What is multifocal motor neuropathy (MMN), and how does it differ from ALS?

MMN is a very rare form of immunoneuropathy associated with paralysis (paresis) of isolated muscle groups (e.g., the finger extensor or foot flexor muscles). MMN is caused by an immunological disorder in which the immune system mistakenly identifies the body’s own nerve tracts as “foreign” and attacks them (an autoimmune disease). This results in damage to individual nerves and weakness in the muscles innervated by these motor nerves.

The disease usually begins in an isolated location (“focal”) and then spreads to multiple nerves over the course of several months (“multifocal”). Unlike ALS, MMN progresses very slowly and remains limited to specific muscle groups. There is no impairment of swallowing or respiratory function. Electroneurography can reveal typical nerve changes in some patients. In isolated cases, ultrasound of the peripheral nerves may also reveal specific nerve swelling. The symptoms of MMN (particularly muscle weakness) respond to treatment with immunoglobulins. Following infusion therapy, symptoms typically stabilize or improve. This is another difference from ALS, in which there is no long-term response to immunotherapy. It is possible to distinguish ALS from MMN because ALS has a higher rate of progression and is usually accompanied by symptoms of the upper motor neuron (increased reflexes or spasticity), which never occur in MMN.