Early ultrasonographic detection of low-volume intraneural injection.

Background: Intraneural injection of local anaesthetic agents carries a risk of neurological complications. Early detection of intraneural needle-tip position is very important in the initial phase of injection. Ultrasound (US) characteristics for real-time detection of intraneural injections have been described, but only for relatively large volumes (5-40 ml). This study assesses the reliability of various US criteria to detect early low volume (0.5 ml) intraneural injections. Intraneural deposition of an injected dye was confirmed by cryomicrotomy.

Methods: In nine unembalmed human cadavers, 0.5 ml methylene blue was injected intraneurally into the supraclavicular brachial plexus and subgluteal sciatic nerve on both sides. The sites of injection were subsequently removed en bloc. Consecutive cryomicrotomy cross-sections with a 50 µm interval were obtained to assess intraneural presence of the injectate. Two independent experts separately reviewed US video clips of the injections and scored each US criterion.

Results: Of the 36 injections, cryomicrotome cross-sections showed intraneural staining in 33 and extraneural staining in three. The best US criterion was expansion of the nerve cross-sectional surface area together with a change in echogenicity. It was observed in 35 injections, including two false positives. There was one true negative. Test precision was 94% [95% confidence interval (CI), 87-100%]. The mean increase in surface area was 8.7% (95% CI, 5.6-11.9).

Conclusions: Reliable detection of early low-volume intraneural injection using US is possible using expansion of the cross-sectional surface area of the nerve together with a change in echogenicity as markers.