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Purpose: Erector spinae plane (ESP) block is an interfascial blockade used in different clinical scenarios. This study investigated the ventral extent of dye diffusion in ESP block. Methods: The ultrasound-guided ESP block was bilaterally performed with an injection at the T5 vertebral level (21-Gauge, 50 mm needle), using diluted black tissue marking dye (20 mL; 1:4 ratio with standard saline solution) instead of local anesthetic on two fresh-frozen corpses within the body donation program of the University of Padova. Subsequently, the gross anatomical dissection was performed by a combined posterior plus anterior approach, and the histotopographic examination completed. Results: Macroscopically by gross anatomical dissection, the dye spreading ranged on the dorsal side of the chest from T2/3 to T10/11 with an extension up to 10 cm laterally, and on the ventral side of the chest from T2/3–T9/10. Microscopically by histotopographic examination, the dye diffused ventrally to the intercostal spaces (2–3 and 5–6 spaces on the right and left, respectively) by following the blood vessels coupled to the dorsal nerve passing through the costotransverse foramen. Conclusions: The anterior pathway of dye diffusion from the site of injection within the erector spinae muscle group during an ESP block seems to follow the blood vessels and dorsal rami of spinal nerves, suggesting the passing through the costotransverse foramen to reach the anterior paravertebral space and the intercostal nerves. These findings display an anterior histotopographic diffusion of dye resembling a paravertebral block. [ABSTRACT FROM AUTHOR]

Erector Spinae Plane (ESP) block is an interfascial blockade used daily in several clinical scenarios, with variable efficacy and reproducibility [1]. This study investigated the anatomical target of ESP block and its pathway of diffusion. Experienced anesthetists bilaterally performed the ultrasound-guided ESP block with an injection at the T5 vertebral level [2]. Diluted black tissue marking dye (20 mL) was injected in two fresh frozen bodies within the Body Donation Program of the University of Padova [3]. The gross anatomical dissection followed through a combined posterior plus anterior sequential approach. The histotopographic with morphometric examination completed the investigation. By gross anatomical dissection, the dye spreading macroscopically was observed on the dorsal side of the chest from T2/3 to T10/11 with an extension up to 10 cm laterally, and on the ventral side of the chest from T2/3 to T9 10. By histotopographic examination, the dye diffused ventrally to the intercostal nerves spaces (2-3 and 5-6 nerves spaces on the right and left, respectively) by following the blood vessels coupled to the dorsal nerve passing through the costotransverse foramen. The anterior dye diffusion from the injection site within the erector spinae muscles group during an ESP block reached the intercostal nerves. The dye followed the blood vessels and dorsal branches of spinal nerves, suggesting the passing through the costotransverse foramen to reach the anterior paravertebral space and the intercostal nerves, somehow resembling a paravertebral block. The authors express their deepest gratitude to the people adhering to the Body Donation Program 'Donation to Science' of the University of Padova, thanks to whom this research was possible. [ABSTRACT FROM AUTHOR]