Measurement of tissue impedance in dorsal root entry zone surgery for pain after brachial plexus avulsion and herpes zoster.

Fifteen patients with brachial plexus avulsion and five patients with postherpetic pain underwent dorsal root entry zone surgery with intraoperative impedance monitoring. The usual range of initial impedance values recorded in the superficial layers of the normal cord is from 1,000 to 1,500 omega. The recordings usually decrease to less than 1,000 omega in the segments of root avulsion injuries. Some variation in recordings occurs owing to atrophy and scarring of the damaged cord. In postherpetic neuralgia, measurements of impedance are abnormally low in the involved area, in which the roots appear macroscopically abnormal. In this study, tissue impedance was correlated with gross pathologic changes in the spinal cord.