Is the erector spinae plane ( ESP ) block a sheath block?

screen and enhanced contrast, the block performers stopped injecting when a hypo-echoic volume was seen clearly on the screen. We also compared needle penetration with the needle tip position in, or outside of, the nerve. Histologically, the risk of ink contamination from one site to another clearly exists and was observed outside the epineurium. Our pathologists made multiple sections of the nerve and could not observe any sub-epineurial contamination. We used fresh cadavers and this could be different with embalmed specimens. Our study does not suggest that targeting the nerve tangentially, and rolling the nerve as the needle passes above or below its ill-defined border is preferable to slowly skewering it. Instead, our study suggests that it is preferable to approach the nerve at its lower/upper border (tangential) and to stop advancing the needle when this border is reached. However, if an accidental advancement ‘beyond the border’ occurs, the risk of nerve penetration and sub-epineurial injection is lower than using a direct approach. As such, we agree with Szerb and Kwesi Kwofie that the border of the nerve should be avoided in all procedures. Szerb et al.’s study and ours complement each other in improving the safety of interscalene blocks.