continue indefinitely and rarely goes into long-term remission (a period without symptoms)^[5].

The progression of MMN varies from person to person. In some cases, symptoms are so mild that treatment may not be necessary^[1]. However, most people experience a progressive worsening of strength, especially in the hands and arms, which can make even simple daily tasks like writing, washing, or dressing difficult^[1].

Without treatment, MMN can lead to significant disability. Loss of function in the hands can affect your ability to work and perform everyday tasks, while foot drop can make it difficult to stand and walk. Some people may need aids like canes, splints, or walkers^[7].

However, with treatment, many people can work and stay active for many years after diagnosis^[2]. Treatment is usually very effective in helping slow down the disease and improve muscle function^[2]. When treatment begins early, patients often have better outcomes^[17].

It’s important to have regular medical monitoring. Your doctor will track disease progression and adjust your treatment as needed^[2]. Many people who receive treatment experience only mild symptoms over prolonged periods^[7].

Living with MMN

Living with a chronic condition like MMN can feel overwhelming at times, but you are not alone^[15]. Having a good support system and taking care of yourself can make a significant difference in your quality of life.

Working with your healthcare team is essential. Your team may include specialists such as a neurologist, physical therapist, occupational therapist, and mental health professional^[15]. Keep in regular communication with your doctors about your symptoms and how you feel while taking your medication. Don’t stop treatment on your own—if something isn’t working, talk to your doctor about adjustments^[15].

Lifestyle adjustments can help you manage daily life with MMN^[15][19]:

• Stay active with low-impact exercises to maintain muscle strength and flexibility
• Adapt your environment using assistive devices
• Balance activity with rest periods to avoid overexertion
• Eat a healthy, balanced diet
• Practice stress management through relaxation techniques

Emotional support is important for your well-being. Living with a chronic condition can be stressful, and ongoing stress can affect your health^[15]. Consider joining a support group to connect with others who understand what you’re going through. Organizations like the GBS CIDP Foundation and the Neuropathy Action Foundation offer information on finding support groups for people with neuropathy^[15]. Talking with a therapist who specializes in working with people who have chronic conditions may also be beneficial^[15].

Many people with MMN find strength in connecting with others who have the disease. As one patient shared, “MMN has taught me to not take life for granted. I truly appreciate what I can do”^[15]. Online communities and forums can provide a place to read others’ experiences and share your own journey.

Financial assistance may be available to help with treatment costs. Some organizations provide help with copays, infusion and nursing costs, and travel expenses related to medical care^[14].

Remember that while MMN may change how you do things, it doesn’t define who you are. With treatment and support, many people with MMN continue to live full, meaningful lives^[15].