Dexmedetomidine Prolongs the Duration of Ropivacaine Corneal Anesthesia After Subconjunctival Injection in Healthy Horses.

Abstract

OBJECTIVE: To determine whether dexmedetomidine prolongs corneal anesthesia following subconjunctival ropivacaine injection in healthy equine eyes. METHODS: Ten horses with normal ophthalmic exams were enrolled in a randomized, blinded, crossover experiment with a minimum 4-week washout between treatments. Baseline corneal touch thresholds (CTT, cm) were measured via esthesiometry and a subconjunctival injection was performed in the treatment eye with ropivacaine (R, 0.3&#x2009;mL 0.5%) or dexmedetomidine-ropivacaine (DR, 0.1&#x2009;mL 0.5&#x2009;mg/mL&#x2009;+&#x2009;0.2&#x2009;mL 0.5%) and saline (control, 0.3&#x2009;mL) in the contralateral eye. CTT was measured every 10&#x2009;min up to 120&#x2009;min, every 30&#x2009;min up to 360&#x2009;min, and every 60&#x2009;min thereafter until return to baseline. Ambient humidity was concurrently recorded. Subconjunctival hemorrhage was scored 0, 2, and 7&#x2009;days after injection. CTT values were converted to corneal pressure tolerance (CPT, g/mm) using humidity-adjusted exponential regression. Data were compared between treatments using mixed model analysis (significance p&#x2009;<&#x2009;0.05). RESULTS: Mean&#x2009;&#xb1;&#x2009;standard deviation total anesthesia time was significantly longer with DR (196&#x2009;&#xb1;&#x2009;121 vs. 103&#x2009;&#xb1;&#x2009;47&#x2009;min R, p&#x2009;=&#x2009;0.02). Evidence of significant differences between treatments was not seen for minimum CTT, duration of minimum CTT, maximal CPT, or hemorrhage scores. However, time to minimum CTT was longer for DR (32&#x2009;&#xb1;&#x2009;12 vs. 16&#x2009;&#xb1;&#x2009;10&#x2009;min R, p&#x2009;<&#x2009;0.001). Subconjunctival hemorrhage resolved within 7&#x2009;days of injection. CONCLUSIONS: Dexmedetomidine prolongs corneal anesthesia from subconjunctival ropivacaine in horses but delays the onset. Further studies are needed to assess efficacy in diseased eyes.