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Background and Aims Paravertebral block (PVB) achieves anesthesia by injecting local anesthetic beneath the superior costotransverse ligament (SCTL), targeting the anterior ramus of the spinal nerve. Although PVB typically affects multiple thoracic levels, the mechanism underlying this spread remains controversial. It is often attributed to a ‘paravertebral space,’ yet no anatomically distinct space has been definitively described. This study aimed to determine the actual anatomical route responsible for local anesthetic spread during PVB.Methods Two embalmed cadavers were dissected through the internal thoracic wall. At the 7th intercostal space, the neurovascular bundle was identified between the anterior and posterior components of the SCTL. After careful removal of the intercostal artery and vein, the spinal nerve located beneath the superior rib was exposed. Methylene blue dye was injected using a paravertebral approach at the 6th intercostal space. The spread of the dye was subsequently evaluated by dissection, focusing on its course through the intervertebral foramen and adjacent structures.Results In both cadavers, the dye extended from the medial aspect of the spinal ganglion toward the spinal nerve, indicating spread from the epidural space. In the first specimen, surrounding connective and adipose tissue were removed to reveal the spinal ganglion and rami communicantes. Dye distribution filled the intercostal groove from medial to distal. In the second specimen, the posterior SCTL was broader, and the anterior component again covered the ganglion; the dye followed a similar path.Abstract P167 Figure 1Dissection to anterior of SCTL, spinal ganglion and nerve inspect from anterior chest wallAbstract P167 Figure 2After dye injectionConclusions Although anatomically there is no true paravertebral space, this study demonstrated that multi-segmental—and occasionally contralateral—spread may occur via lateral extension of the epidural space beneath the SCTL. The presumed wedge-shaped paravertebral space appears not to be cranio-caudal but instead represents segmental projections of the epidural space. Larger cadaveric studies are warranted to validate these findings.