Multifocal motor neuropathy (MMN) was distinguished from other motor neuropathies in 1986 and has since become a well defined condition due to its distinctive clinical presentation and response to treatment, as well as its characteristic laboratory and electrophysiologic features. It is characterized by slowly progressive, predominantly distal, asymmetric limb weakness and wasting, predominantly in the arms, as well as muscle cramps and fasciculations within an anatomical distribution of individual motor nerves, with minimal or no sensory involvement."

Source : Current Opinion in Neurology 18(5) pp 567-573,

The cause of MMN is currently unknown, though it's thought very likely to be an auto-immune disorder. Patients often test positive for antibodies directed against GM-1 - an important naturally-occurring lipid molecule present on normal nerve-fibres' myelin sheaths.

Notes :

2) MMN is often mistaken for Amyotrophic Lateral Sclerosis (ALS). also known as Motor Neurone diseaseplugin-autotooltip__plain plugin-autotooltip_bigMotor Neurone disease

Motor Neurone disease, also known as Amyotrophic lateral sclerosis (ALS) and Lou Gehrig's disease, is a rare, progressive, neurological condition that involves degeneration of the nerve cells (neurons) responsible for controlling voluntary muscle movement.

2) Unlike Motor Neurone Disease, MMN is not ultimately fatal, and does not decrease life expectancy.