Ultrasound-guided thoracic erector spinae plane injections in cat cadavers: assessment of two injectate volumes using dissection and computed tomography.

OBJECTIVE: To compare the distribution of two injectate volumes following thoracic erector spinae plane (ESP) injections in cat cadavers, using dissection and computed tomography (CT). STUDY DESIGN: Prospective, randomized, experimental cadaveric study. ANIMALS: One formalin-preserved and 13 thawed cat cadavers. METHODS: Ultrasound-guided ESP injections were performed at the fifth thoracic vertebra in thawed cat cadavers using high (HV, 0.5 mL kg-1) and low (LV, 0.3 mL kg-1) volumes, one per side. A total of 10 cadavers underwent dissection after methylene blue injection to assess staining of nerves and adjacent spaces. Three underwent CT following iohexol injections to evaluate craniocaudal spread within the epaxial musculature across vertebral segments, craniocaudal spread within the ESP, and involvement of adjacent structures. RESULTS: Craniocaudal injectate spread was similar between groups. HV and LV dye injections stained, respectively, a median (range) of 4 (2-6) and 3.5 (1-5) medial branches (p = 0.757), and 3 (2-4) and 2 (1-3) lateral branches (p = 0.013) of the dorsal branches of the spinal nerves (DBSN). HV dye injections stained the intercostal nerve (2/10 injections), intercostal space (3/10), and sympathetic trunk (1/10). LV stained the intercostal nerve (1/10), intercostal space (3/10), and sympathetic trunk (2/10). CT confirmed epaxial distribution over 10 (8-11) and 10 (5-10) vertebral segments for HV and LV, while spread specifically within the ESP was confined to 5 (5-6) and 5 (4-6) vertebral segments, respectively. Contrast reached the intervertebral foramina (3/3 HV, 2/3 LV), paravertebral and intercostal spaces (2/3 HV, 1/3 LV), and epidural space (1/3 in each group). CT demonstrated continuity between these structures. CONCLUSIONS AND CLINICAL RELEVANCE: Thoracic ESP injections in cats consistently reached DBSN branches, with unpredictable spread to ventral branches and adjacent structures. Both volumes achieved similar craniocaudal spread, but the higher volume increased staining of the lateral branches of the DBSN.