Severe toxic damage to the rabbit spinal cord after intrathecal administration of preservative-free S(+)-ketamine

Background: Ketamine and S(+)-ketamine have been advocated for neuraxial use in the management of postoperative pain and severe intractable pain syndromes unresponsive to opioid escalation. Although clinical experience has accumulated with S(+)-ketamine, safety data on toxicity in the central nervous system after neuraxial administration of S(+)-ketamine are conflicting. In this study, neurologic and toxicologic effects on the spinal cord from repeated daily intrathecal administration of commercially available, preservative-free S(+)-ketamine were evaluated against placebo in a randomized, blinded design. Methods: Eighteen white New Zealand rabbits were assigned to two groups receiving either 0.5 ml intrathecal s(+)-ketamine, 0.5% solution (12 rabbits), or 0.5 ml saline (6 rabbits) once a day for 7 consecutive days. During general anesthesia, an intrathecal catheter was placed between the fifth and sixth spinous processes (lumbar). Neurologic (according to Tarlov criteria) and histopathologic assessments were performed after 7 days of treatment. Results: Postmortem investigation of the spinal cord and nerve roots revealed histopathologic lesions suggestive of toxic damage in 11 rabbits, from the group of 12 animals receiving s(+)-ketamine. These results were significantly different compared with 5 control animals (no histologic changes observed). There was no significant difference in neurologic status between the two groups after 7 days of intrathecal treatment. Conclusions: The authors conclude that repeated intrathecal administration of preservative-free s(+)-ketamine in a clinically relevant concentration and dosage has, considering the extent and severity of the lesions, a toxic effect on the central nervous system of rabbits.