The endoneurial response to neurolytic agents is highly dependent on the mode of application

Background and objectives. The variability and predictability of neurolytic neural blocks were studied using an experimental rat sciatic nerve model. The goal of the study was to compare endoneurial and clinical responses to commonly used neurolytic agents. Methods. The sciatic nerves of 80 rats wer e treated either with intra- or perineural injections of 7% phenol-aqua anh ydrous glycerol, or 5% phenol-glycerol. Lidocaine and saline injections wer e used as controls. Muscle function and trophic changes of the bind limbs w ere evaluated, and samples for morphologic analysis were taken 1, 2, 4, and 8 weeks after the injections. Results. Intra- and perineural injections of 7% phenol-aqua resulted in gross endoneural damage of the sciatic nerve an d bind limb paresis. Perineural 5% phenol-glycerol and anhydrous glycerol i njections caused subperineural damage with slight paresis; gross endoneural damage and noticeable paresis were present only after intraneural injectio ns. When 7% phenol-aqua was compared to other neurolytic agents, the differ ences in the lesion size (P < .0001) were statistically significant after p erineural injections; Regeneration occurred in a stereotypic fashion in all neurolytic groups. Axonal sprouts were noted at the injured area 2 weeks a fter intraneural and 1 week after perineural injections. Motor function had partially recovered at 8 weeks. Conclusion. There were no differences in t he effects of clinically used neurolytic agents after intraneural injection s. Although the perineurally applied 7% phenol;aqua induced marked endoneur al damage, the destructive effect of glycerol and phenol-glycerol injection s seemed to be prevented by the perineurium; phenol-glycerol and glycerol t reatments induced subperineural damage only after perineural injections. Th e ability to penetrate the perineurium favors the use of 7% phenol-aqua in peripheral perineural blacks when complete neurolysis is the goal.