Multifocal motor neuropathy (MMN) is a rare, chronic disorder where the immune system attacks the body’s peripheral motor nerves, which transmit signals from the brain to the muscles. Unlike many other neuropathies, MMN does not affect sensory nerves, so the ability to feel touch, temperature, and pain remains intact. It is an acquired condition, not inherited, that emerges in individuals between 40 and 50, affecting men more than women. Although MMN causes progressive muscle weakness, it is a treatable condition.
Hallmark Symptoms of MMN
The initial signs of MMN appear subtly with progressive, asymmetric muscle weakness, affecting different muscles on each side of the body. This weakness frequently begins in the hands and arms, causing difficulty with fine motor skills like buttoning a shirt or writing. A common manifestation is “wrist drop,” an inability to extend the wrist, or a weakened grip.
As the condition progresses, weakness can extend to the lower limbs, causing “foot drop,” which leads to tripping and an unsteady gait. Involuntary muscle twitches (fasciculations) and cramping often accompany the weakness. Over time, the affected muscles may decrease in size, a process called muscle atrophy. A defining feature of MMN is this lack of sensory symptoms like numbness or tingling.
The Diagnostic Pathway
Diagnosing MMN involves distinguishing it from other conditions with similar symptoms, such as amyotrophic lateral sclerosis (ALS). A diagnosis starts with a clinical examination and a review of the patient’s symptom history, focusing on the pattern of asymmetric weakness without sensory loss.
The most definitive test is a nerve conduction study (NCS), which measures the speed and strength of electrical signals in the nerves. In MMN, the characteristic finding is a “conduction block,” where the signal along a motor nerve is disrupted or stopped. An electromyography (EMG) may also be performed to assess muscle health and rule out disorders that directly impact the muscles.
Blood tests are also conducted to search for specific antibodies. About half of individuals with MMN have elevated levels of anti-GM1 antibodies. While these antibodies are a strong indicator of the condition, their absence does not rule out an MMN diagnosis. This combination of clinical findings, electrophysiological evidence, and antibody testing helps neurologists confirm the diagnosis.
The Autoimmune Origin
MMN is an autoimmune disorder where the immune system mistakenly attacks the peripheral motor nerves. The attack is directed at the myelin sheath, the protective layer covering nerve fibers, or at related structures. These structures, near the nodes of Ranvier, are gaps in the myelin that facilitate rapid nerve signal conduction.
This immune assault on the myelin disrupts the nerve’s ability to transmit electrical signals, causing a conduction block. The body produces autoantibodies that are believed to play a direct role in this damage. By compromising the motor nerves, the immune system’s actions result in the muscle weakness and other motor symptoms of MMN.
Primary Treatment Options
The primary treatment for MMN is Intravenous Immunoglobulin (IVIg) therapy. IVIg is a blood product from the plasma of healthy donors that contains a concentrated mix of antibodies. It is thought to work by modulating the patient’s immune response and inhibiting the attack on motor nerves. Around 80% of patients show a positive response to this treatment.
Treatment begins with an initial series of infusions, followed by regular maintenance infusions to sustain the benefits. The frequency of these maintenance doses varies among individuals, ranging from weekly to every few months, depending on their response.
Treatments used for other autoimmune neuropathies are not effective for MMN. Corticosteroids like prednisone and plasma exchange (plasmapheresis) provide no benefit and can sometimes worsen symptoms. For this reason, IVIg remains the frontline treatment.
Living with MMN: Prognosis and Management
The long-term outlook for individuals with MMN is favorable, as the condition is not fatal and does not reduce life expectancy. MMN is a chronic and slowly progressive disease with no cure. However, available treatment can improve muscle strength and slow the disease’s advancement.
With consistent IVIg therapy, many people with MMN manage their symptoms and maintain a high level of function. However, some disability can develop over time. Persistent weakness in the hands or feet may require adaptive equipment like canes or splints to maintain independence.
Physical and occupational therapy are also part of a management plan. These therapies help individuals learn strategies to compensate for muscle weakness and improve mobility. This helps them find new ways to perform tasks, preserving function and quality of life.