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

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.

The sciatic nerves of 80 rats were treated either with intra- or perineural injections of 7% phenol-aqua, anhydrous glycerol, or 5% phenol-glycerol. Lidocaine and saline injections were used as controls. Muscle function and trophic changes of the hind limbs were evaluated, and samples for morphologic analysis were taken 1, 2, 4, and 8 weeks after the injections.

Intra- and perineural injections of 7% phenol-aqua resulted in gross endoneural damage of the sciatic nerve and hind limb paresis. Perineural 5% phenol-glycerol and anhydrous glycerol injections caused subperineural damage with slight paresis; gross endoneural damage and noticeable paresis were present only after intraneural injections. When 7% phenol-aqua was compared to other neurolytic agents, the differences in the lesion size (P < .0001) were statistically significant after perineural injections. Regeneration occurred in a stereotypic fashion in all neurolytic groups. Axonal sprouts were noted at the injured area 2 weeks after intraneural and 1 week after perineural injections. Motor function had partially recovered at 8 weeks.

There were no differences in the effects of clinically used neurolytic agents after intraneural injections. Although the perineurally applied 7% phenol-aqua induced marked endoneural damage, the destructive effect of glycerol and phenol-glycerol injections seemed to be prevented by the perineurium; phenol-glycerol and glycerol treatments induced subperineural damage only after perineural injections. The ability to penetrate the perineurium favors the use of 7% phenol-aqua in peripheral perineural blocks when complete neurolysis is the goal.