Relative neural toxicity of local anesthetics

In rat sciatic nerve, relative neural toxicity and relative motor nerve conduction blockade were assessed for two amide-linked local anesthetics (etidocaine and lidocaine) and two ester-linked local anesthetics (chloroprocaine and procaine). As measures of neural toxicity, nerve fiber injury and edema were assayed by light microscopic examination of nerve tissue sampled 2 days after perineural (next to the sciatic nerve) injection of various concentrations of the local anesthetics. Both nerve injury and edema increased with concentration of local anesthetics, but injury was frequently present in nerve fascicles with little or no edema. In parallel studies, the amplitude of the electrical activity elicited from the interosseous muscles of the foot following ipsilateral electrical stimulation at the sciatic notch was monitored for up to 15 minutes to assess the extent of motor nerve blockade. The resulting log concentration-response curves were analyzed for differences in potency. Both for injury and for conduction block, the order of decreasing potency was: etidocaine, lidocaine, chloroprocaine, procaine. These results are not consistent with the proposal that ester-linked agents are more likely than other local anesthetic agents to cause nerve injury.