There are similarities in clinical features and response to treatment between the two entities, but the purely motor nature of MMN (see Delmont et al), makes it useful to separate them. Multifocal motor neuropathy, but not multifocal conduction block, is associated in half or more of cases with a particular IgM antibody, anti-GM1, directed against a ganglioside component of peripheral myelin (Pestronk et al). For this reason, some view this illness as belonging to the class of paraproteinemic neuropathies (see earlier and Simmons et al) and it is distinctive enough clinically to be categorized separately. Its importance as a clinical entity lies in the similarity of the clinical picture to a purely lower motor neuron type of amyotrophic lateral sclerosis (ALS) and, unlike ALS, its potential responsiveness to treatment. The pathophysiologic role of anti-GM1 antibodies is displayed by a case of transplacental transmission of a motor neuropathy to a neonate (Attarian et al).
Multifocal motor neuropathy and motor conduction block predominate in men. They usually begin with an acute or subacute motor mononeuropathy, manifest, for example, as weakness of the wrist or foot-drop, and are often joined insidiously by another focal motor palsy. The process is painless, unlike vasculitic mononeuritis multiplex, involves the nerve incompletely, and, in its usual form, is unaccompanied by any sensory symptoms such as paresthesias or numbness. Despite the initially demyelinating character of the disorder, there is almost always atrophy of the weakened muscle within months and there may be a few fasciculations, thus simulating ALS. Nevertheless, the weakness tends to be disproportionate to atrophy. Usually, the tendon reflex is lost or muted in an affected region, but for unexplained reasons, some patients have one or more brisk reflexes. Our experience has been that this latter reflex change does not reach the point of appearing “pathologic” and that clonus and Babinski signs are categorically not part of the illness, as they are in ALS.
When there is an association of the motor features with sensory symptoms or sensory loss and there is slowing of sensory conduction in regions of motor conduction block (multifocal conduction block), the acronym MADSAM (multifocal acquired demyelinating sensory and motor neuropathy) has been used as noted earlier, but the disorder, while similar to multifocal conduction block, more resembles CIDP. This conforms to what had been called Lewis-Sumner syndrome (see Lewis et al 1982) based on the description by these authors and their colleagues of subacute, painless asymmetric, distal multiple mononeuropathies. The ulnar and median nerves were involved in their patients and there was motor conduction block and sensory slowing in affected nerves. Curiously, 2 of their 5 original patients had optic neuritis, a feature not reported subsequently. The disease is not directly connected to antibodies against GM1, but a few patients with the sensorimotor disorder will display them.
Treatment For multifocal motor conduction block and motor neuropathy, with or without anti-GM1 antibodies, IVIg infusions have been effective, albeit temporarily, in more than half of patients. Some authoritative clinicians favor the early addition of rituximab in treatment-resistant cases or when the frequency of infusions is unsustainable and if that fails, cyclophosphamide. Other immune-modulating drugs have been tried in small series with various results. There is no response to corticosteroids. The MADSAM illness responds similarly to corticosteroids, IVIg, or plasma exchange, similar to the effects of these approaches in CIDP.