Local anaesthetic delivery through indwelling retrobulbar catheters for ocular pain control in horses

Equine corneal disease is common and painful. Current pain mitigation strategies are often suboptimal, especially early in the disease. To evaluate the effects of local anaesthetic delivery via indwelling retrobulbar catheter on corneal sensitivity, pupil size, pupillary light responses (PLRs), and ocular motility in normal horses. Randomised, controlled crossover experiment. One eye was randomly selected from seven horses. A 20-gauge catheter was placed in the retrobulbar space and injected with 10 mL of 0.5% bupivacaine or 0.9% saline. Cochet-Bonnet esthesiometry (CBE), pupil photogrammetry, PLRs, and oculocephalic reflexes were evaluated before injection (t = 0) and t = 0.25, 1, 3, 6, 9 and 12 h after injection. Following a 7-14 day washout period, this procedure was repeated using the alternative injection solution. Corneal touch thresholds (CTTs) derived from CBE and pupillary areas (PA; as measured from photographs) were compared across time for each group. PLRs and oculocephalic reflexes were compared between groups at each evaluation time point. Saline did not affect CBE, PA, PLRs or oculocephalic reflexes at any time point. Bupivacaine reduced CTT compared with saline for 6 h, with maximum reduction at t = 1 h (bupivacaine: 0.08 ± 0.20 cm; saline: 4.21 ± 0.39 cm; p < 0.001). Bupivacaine increased PA compared with saline for 3 h, with maximum increase at t = 3 h (bupivacaine: 349.75 ± 84.80 mm2; saline: 194.65 ± 27.03 mm2; p = 0.04). Bupivacaine injection reduced PLRs for 9 h and oculocephalic reflexes for 3 h; saline had no effect. Mild adverse effects included chemosis, blepharoedema, and transiently reduced palpebral reflex. Reduction in CTT may not directly translate to pain control in clinical horses with corneal disease. Adverse effects of repeated injections were not evaluated. Injection of bupivacaine via an indwelling retrobulbar catheter in horses reduces corneal sensitivity and may be useful in reducing pain in horses with corneal disease.