External oblique intercostal block: an anatomical study in soft- embalmed Thiel cadavers

Background The external oblique intercostal (EOI) block has been reported to provide analgesia for the anterior and lateral upper abdominal walls. However, the anatomical distribution of the injectate after the EOI block has not been well established. This pilot cadaveric study aimed to evaluate the spread of dye following an ultrasound-guided EOI block. Methods Ultrasound-guided EOI injections (n=8) were performed bilaterally on four soft-embalmed Thiel cadavers. Thirty milliliters of water-soluble dye was injected into the plane beneath the external oblique muscle at the sixth intercostal space between the midclavicular and anterior axillary lines. The cadavers were subsequently dissected to assess the dye spread. Results In all injections, the spread of the dye into the interspace between the rectus abdominis and the costal cartilage was restricted at the transformation of the external oblique fascia into the aponeurosis and at the costal arch. Consequently, the anterior branches of the thoracoabdominal nerves were not stained. The anteroposterior spread was consistent between the midclavicular and midaxillary lines, but anterior spread beyond the midclavicular line was observed in only 25% of the dissections. Conclusions In this cadaveric study, the EOI block demonstrated limited anterior spread, and the anterior branches of the thoracoabdominal nerves were not involved. These findings suggest that the EOI block may not reliably provide midline sensory blockade and may have limited utility for mid-abdominal analgesia. The implications of these cadaveric findings warrant further validation in clinical studies.