Multifocal Motor Neuropathy (MMN) is a rare condition in which multiple motor nerves are attacked by one’s immune system. This causes weakness without loss of sensation. The specific nature of the attack is unique and perplexing, since motor and sensory fibers are intermingled within the nerve trunks of the arms and legs, but only the motor nerves become involved.
To understand MMN it is important to understand how nerves transmit impulses. Nerve fibers are similar to electric wires, in that they are composed of axons (the wires) and myelin (the insulation). Unlike wires, however, the myelin is spaced along the nerves with gaps called Nodes of Ranvier. These nodes sit between segments of nerve that are wrapped with myelin. Nerves propagate electrical current, called action potentials by “jumping” from one Node of Ranvier to the next. This remarkable arrangement allows microscopic axons to transmit impulses at very high speeds; in adult humans, at rates of 50 meters/second.
A defining feature of MMN is conduction block; Conduction block is when the impulse fails to travel all the way down the nerve. There are many reasons for this but the exact one responsible in MMN is unknown. The encouraging aspect of conduction block is that it is potentially reversible, restoring function without any significant long-term effects.
In MMN, it is thought, although not proven, that antibodies directed against a constituent of the Node of Ranvier (GM 1 ganglioside) cause the sodium channels at the nodes to stop functioning.
Although conduction block is reversible, continued attacks on the nodes can cause destruction of the myelin and axons. These different mechanisms explain why some people with MMN respond to therapy very quickly, while others may develop prolonged or permanent weakness and shrinking of muscles from damage to the nerve (atrophy). As such, the goals of any treatment are to prevent the axonal degeneration and reverse the conduction block.
The diagnosis of MMN depends on demonstrating that a patient has a purely motor disorder affecting individual nerves, that there are no UMN signs, that there are no sensory deficits, and that there is evidence of conduc¬tion block. These criteria are designed to differentiate the disorder from ALS (purely motor but with UMN signs), the Lewis-Sumner Syndrome variant of Chronic Inflammatory Demyelinating Polyneuropathy (CIDP) (similar to MMN but usually with significant sensory loss), and “vasculitis” (a type of multiple mononeuropathy syndrome caused by inflammatory damage to the blood vessels in nerves that also causes sensory and motor symptoms).
A neurologist is usually needed to determine the diagnosis, which is based on the history and physical examination along with the electrodiagnostic study, which includes nerve conduction studies (NCS) and needle electromyography (EMG).
The NCS usually demonstrate conduction block. This can be done by showing that the nerve signal cannot conduct past a “lesion” at some point along the nerve. For example, if the nerve is blocked in the forearm, an electrical impulse can easily get from the wrist to the hand if the stimulus is placed at the wrist. However, the signal will be blocked from reaching the hand if the stimulus is applied at the elbow. In MMN, sensory conduction along the same path should be normal. The EMG portion of the test looks for signals in the way muscles fire. In MMN it will most likely reveal abnormalities suggesting that some percentage of the motor axons have been damaged.
Laboratory testing for GM1 antibodies is frequently done and can be very helpful if they are abnormal. However, since only a third of patients with MMN have these antibodies, a negative test does not rule out the disorder. Spinal fluid examination is not usually helpful.
For more information, visit the Neuropathy Action Foundation: www.neuropathyaction.org