Multifocal motor neuropathy (MMN): a rare form of nerve damage (neuropathy) characterized by progressive and asymmetric muscle weakness and wasting (atrophy). The cause of MMN is poorly understood, but it may be related to an abnormal immune response. Signs and symptoms of MMN include weakness in the hands and lower arms; muscle cramping; involuntary muscle contractions or twitching; and wasting of the involved muscles. The leg muscles may also be affected in some people. Also known as Multifocal Motor Neuropathy with Conduction Block (MMNCB).