Multifocal motor neuropathy (MMN) is a progressively worsening condition where muscles in the extremities gradually weaken. The disorder, a pure motor neuropathy syndrome, is sometimes mistaken for amyotrophic lateral sclerosis (ALS) because of the similarity in the clinical picture, especially if muscle fasciculations are present. MMN is thought to be autoimmune.
It was first described in the mid-1980s. Unlike ALS which affects both upper and lower motor nerves, MMN involves only lower motor nerves. Nevertheless, definitive diagnosis is often difficult, and many MMN patients labor for months or years under an ALS diagnosis before finally getting a determination of MMN. MMN usually involves very little pain; however, muscle cramps, spasms and twitches can cause pain for some sufferers. MMN is not fatal and does not diminish life expectancy.
Many patients, once undergoing treatment, only experience mild symptoms over prolonged periods, though the condition remains slowly progressive. MMN can however, lead to significant disability, with loss of function in hands affecting ability to work and perform everyday tasks, and “foot drop” leading to inability to stand and walk; some patients end up using aids like canes, splints and walkers.