Effects of Intrathecal Saline and Ketamine with and Without Preservative on the Spinal Nerve Roots of Monkeys

Ketamine, a cataleptic analgesic substance which theoretically may block pain receptors in the spinal cord, providing pain relief without respiratory depression, was injected intrathecally with or without benzethonium chloride 0.1 mg/ml (a preservative) into 8 monkeys under anaesthesia. Two monkeys received saline (control group). No assessment as to relief from experimental pain was possible. All Monkeys recovered normally from anaesthesia and were found moving about in their enclosures without gross evidence of neurological impairment. No adverse reactions were noted. Ten days after the subarachnoid injection the monkeys are sacrificed. Autopsy was performed within 30 minutes. No macroscopic abnormality of the cord was noticed. Microscopic examination revealed oedema of a few nerve roots in all animals, irrespective of whether ketamine or saline was injected intrathecally. Focal degeneration with loss of myelin and axoplasm was observed within a solitary nerve root in 2 monkeys that had received ketamine, 1 with and 1 without preservative. However, in these animals lumbar puncture proved difficult and bloody taps ensued. Hence trauma could have been a contributing factor in these 2 cases. None of the other monkeys showed these changes, regardless of group.