Comparison of ultrasound-guided subzygomatic and supratemporal approaches for the retrobulbar nerve block in cats : a cadaveric tomographic study

Abstract

Ultrasound (US)-guided locoregional anaesthesia techniques are increasingly used in veterinary medicine. Their aim is to provide efficient analgesia by improving safety and accuracy of local anaesthetic injection thanks to needle and anatomical landmarks' visualization. Despite several recent publications, the most effective retrobulbar nerve block (RBA) technique has yet to be established for cats. Therefore, the aims of the present study were to evaluate the feasibility and safety as well as compare the injectate distribution of US-guided subzygomatic (SZ) and supratemporal (ST) approaches for RBA in cats. Since cross-sectional imaging was used as an assessment technique in this study, the normal feline orbital tomographic anatomy was first described. Seven feline cadavers were used in a preliminary investigation to determine external and ultrasonographic anatomical landmarks, body positioning and needle direction for the US-guided SZ and ST approaches. Subsequently, fourteen feline cadavers were included in a prospective randomized cadaveric study. Fourteen cadavers (28 orbits) were injected with 0.1 mL/kg of a 1:1 mixture of contrast product and saline solution using either the SZ (n =14) or ST (n = 14) approach. Both the SZ and ST techniques were performed under US-guidance using a 4-10 MHz microconvex probe. Contrast spread was assessed by computed tomography (CT), immediately after injections. Injectate localization (intraconal/extraconal), injectate spread within the intraconal compartment and globe (percentage), and intraconal perineural angular extension of the injectate spread around the optic nerve (degrees) were evaluated. The distance between the contrast medium and orbital fissure was also measured. Needle and ultrasonographic landmarks’ visualization were superior with the SZ approach. Injectate spread was predominantly intraconal [12/14 (86%) for SZ, 9/14 (64%) for ST], although one ST injection failed. Overall injectate diffusion within the intraconal space was limited for both approaches. Perineural injectate spread around the optic nerve was moderate-to-large (>180°) in 18/28 (64%) of the injections. No globe perforation or intrathecal injection was noted. The US-guided SZ and ST approaches are feasible in cadaveric cats and result in similar injectate distribution. The SZ approach allows for superior visualization of ultrasonographic landmarks. Further clinical studies are needed to assess both techniques in live animals.