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Assessing Focal Nerve Lesions Using Evoked Nerve Signals
Shaojun Xiao, PhD; Kevin C McGill, PhD; Vincent R Hentz, MD
Objective - Physicians treating peripheral nerve injuries need objective methods for assessing the extent of the injury and the progress of recovery. A practical assessment method should be similar to conventional non-invasive techniques for testing nerve conduction and should accurately characterize the state of the injury. Such a method would help the physician make decisions regarding the treatment of the nerve, and would also allow objective evaluations of new nerve-repair techniques.
Approach - We have developed a method for assessing nerve injuries that involves electrically stimulating the nerve near the site of the injury and recording the evoked nerve signals (compound action potentials or CAPs) from a section of healthy nerve proximal to the injury (Figure 1). The stimulation and recording are done with skin-surface electrodes. We assume that for each individual nerve fiber, conduction at the lesion site is either delayed - with the length of the delay reflecting the sickness of the fiber - or else blocked altogether. We attempt to estimate the range of delays (distribution of added delays or DAD) and the fraction of non- conducting fibers by comparing the signals evoked by stimulating on the near and far sides of the injury. The analysis is complicated by the fact that the individual nerve fibers exhibit a range of conduction velocities related to their range of sizes. We try to estimate the fraction of fibers having each conduction velocity (the distribution of conduction velocities or DCV) from the recorded signal, using a mathematical model of the signals generated by fibers of different conduction velocities. While previous models have generally assumed that the limb is flat or cylindrical and that the nerve travels in a straight line parallel to the surface, our model can handle the more complicated geometries that occur in actual nerves and limbs.
Status - We have used the DAD method to help evaluate different experimental nerve repair techniques in monkeys. The DAD results correlated well with conventional invasive electrophysiological methods of nerve integrity as well as with histologic estimates of nerve health. We are now ready to apply the method to human subjects.
Figure 1. Nerve lesion assessment method. Compound action potentials
are evoked by stimulating proximal and distal to the lesion. These are analyzed
to estimate the distribution of conduction velocities (DCV) and the
distribution of the added delays (DAD).
Republished from the 1994 Rehabilitation R&D Center Progress Report. For
current information about this project, contact
Kevin McGill.
