It is generally assumed that clinical concentrations of local anesthetics are not harmful to the tissues into which they are injected; however, neurologic injury has recently been reported following spinal anesthesia in the clinic and injection near peripheral nerves in experimental research. This proposal will further investigate the neurotoxicity of local anesthetics in the peripheral nervous system in which the perineurial barrier system, nerve fibers, vessels, and supporting cells are interdependent and subject to pathologic change. Having shown that local anesthetics alter perineurial permeability, induce edema and nerve fiber injury as well as functional disturbances, we now propose to investigate the mechanisms of nerve fiber injury using a combination of new techniques for research into the nerve microenvironment combined with classic methods such as electrophysiology, autoradiography, electron microscopy, and morphometry. We will evaluate the effects of both the anesthetics and a common adjunct, epinephrine, on nerve blood flow and the subsequent changes in anesthetic distribution as well as a hypothesized role of ischemia in the potentiation of nerve injury. Additonally, we aim to determine the role of anesthetic- mediated increased perineurial permeability on the facilitation of anesthetic action and subsequent toxicity. Local anesthesia and the early toxic effects will be distinguished from more slowly developing electrophysiological deficits resulting from complications such as perineurial thickening, endoneurial fibrosis, and demyelination due to Schwann cell injury. We will also be using new techniques to test the role of endoneurial fluid changes in precipitating nerve injury, the hypothesis of Fink that nerve conduction might be decreased indirectly by a hypotonic environment, and the hypothesis that endoneurial fluid hypotonicity is a consequence of altered perineurial permeability. Finally, we plan to test metabolic, osmotic, vascular, and hypertensive treatments which might protect nerve fibers of facilitate recovery following injury by maintaining the endoneurial environment. These studies will help to define the boundaries of local anesthetic-induced injury to peripheral nerve as well as possible therapies for the treatment of those injuries.
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