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Mechanism of Differential Epidural Block

Received from the Department of Anesthesiology, University of Washington, and Cancer Pain Research Center, Fred Hutchinson Cancer Research Institute, Seattle, WA 98195.

Abstract

Pain block in obstetric epidural anesthesia is usually accompanied by relatively persistent retention of motor power; however, the reason for this is not entirely clear. The differential effect has been ascribed to a relatively high resistance of large fibers to conduction block. However, recent results from individual axons are incompatible with that explanation and have prompted the suggestion that greater vascularity in ventral than in dorsal roots might account for the phenomenon. To test this hypothesis, transverse sections of five sets of human dorsal and ventral C-8, T-7, and L-5 roots were examined by an immunohistochemical method that is specific for factor VIII and stains the vascular endothelium. A naive tally of the stained vessels failed to reveal any excess vascularity of the ventral roots, and the hypothesis was therefore rejected. However, it was noted at autopsy that the thin dural sleeve surrounding the roots, where epidural anesthetic is most likely to penetrate to the nerve fibers, may measure less than 5 mm from the dural sac to the intervertebral foramen, a distance probably too short to contain three internodes of motor fibers, the minimum length of fiber necessary for block by local anesthetic. This anatomy might help explain the clinical differential.

Key Words: ANATOMY—spinal nerve root, vascularity • ANESTHETIC TECHNIQUES, EPIDURAL—vascularity of nerve roots

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999