A rare autoimmune disorder in which the muscles worsen gradually is called multifocal motor neuropathy (MMN). While it mostly affects the hand and wrist, it may also impact the legs. Unfortunately, there is no way to cure this disorder. That’s why the treatment goal is to reduce the symptoms and prevent serious complications. Doctors often recommend IVIG therapy to manage MMN.
In other words, MMN occurs when the immune system mistakenly attacks healthy tissue in the body instead of viruses, bacteria, and other germs that can make you ill. In such cases, the immune system attacks multiple motor nerves. Generally, this condition occurs quite rarely, and it affects about 1 in 100,000 people worldwide. In some cases, this disorder is called multifocal motor neuropathy with conduction block (MMNCB).
Symptoms
While the main symptom of this autoimmune condition is muscle weakness, it may also cause other symptoms. Usually, weakness affects only one part of the body. Check for other symptoms of MMN below:
- Problems with bending or extending the wrist upward
- Difficulty performing fine motor skills, such as turning a doorknob or fastening buttons
- Dropping objects
- Finger weakness
- Muscles twitch or cramps
- Foot drop
- Decreased toe movement
- Low muscle tone
When the disease advances, it may also cause severe fatigue, especially in the affected muscles. Thus, it affects your ability to perform physical exercise and endurance. Most of the time, this disorder does not cause sensory problems, including numbness, paresthesia, and pain, which are common for other neurological disorders.
Causes and Risk Factors
This condition happens when the immune system mistakenly attacks multiple motor nerves, which control the signals between the brain and muscles. However, experts do not fully understand what exactly causes this autoimmune disorder. Usually, about 30% to 50% of people have specific antibodies known as anti-GM1. This is a malfunctioning type of antibody. In normal circumstances, antibodies (proteins produced by the immune system) attack viruses, allergens, toxins, and other germs that can make you ill.
Furthermore, this specific antibody attacks GM1 gangliosides, which are fatty materials in the peripheral nerves. Nowadays, this is the primary theory that researchers have explained about what is happening in people with MMN. However, more research is needed to prove this theory.
In addition, certain factors may increase your risk of developing MMN. Check some of them below:
- Existing autoimmune disorders – Sometimes, the following disorders are associated with MMN. These include type 1 diabetes, Hashimoto’s thyroid disease, and others.
- Genetic factors – Recently, physicians have identified a possible connection between HLA-DRB1 and MMN.
- Specific antibodies – The anti-GM1 antibodies could play a role.
- Gender – Males are slightly more prone to MMN compared to females.
What Are The Long-term Effects of Multifocal Motor Neuropathy?
People with MMN may also experience some complications, especially if they leave it unmanaged. Check some complications below:
- Progressive muscle weakness
- Muscle atrophy (wasting of muscles)
- Involuntary movements
- Permanent nerve damage
- Decreased range of motion
- Disability
This article does not contain a full list of complications, but doctors may try to reduce the risk or even prevent them. In addition, this autoimmune disorder cannot be prevented because healthcare providers do not know the exact cause.
Diagnosis
In most cases, it is difficult to diagnose MMN because it progresses slowly. However, doctors often start the diagnosis with a physical and neurological examination to check for abnormalities linked to the disease. They may also ask some questions about your medical and family history to gather more information about your disease. Check below some tests often used to confirm MMN or exclude other health conditions that cause similar symptoms:
- Laboratory tests – These include blood, urine, and cerebrospinal fluid (CSF) tests to check for anti-GM1 antibody.
- Nerve conduction study and electromyography (EMG) – This test is used to determine how well your nerves are functioning. MMN usually affects the median or ulnar nerves.
- Nerve ultrasound and MRI (magnetic resonance imaging) – These are imaging tests used to get detailed images of different tissues and organs in the body (including motor nerves).
Health conditions that cause similar symptoms to MMN include amyotrophic lateral sclerosis (ALS), Lewis-Sumner syndrome, progressive muscular atrophy (PMA), and chronic inflammatory demyelinating polyneuropathy (CIDP).
Treatment
While the main treatment option for people with MMN is intravenous immunoglobulin (IVIG), doctors may also recommend other treatments. Generally, IVIG is a product made from healthy donors that you receive through a vein. It often helps people with this autoimmune condition improve their symptoms. Unfortunately, about 20% of people with this disorder stay in remission after a single dose of IVIG. Others usually require multiple infusions to maintain their muscle strength.
Frequently Asked Questions
Is there a way to cure multiple motor neuropathy?
No, it is not possible to cure this disorder. That’s why the treatment goal is to reduce the symptoms, prevent serious complications (such as disability), and improve your quality of life.
What is the difference between MMN and ALS?
While both disorders cause similar symptoms, there is a difference between them. ALS is a type of motor neuron disease that negatively affects muscles throughout the body. Furthermore, the worsening of the symptoms happens more slowly in MMN compared to ALS.
At what age do people get multifocal motor neuropathy?
Approximately 80% of people experience first symptoms between the ages of 20 and 50. However, the average age at onset is about 40 years. If you have any other questions, ask your healthcare provider.
