Multifocal Motor Neuropathy (MMN)

Understanding MMN

Multifocal motor neuropathy (MMN) is rare and believed to be an autoimmune disorder. An autoimmune disorder develops when the immune system malfunctions and creates antibodies that attack and cause damage to a part of the body. In MMN, the myelin sheath of motor nerves is attacked. The myelin sheath is the protective covering over nerves, which aids in sending nerve signals from spine to muscles. This attack on the myelin sheath results in muscle weakness.

Symptoms of MMN

Symptoms of multifocal motor neuropathy (MMN) include weakness usually beginning in a single limb, for example, the hand or the ankle. The weakness is usually different from one side of the body to the other. Weakness will interfere with and make everyday activities such as writing, holding eating utensils or walking difficult. Other symptoms frequently seen are muscle wasting, cramping and twitching.

Diagnosing MMN

Diagnosing multifocal motor neuropathy (MMN) can be difficult for many reasons. Because MMN may only affect a single nerve and cause minimal weakness in one limb, it may take a long time to determine MMN as the cause. MMN only affects motor nerves and the symptoms may mimic Amyotrophic Lateral Sclerosis (ALS), also known as Lou Gehrig's disease. Many people ultimately diagnosed with MMN are initially told they have ALS, which can be extremely frightening and stressful. Nerve conduction studies may show a nerve conduction block in MMN, but not with ALS. Conduction blocks are difficult to detect and may contribute to misdiagnosis. Additionally, not all patients with MMN have a conduction block. Because MMN is an autoimmune disorder, there may be antibodies present that are detectable with a blood test. However, they may only be present in half of the people with MMN.

Diagnostic testing may include:

• Electromyography (EMG) to measure muscle activity.
• Nerve conduction studies (NCS) to measure efficiency and speed of signals of the nerves.
• Blood work to test for anti-GM1 antibody which may be present in MMN.

Treating MMN

The primary treatment for multifocal motor neuropathy (MMN) is immune globulin therapy administered into the blood stream intravenously (IVIG). The goal of IVIG in MMN is to stop the attack on the myelin sheath. This will then cause the symptoms to subside. Strength will increase and tasks that may have become difficult will become easier. Unlike ALS, MMN is treatable. It may take one to three courses of IVIG for patients to experience improvement. Your doctor will assess your response to therapy and determine how much IVIG you need to maintain your response. Most people with MMN usually require long-term/lifetime therapy.

Resources:

• American Academy of Neurology
• Multifocal Motor Neuropathy Information Page: National Institute of Neurological Disorders and Stroke (NINDS)
• Neuromuscular Center at Washington University
• Neuropathy Action Foundation

Support Groups:

• ALS/MND forums for Multifocal Motor Neuropathy

Reference:

• Multifocal motor neuropathy (MMN) or MMNCB | Autoimmune Association