MMN

MMN with Conduction Block (MMNCB): This is the classic form where there is significant conduction block on nerve testing (nerve conduction studies), indicating areas where nerve impulses are not properly transmitted.

MMN without Conduction Block: Some patients may present with symptoms of MMN but without the detectable conduction block on electrodiagnostic studies. These cases can be more challenging to diagnose.

The exact cause of MMN is not fully understood. However, evidence suggests that MMN occurs when the immune system mistakenly attacks parts of the nervous system, making it an immune-mediated neurological condition. It is believed that the immune attack in MMN occurs in the small gaps (nodes) between sections of myelin sheath, which provides insulation around motor nerves to ensure adequate function. MMN does not appear to be hereditary, and no specific genes have been linked to the disorder.

Multifocal motor neuropathy (MMN) typically progresses slowly, causing asymmetric limb weakness. Patients often first notice weakness in their hands, leading to difficulties such as dropping objects or trouble turning a key. In some patients, the weakness can start or then evolve to include lower extremity muscles. This weakness corresponds to specific nerve territories. Unlike many other neurological conditions, MMN rarely involves numbness, tingling, or pain. However, patients may experience muscle twitching (fasciculations) and muscle atrophy.

While some patients may eventually need a cane or brace, they usually do not become wheelchair-dependent and can continue working unless the job is physically demanding. Muscles used for swallowing, speech, and chewing are not typically affected.

The diagnosis of multifocal motor neuropathy (MMN) involves several steps, including clinical evaluation, electrodiagnostic studies, and laboratory tests. The clinical evaluation should ideally consist of a complete neuromuscular review of the patient’s medical history and a physical examination focusing on detecting muscle weakness, atrophy, and fasciculations.

To confirm a diagnosis, physicians may undertake the following tests:

Nerve conduction studies. This test, sometimes called an electromyogram, uses low electrical currents to assess nerve function and detect myelin damage in peripheral nerves. It is used to measure the speed and strength of electrical signals as they travel through the nerves.

Electromyography to assess the electrical activity of the muscles.

Blood tests, testing for serum IgM anti-ganglioside (anti-GM1). These antibodies are present in approximately half of MMN patients.

MRI and ultrasound. Contrast MRIs and peripheral nerve ultrasound can be used to detect nerve root enhancement or swelling, however, are not conventional tests and do not always consistently show abnormalities in MMN.

Lumbar Puncture. In this procedure, a small needle is inserted into the back to withdraw a sample of cerebrospinal fluid (CSF), the fluid surrounding the spinal cord. This test helps detect elevated levels of specific proteins related to MMN, which can occasionally occur.

Because of its overlapping symptoms, MMN is challenging to diagnose and can sometimes be misdiagnosed as amyotrophic lateral sclerosis (ALS) and variants, demyelinating polyradiculoneuropathy (CIDP), Hereditary Motor Neuropathy, or peripheral neuropathy due to diabetes or other medical conditions.

Many MMN patients show short- and long-term improvement with immunoglobulin (Ig) therapy, administered either intravenously (IVIg) or subcutaneously (SCIg), making it the best first-line treatment. Most patients require repeated doses to maintain improvement, though gradual deterioration over years is possible.

Other therapies, such as corticosteroids, are less effective and can worsen symptoms in some MMN patients. New immunotherapies are currently being investigated for treatment of MMN.

Physical and occupational therapy can help maintain daily activities through exercise and the use of aids such as hooks for buttoning shirts. 

MMN is considered a treatable condition with a good prognosis. It typically does not shorten an individual’s lifespan, but quality of life can be compromised due to symptoms like poor dexterity, which affects daily activities.

Physical and occupational therapy are beneficial for maintaining function through targeted exercises and the use of adaptive aids. Patients should be cautious about intense physical exercise as it can lead to increased muscle pain and decreased muscle function if not properly monitored.

Feelings of sadness or depression are common among those with physical disabilities. If a patient experiences these emotions, their healthcare provider may recommend seeing a mental health professional. Antidepressants, psychotherapy, and support groups for people with chronic health conditions can be beneficial.

To manage the stress associated with MMN, it is recommended that patients consider these suggestions:

•     Maintain a strong support system of friends and family
•     Contact a support group or organization
•     Discuss feelings and concerns with a counsellor