Nerve conduction studies in polyneuropathy: practical physiology and patterns of abnormality

Nerve conduction studies are an essential part of the work-up of peripheral neuropathies. Many neuropathic syndromes can be suspected on clinical grounds, but optimal use of nerve conduction study techniques (in combination with needle electromyography) allows diagnostic classification and is therefore crucial to understanding and separation of neuropathies. Multifocal motor neuropathy, for example, may clinically present as ALS. Detection of evidence of demyelination (conduction blocks) leads to the correct diagnosis and to proper treatment. Nerve conduction studies provide essential information on (1) the spatial pattern of neuropathy, (2) the pattern of abnormalities distinguishing between primarily axonal and demyelinating pathology, and (3) the severity of neuropathic damage. This information is very comprehensive since many nerves and long segments of individual nerves can be sampled. Moreover the information is extremely detailed to the extent that the cellular pathology of a patient's neuropathy is usually defined best by physiological testing rather than by biopsy. Neuropathies can be generalized, focal, or multifocal; they can be symmetric or asymmetric; they can be distally predominant or proximal and distal. Primarily axonal neuropathies mainly affect sensory nerve and compound muscle action potential amplitudes, whereas demyelinating neuropathies lead to slowing of nerve conductions, and to increased temporal dispersion or conduction block. Usually, the pattern of demyelination allows to distinguish hereditary (uniform demyelination) from acquired (segmental demyelination) neuropathies. Electrodiagnostic criteria for primary demyelination are helpful to identify acquired demyelinating neuropathies.