Background: Ultrasonographic (US) images of apparent intraneural injection of local anesthetic solutions have been reported. We aimed to define US signs of intraneural (ie, subepineural) injection using a histologic standard in an animal model and compare these signs with other potential markers of intraneural injection, including low nerve stimulation current thresholds and high injection pressures.

Methods: In 6 anesthetized adult swine, bilateral brachial plexus and femoral nerves were contacted by needles and penetrated. India ink was injected intraneurally under US monitoring. The minimum current that elicited a motor response was recorded. Injection pressures were measured using a digital manometer. Nerves were then excised, processed, and subjected to histologic analysis.

Results: Nerve expansion during injection was visualized under ultrasonography in all procedures. Electrical current intensity to elicit motor response to nerve stimulation varied between 0.2 and 3.3 mA with the needle tip positioned intraneurally. The mean injection pressure was 7.40 ± 8.07 psi (range, 0.07-31.5 psi), with 80% of injections between 0.61 and 15.0 psi. None of 24 intraneural injections resulted in histologic evidence of intrafascicular injection (95% confidence interval, 0.0%-16.3%).

Conclusions: Ultrasonographic images compatible with nerve swelling during an injection are consistent with true intraneural injections as demonstrated by histologic analysis. Under the conditions studied, intensity of the stimulating current required to elicit motor response was not associated with intraneural needle placement. In the absence of fascicular injury, intraneural injections were associated with low injection pressure, although false-positive results can occur.