Histopathologic changes in primate spinal cord after single and repeated epidural phenol administration

Epidural phenol for control of pain and spasticity has been advocated for clinical use. This study determined the histopathologic changes that follow single and repeated epidural administration of phenol in saline in nonhuman primates.

Nine primates received 0.5 mL of either 3% phenol in saline (n = 4) or 6% phenol in saline (n = 5) via lumbar epidural injection. Two additional primates received three consecutive daily epidural doses of 0.5 mL of 3% phenol in saline. Finally, 5 unoperated primates and 5 primates that received only 2 mL of radiographic contrast material served as control subjects. Two weeks after the epidural injection, spinal cords were removed and processed for histopathologic study by a neuropathologist blinded to the solution administered.

None of the control animals demonstrated histopathologic changes. One animal that received 6% phenol died 3 days after injection. All phenol-treated animals demonstrated predominantly posterior root damage. Spinal cord damage was seen in all animals receiving 6% phenol, in 2 animals receiving 3% phenol single doses, and in neither animal receiving 3% phenol multiple doses. Anterior root damage occurred in all phenol-treated animals except the 4 that received single 3% phenol injections. Animals that received 6% phenol demonstrated greater lower extremity motor weakness than those in the other groups, but no clear correlation existed between extent of histopathologic changes and motor weakness.

Motor weakness, anterior root damage, and direct cord injury were noted in primates following epidural administration of phenol in concentrations below what has been reported for clinical use in humans. Since it is more difficult to control the spread of epidural versus subarachnoid phenol, the risks of epidural phenol may outweigh the benefits relative to subarachnoid administration.