• dog
• loco-regional anesthesia
• retrobulbar block anesthesia (RBA)
• ultrasound-guided technique

Objective: the aim of this study was to describe an ultrasound-guided approach for the retrobulbar block anesthesia (RBA) in the dog, identifying a new distinctive ultrasound image that allow to visualize the needle in-plane during the entire procedure.
Material and Methods: The study was divided in two phases. Phase 1: an ultrasound image has been sought on a dolichocephalic dog skull, with a water immersion ultrasound study, suitable for the visualization of specific landmarks in order to allow the development of an ultrasound-guided retrobulbar loco-regional anesthesia technique with an in-plane approach. Once the ultrasound image corresponding to the bone profile of the orbital fissure was identified, the landmarks were used to develop the technique on cadavers in the next phase. Phase 2: 11 cadavers of dogs of different breeds and weights were used in order to assess the applicability of the proposed technique. A microconvex ultrasound probe was used to search for the correct ultrasound window. Once the retrobulbar cone and the orbital fissure were identified, an injection with an echogenic needle was performed with an in-plane technique, using a volume of 0.03 ml/kg of methylene blue solution 2%. Following the injection, an anatomical dissection of the area subjected to the injection was performed in order to evaluate the spread of methylene blue obtained and verify its absence in the brain.
Results: The first phase has allowed to identify the orbital fissure and to obtain the visualization of the entire body of the needle and its tip with an in-plane positioning. The second phase led to the correct identification of an ultrasound image showing the orbital fissure and the retrobulbar orbital cone on the cadavers. This ultrasound image allowed the execution of a satisfactory injection of the dye solution with a in-plane approach of the needle in 18/21 (86%) of the total injections. Anatomical dissections confirmed the absence of spread of the dye in the brain in all cases.
Conclusions: The study demonstrated the applicability, on cadavers, of a new ultrasound-guided retrobulbar injection technique with an in-plane approach which allows to increase the safety and precision of performing retrobulbar injections through a single injection site with a volume of 0.03 ml/kg of solution.