Background Injection techniques for retrobulbar anaesthesia are published in horses, but neither safety nor anaesthetic efficacy and duration have been evaluated objectively in vivo. Objective To characterise the safety and efficacy of one published technique for retrobulbar anaesthesia. Study design Randomised, controlled descriptive experiment. Methods Unilateral retrobulbar injection with 10 mL lidocaine (2%) was performed in eight sedated adult mares. Contralateral eyes served as untreated controls. Neurophthalmic parameters, intraocular pressure (IOP), and corneal and periocular sensation were measured awake, post‐sedation and at periodic time points for 24 hours following injection. Adverse effects were documented. Results Injection of 10 mL lidocaine significantly increased IOP for up to 2 hours (P < .05) maximally at 30 min (mean [95% CI]: 6.0 [2.7, 9.2] mm Hg, P < .001). Six of the eight treated eyes developed mild to moderate reversible chemosis for 2 to 24 hours. One eye developed severe chemosis and superficial corneal ulceration at 24 and 48 hours following injection respectively. Corneal sensitivity significantly decreased for 6 hours (P < .05), maximally at 10 min (−44.4 [−34.6, −54.1] mm, P < .001). Periocular sensitivity (measured as increase in applied force) significantly decreased dorsally and medially for up to 2 hours (maximal at 2 hours (367.1 [238.5, 495.7] g, P < .001, and at 30 min: 345.8 [202.6, 488.9] g, P < .001) respectively). Ventral and lateral sensitivity were not effectively decreased beyond 30 min. Optic nerve function was not consistently reduced following injection. Main limitations Investigators were not masked to the treated eye. Conclusions Retrobulbar injection using 10 mL lidocaine is safe in normal eyes of adult horses, but carries risk in structurally compromised or glaucomatous eyes due to transient IOP increase. Reversible chemosis commonly develops 2‐4h following injection, and may be severe in some horses with risk for corneal ulceration. Corneal anaesthesia is rapid and prolonged, but all periocular regions are not consistently anaesthetised. Retrobulbar injection should be combined with other local anaesthetic injections for eyelid surgeries or enucleations.

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