Neuropathies associated with conduction block

Two disorders, Lewis-Sumner syndrome and multifocal motor neuropathy, are defined by the demonstration of conduction block. These two entities have been intertwined since their description but there are important distinctions between them. It is therefore timely to reconsider these disorders and the physiologic process that defines them.

Understanding of the pathophysiology of conduction block has evolved with better understanding of the structure of the node of Ranvier and surrounding apparatus. Axonal excitability studies have begun to give insights into why multifocal motor neuropathy has only motor conduction block. The many published criteria for conduction block vary in sensitivity and specificity and the electromyographer must be aware of the difficulties in determining physiologic block. The distinctions between Lewis-Sumner syndrome and multifocal motor neuropathy have become increasingly clear. Evidence suggests that Lewis-Sumner syndrome is a multifocal variant of chronic inflammatory demyelinating polyneuropathy but that multifocal motor neuropathy is distinct.

It is important to recognize the distinctions between these disorders, not only because there are important therapeutic issues, but also because a true understanding of the nature of these illnesses will only be accomplished if the overlaps and differences are carefully considered.