• peripheral nerve injuries
• regional anesthesia
• diagnostic techniques and procedures
• nerve block

Dear editor

We read with great interest the technical report from Mejia et al 1 as they provide interesting insights regarding the use of opening injection pressure (OIP) to detect intraneural injection during peripheral nerve block. The authors conclude that in-line pressure monitors may not prevent intraneural injection based on an isolated cadaveric nerve models (tibial and common peroneal nerves).

We agree with the authors as OIP alone does not seem to discriminate between intraneural and extraneural needle tip position since, based on their observations, 15 PSI is not an absolute threshold for a safe OIP. Consequently, what arises from this report is a legitimate doubt regarding the usefulness of OIP as a parameter to be monitored in a framework of a safer peripheral nerve block. However, we fear that these results could mislead the reader to underestimate the importance of OIP monitoring.

As previously demonstrated by Krol et al, not all nerves share the same OIP values for intraneural injections.2 Peripheral nerves, such as distal median nerve or tibial nerve, are rich in adipose and connective tissue. This allows a short bevel needle to penetrate the nerve but not the fascicles and OIP is generally lower. Intraneural injections in more dense structures, such as brachial plexus trunks, are deemed more dangerous and generate higher pressures.2 For this reason, we believe that the Mejia experience could not give the same results if the isolated nerve used was a brachial plexus trunk, that is, during an interscalene block execution. Furthermore, knowledge about safety of peripheral nerve blockades is constantly evolving, either from an anatomical or methodological point of view.3

Although OIP may not detect an extrafascicular tip position inside the nerve, it is very sensitive in detecting both a needle nerve contact and an intrafascicular tip position, which unavoidably leads to nerve damage.4 On the other hand, although intraneural/extrafascicular injections appear safe, a subclinical injury always occurs.5 Hence, we think that OIP should be considered in the context of a multimodal monitoring which includes ultrasound guidance and nerve stimulation, aiming to limit unintentional intraneural injections, as already suggested.6

In conclusion, we congratulate the authors for pointing out an OIP limitation. It should still be used as part of a ‘safety pack’ for regional anesthesia, looking forward for further studies confirming the efficacy of this practice.