Retrobulbar and peribulbar regional techniques in cats: A preliminary study in cadavers

Objective: To compare injectate distribution and potential complications of retrobulbar and peribulbar injections in cat cadavers. Study design: Prospective randomized masked study. Animals: Ten cat cadavers (20 eyes). Methods: A dorsomedial retrobulbar injection (RB) of 1 mL of 0.5% bupivacaine and iopamidol (1:1) was performed in seven eyes. A dorsomedial peribulbar injection (PB-1) of 4 mL of the same injectate was performed in seven eyes, and two peribulbar injections (PB-2) of the same injectate, divided equally between the dorsomedial and ventrolateral regions (2 mL each) were performed in six eyes. Intraocular pressure (IOP) was measured before, immediately and 15 minutes after injection. Cadavers underwent computed tomography before and following injections. A radiologist scored injectate distribution within the intraconal space (none, moderate, or large) and around the optic nerve (degrees). An injection was defined as likely to provide adequate regional anesthesia if the volume of distribution of intraconal injectate was 'large' and it contacted over 270° of the optic nerve circumference. Results: The success rate (95% confidence interval) of RB, PB-1, and PB-2 injections was 71% (29.0-96.3%), 86% (42.1-99.6%), and 67% (22.3-95.7%), respectively. With all three techniques, IOP increased significantly after injection, but returned to baseline by 15 minutes following RB injection. No intraocular, intravascular, intrathecal, or intraneural injectate was observed. Conclusion and clinical relevance: The single-peribulbar injection technique may be superior to retrobulbar or double-peribulbar injections, however, all techniques require further studies in live cats to determine safety and efficacy prior to clinical use.