Mechanism of differential epidural block.
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.
D000328:Adult; D000767:Anesthesia, Epidural; D001808:Blood Vessels / Q000033:anatomy & histology / Q000648:ultrastructure; D002196:Capillaries / Q000033:anatomy & histology; D006801:Humans; D008854:Microscopy, Electron; D013126:Spinal Nerve Roots / Q000098:blood supply*