Multifocal Motor Neuropathy (MMN) is a rare condition that causes weakness without significant loss of sensation. MMN (Multifocal Motor Neuropathy) is a rare disorder in which focal areas of multiple motor nerves are attacked by one's own immune system. Multifocal motor neuropathy (MMN) disease can cause a weakness of the arms and legs. Multifocal motor neuropathy (MMN) is a disease that affects your body's motor nerves. MMN is rare and believed to be an autoimmune disorder.