Background and Aims Inadvertent intraneural injection is not infrequent during peripheral nerve blocks. To this end, injection pressure monitoring is suggested as a safeguard method that warns of a potentially hazardous needle tip location. However, doubts remain if this method is superior to the sonographic nerve swelling in terms of earlier detection of the intraneural injection.

Methods A cadaveric study was designed to assess injection pressures during an ultrasound-guided intraneural injection of the median nerve. We hypothesized that the sonographic swelling occurred first than elevated injection pressures (>15 pound per square inch) using an in-line monitor. 33 injections of 11 median nerves from unembalmed human cadavers were performed at proximal, mid and distal locations. 1 ml of a mixture of local anesthetic and methylene blue was injected at 10 ml/min. Afterwards, dissection was performed to assess spread location. Videos of the procedures including ultrasound images were blindly analyzed to evaluate nerve swelling and injection pressures.

Results 31 injections were analyzed (2 were excluded due to uncertain needle tip location). >15 pound per square inch was attained in 6 injections (19%) following a mean volume of 0.7 ml. Nerve swelling was evident in all 31 injections (100%) with a mean volume of 0.4 ml. Upon dissection, spread was confirmed intraneural in all injections, with a proximal-distal longitudinal diffusion of an average 6 cm per injection.

Conclusions Ultrasound is a more sensitive and earlier indicator of the intraneural injection than injection pressure monitoring. Further research is required to consolidate the role of pressure monitors in the clinical setting.