Intraneural injection of a test dose of local anesthetic in peripheral nerves - does it induce histological changes in nerve tissue?

Background & Objectives: Most anesthesiologists use the injection of a test dose of local anesthetic in order to evaluate the final needle tip position. Thus, the intraneural injection of a full dose can be avoided. The aim of this study was to analyze whether an intraneural injection of a test dose of bupivacaine could trigger histological changes.

Methods: Intraneural injections under direct vision were performed in 40 brachial plexus nerves in seven anesthetized pigs. Tibial nerves served as positive and negative controls. Two milliliter of bupivacaine 0.5% was injected in three nerves on the left brachial plexus. For control of local anesthetic's toxicity Ringer's solution was applied intraneurally on the right side. After maintaining 48 h of general anesthesia, the nerves were resected. The specimens were processed for histological examination and assessed for inflammation (hematoxylin and eosin stain, CD68-immunohistochemistry) and myelin damage (Kluver-Barrera stain). The degree of nerve injury was rated on a scale from 0 (no injury) to 4 (severe injury).

Results: Statistical analysis showed no significant differences between the bupivacaine group [median (interquartile range) 1 (1-1.5)] and the Ringer's solution group [1 (0.5-2) P = 0.772]. Mild myelin alteration was found in 12.5% of all specimens following intraneural injection, irrespective of the applied substance.

Conclusions: "In our experimental study, intraneural injection of 2 ml of bupivacaine or Ringer's solution showed comparable mild inflammation. Nevertheless, inflammation can only be prevented by strictly avoiding nerve perforation followed by intraneural injection, as mechanical nerve perforation is a key factor for evolving inflammation.