Intraneural or extraneural: diagnostic accuracy of ultrasound assessment for localizing low-volume injection.

Background And Objectives: When one is performing ultrasound-guided peripheral nerve blocks, it is common to inject a small amount of fluid to confirm correct placement of the needle tip. If an intraneural needle tip position is detected, the needle can then be repositioned to prevent injection of a large amount of local anesthetic into the nerve. However, it is unknown if anesthesiologists can accurately discriminate intraneural and extraneural injection of small volumes. Therefore, this study was conducted to determine the diagnostic accuracy of ultrasound assessment using a criterion standard and to compare experts and novices in ultrasound-guided regional anesthesia.

Methods: A total of 32 ultrasound-guided infragluteal sciatic nerve blocks were performed on 21 cadaver legs. The injections were targeted to be intraneural (n = 18) or extraneural (n = 14), and 0.5 mL of methylene blue 1% was injected. Cryosections of the nerve and surrounding tissue were assessed by a blinded investigator as "extraneural" or "intraneural." Ultrasound video clips of the injections were reviewed by 10 blinded observers (5 experts, 5 novices) independently who scored each injection as either "intraneural," "extraneural," or "undetermined."

Results: The mean sensitivity of experts and novices was measured to be 0.84 (0.80-0.88) and 0.65 (0.60-0.71), respectively (P = 0.006), whereas mean specificity was 0.97 (0.94-0.98) and 0.98 (0.96-0.99) (P = 0.53).

Conclusions: Discrimination of intraneural or extraneural needle tip position based on an injection of 0.5mL is possible, but even experts missed 1 of 6 intraneural injections. In novices, the sensitivity of assessment was significantly lower, highlighting the need for focused education.