Multifocal motor neuropathy with conduction block (MMN) is a rare, immune-mediated, demyelinating motor neuropathy. In some cases, it is associated with anti-GM1 antibodies. Patients present with subacute onset of asymmetric weakness in the distribution of single nerves without associated sensory loss. Weakness typically begins as a focal mononeuropathy such as a wrist drop. Other symptoms include fasciculations, muscle atrophy, and decreased deep tendon reflexes. MMN has a predilection for upper limb involvement that may resemble amyotrophic lateral sclerosis (ALS). Patients with MMN do not have sensory abnormalities, bulbar symptoms, or upper motor neuron findings on examination.

Diagnosis requires progressive, focal, asymmetric limb weakness in the distribution of two nerves for more than one month and absence of sensory abnormalities. Treatment with intravenous immunoglobulin (IVIG) is recommended for all patients as it can improve weakness. Without treatment, weakness and motor symptoms will continue to progress resulting in increasing disability.

Disease typically presents before the sixth decade, with an average age of onset of 40 years (range 20-70 years); the condition very rarely presents during childhood. It has a 3:1 male predominance.