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

Abstract

BACKGROUND: Equine corneal disease is common and painful. Current pain mitigation strategies are often suboptimal, especially early in the disease. OBJECTIVES: 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. STUDY DESIGN: Randomised, controlled crossover experiment. METHODS: One eye was randomly selected from seven horses. A 20-gauge catheter was placed in the retrobulbar space and injected with 10&#x2009;mL of 0.5% bupivacaine or 0.9% saline. Cochet-Bonnet esthesiometry (CBE), pupil photogrammetry, PLRs, and oculocephalic reflexes were evaluated before injection (t&#x2009;=&#x2009;0) and t&#x2009;=&#x2009;0.25, 1, 3, 6, 9 and 12&#x2009;h after injection. Following a 7-14&#x2009;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. RESULTS: Saline did not affect CBE, PA, PLRs or oculocephalic reflexes at any time point. Bupivacaine reduced CTT compared with saline for 6&#x2009;h, with maximum reduction at t&#x2009;=&#x2009;1&#x2009;h (bupivacaine: 0.08&#x2009;&#xb1;&#x2009;0.20&#x2009;cm; saline: 4.21&#x2009;&#xb1;&#x2009;0.39&#x2009;cm; p&#x2009;<&#x2009;0.001). Bupivacaine increased PA compared with saline for 3&#x2009;h, with maximum increase at t&#x2009;=&#x2009;3&#x2009;h (bupivacaine: 349.75&#x2009;&#xb1;&#x2009;84.80&#x2009;mm; saline: 194.65&#x2009;&#xb1;&#x2009;27.03&#x2009;mm; p&#x2009;=&#x2009;0.04). Bupivacaine injection reduced PLRs for 9&#x2009;h and oculocephalic reflexes for 3&#x2009;h; saline had no effect. Mild adverse effects included chemosis, blepharoedema, and transiently reduced palpebral reflex. MAIN LIMITATIONS: Reduction in CTT may not directly translate to pain control in clinical horses with corneal disease. Adverse effects of repeated injections were not evaluated. CONCLUSIONS: 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.