Histomorphometric differences between perineural and intraneural application of 0.5% ropivacaine in Wistar rats

Abstract

Background: Ropivacaine is a long-acting amide local anaesthetic agent and first produced as a pure enantiomer. Ropivacaine is thought to be one of the safest local anesthetics however, there have been several studies detailing possible neurotoxic effects. At present, the exact molecular mechanism of ropivacaine-mediated neurotoxicity is unknown. We postulated that intraneural injection of 0,5% ropivacaine results in greater nerve injury and this would be proved by objective quantitative histological analysis.

Methods: We used quantitative histomorphometric analyses to evaluate the neurotoxic effect of 0.5% ropivacaine after intraneural and perineural injection of sciatic nerve in Wistar rats. 30 Wistar rats were studied. After general anesthesia, the sciatic nerves (n=60) were exposed bilaterally. Under direct vision, a 27-gauge needle was placed perineurally (n=30) or intraneurally (n=30) and 4 ml of 0.5% ropivacaine was injected using automated pump at speed 3 ml/min. Manometer wes used to measure injection pressure. Three days after application, animals were euthanized and nerves samples were taken for histomorphometric analysis.

Results: All intraneural injections showed significantly smaller number of nerve fibers (p<0,001), smaller nerve fiber (p<0,002) and axon diameter (p<0,001) and myelin thickness (p<0,002) compared with perineurally administarated 0.5% ropivacaine. Maximum achieved injection pressures were 20,3 psi (intraneural) vs 6,7 psi (perineural).

Conclusion: Intraneural injection of 0,5% ropivacaine resulted in an indistiguishable quantitative histologic changes. Intraneural injection is associated with significantly higer injection pressure as compared to the perineural injection. Ropivacaine administered intrafascicularly is neurotoxic, similar to other local anesthetics.