Chemical neurolysis can be an extremely effective intervention for reducing spasticity. Phenol nerve block as a method of chemical neurolysis has been used over 40 years, nevertheless, many clinicians remain reluctant to perform this procedure. This is largely due to the fear of its side effects and complications such as excessive muscle weakness or increased spasticity of antagonistic muscles.

The purpose of this study was to titrate the nerve block effects of phenol with different concentrations and different volumes of the phenol solution. Left tibial nerves of forty eight adult rabbits were injected with phenol solution of different concentrations(5%, 4%, 3%) and volumes(0.3 ml, 0.2 ml, 0.1 ml) into the epineural sheath. Nerve conduction study of the gastrocnemius muscle was performed before and after the nerve blocks(1day, 1week, 2weeks, 4weeks, 8weeks). The proportion of compound muscle action potential(CMAP) amplitudes and areas before and after the nerve blocks was used for the evaluation of nerve block effect.

There was a statistically significant difference in the proportion among three volumes of phenol solution(p＜0.05). However there was no statistical difference in the proportion among three concentrations of phenol solution(p＞0.05) although the higher concentration of phenol solution showed the tendency for smaller proportion. The area of histological degeneration appeared at 2 weeks following nerve block alongside the perineurium. The depth of degeneration area of nerve fascicle varied according to the distance from injection point.

These data suggest that the nerve block effect of phenol can be titrated more easily with the volume of phenol solution rather than the concentration until 8 weeks after the nerve block. The variation of the extent of degeneration with different volumes of phenol solution seems to be the mechanism for the titration.