NEUROLOGICAL PROGRESS Principles and Pitfalls of Nerve Conduction Studies Jun IGmura, MD ~ ~ ~ This report reviews the fundamental principles and the changing concepts of nerve stimulation techniques, and discusses the proper application of these techniques in the differential diagnosis of peripheral nerve disorders. Nerve conduction stlldies help delineate the extent and distribution of the neural lesion and distinguish two major categories of peripheral nerve disease: demyelination and axonal degeneration. Although the method is based on simple princi- ples, pitfalls abound in practice. Variability in nerve conduction measurement may result from temperature change, variations among nerve seg- ments, and the effects of age. Other sources of error include excessive spread of stimulation current, anomalous innervation, temporal dispersion, and inaccuracy of surface measurement. Unlike a bipolar derivation, which selec- tively records near-field potentials, a referential recording may give rise to stationary far-field peaks from a moving source. Overlooking this possibility can lead to an incorrect interpretation of findings. Conventional nerve conduction studies deal primarily with measurements of the distal nerve segments in an extrem- ity. More recent techniques are applicable to less accessible anatomical regions, as illustrated by elicitation of the blink reflex, F wave, and H reflex, and the use of the inching technique. Other methods used to assess special aspects of nerve conduction include the ischemic test and studies of slow-conducting fibers. lmura J. Principles and pitfalls of nerve conduction studies. Ann Neurol 16:415-429, 1984 Nerve conduction studies ate useful in evaluating dis- eases of peripheral nerves. With steady improvement and standardization of methods, they have become a reliable test in clinical settings [33, 551. They are now widely used not only for precise localization of a lesion, but also for accurate characterization of peripheral nerve function [ 14,481. The technique consists of elec- trical stimulation of a nerve and recording of the evoked potentials either from the muscle or from the nerve itself. Although the methods are relatively sim- ple, various technical factors influence the measure- ments and a number of pitfalls can lead to a wrorlg or misleading conclusion [73}. Recognition of the inher- ent limitations of the method can minimize such errors. For quick reference, Table 1 lists the concepts and procedures reviewed with a brief caption summarizing the key points for each heading. Types of Neuropathic Abnormality Seddon [68} defined three degrees of nerve injury: neutapraxia, axonotmesis, and neurotmesis. In neura- praxia conduction ceases without structural change in the axon. Fibers usually regain function promptly within days or weeks, although nerve block from acute entrapment may occasionally last for as long as several years C621. In compressive lesions demyelination may accompany neurapraxia, and remyelination must occur before conduction returns to normal f3 1, 321. Axonot- mesis results from loss of axonal continuity, leading to wallerian degeneration of the distal segment. The nerve fibers regenerate slowly at a rate of 1 to 3 mm per day, often leading to eventual recovery of function. In neurotmesis injury separates the entire nerve, in- cluding the connective elements. With supporting tis- sue lost, regeneration is poorly organized and incom- plete r751. The electrophysiological abnormalities depend on the kind and degree of damage in individual nerve fibers within the nerve. Although different types of abnormality can coexist, the results of conduction stud- ies usually correlate well with the overall structural ab- normalities {30, 33, SS}. In segmental dernyelination, or during partial remyelination, thin myelin increases the internodal capacitance and conductance, leading to loss or diminution of local current. Failure to acti- vate the next node of Ranvier results in conduction block. When function returns, impulses propagate more slowly than normal, because it takes longer for the dissipated current to generate an action poten- tial. Thus, demyelinated axons characteristically show blocking of impulses, substantial decreases in conduc- tion velocity (commonly, although not always, to less From the Division of Clinical Electrophysiology, Department of Neurology, University of Iowa Hospitals and Clinics, Iowa City, 1A 52242. Received December 30, 1983, and in revised form March 20, 1084. Accepted for publication March 23, 1084. Address reprint requests to Dr Kimura. 41 5Table 1. Concepts and Procedares Discussed Concepts and Procedures Comments Types of neuropathic abnormal- ity Variability in nerve conduction Common sources of error Near- vs. far-field potential Newer techniques To assess anatomical regions not otherwise accessible To assess other aspects of conduction Neurapraxia: functional block is characterized by reversible loss of Conduction across the site of lesion; demyelination results in slowing of conduction velocity over the affected segment on return of function, with relative sparing in amplitude of the evoked potential distally Axonotmesis: axonal degeneration causes a reduction in amplitude of the evoked response distally, proportionate to the number of lost axons Neurotmesis: complete separation of a nerve leads to total loss of evoked response distally Temperature: velocity decreases 5% per 1"C, requiring an adjustment if skin temperature falls below 34°C Nerve segments: nerves conduct faster over proximal than over distal segments, in the arms than in the legs, and in shorter than in taller subjects Age: velocity is half the adult value at birth, in the adult range in 3-5 yr, and slightly less after 30 to 40 yr of age, although the decrease is less than 10 mls by age 60 to 80 yr Spread of stimulus current: inadvertent activation of neighboring nerves elicits unintended potentials Anomalies: Martin-Gruber anastomosis provides communication from median to ulnar nerve in the forearm; accessory deep peroneal nerve supplies the lateral half of the extensor digitorum brevis Temporal dispersion: changes in amplitude and area affect the nerve action potential more than the muscle response unless the conduction velocity is quite slow