Comparison of efficacy and duration of topical anesthetics on corneal sensitivity in clinically normal horses

Abstract

Objective- The purpose was to compare the efficacy and duration of 0.5% proparacaine, 0.5% bupivacaine, 2% lidocaine, and 2% mepivacaine on corneal sensitivity in clinically normal horses. Animals- 68 clinically normal horses Procedures- In group 1, 60 horses from the Kansas State University horse unit were assigned to receive one topical anesthetic in a completely randomized design. In group 2, 8 privately owned horses were sequentially treated with each of the topical anesthetics in random order with a one week washout period between drugs. Corneal sensitivity was assessed by corneal touch threshold (CTT) measurements which were taken with a Cochet-Bonnet aesthesiometer before anesthetic application (T0), 1 minute after (T1), every 5 minutes until 60 minutes (T5-T60), and then every 10 minutes until 90 minutes (T70-T90) after application. General linear mixed models were fitted to CTT in each design in order to assess the effects of topical anesthetics over time, accounting for repeated observations within individual horses. Results- Corneal sensitivity, as determined by CTT measurements, decreased immediately following application of the topical anesthetic, with persisting effects until T35 for proparacaine and mepivacaine, T45 for lidocaine, and T60 for bupivacaine. Maximal CTT reduction was achieved following application of bupivacaine or proparacaine, while mepivacaine was least effective. Conclusions and Clinical Relevance- All topical anesthetics reduced corneal sensitivity, though maximal anesthesia and effect of duration differed between drugs. For brief corneal anesthesia, 0.5% proparacaine or 2% lidocaine appeared adequate, while 0.5% bupivacaine may be most appropriate for procedures requiring longer periods of corneal anesthesia.