Effects of topical application of tramadol with/without dexmedetomidine and proparacaine on corneal sensitivity in rats.

Purpose: To evaluate the corneal anesthetic effect following topical application of tramadol alone and in combination with dexmedetomidine, and compare it to proparacaine, in clinically healthy rats.

Methods: A randomized, crossover study was performed. Twenty Wistar albino rats (n = 40 eyes) were used. Corneal touch threshold (CTT) measurements (in mm) were obtained using a Cochet-Bonnet aesthesiometer. CTT measurements were obtained at baseline, 1-min following application of the topical anesthetic agent, and repeated at 5-min intervals up to 75 min. The topical protocol involved 3 treatment conditions, separated by a 2-week washout period: proparacaine, tramadol alone, and tramadol in combination with dexmedetomidine.

Results: CTT values were significantly decreased compared to baseline at each timepoint until completion of the 75-min evaluation in all treated eyes, regardless of the assigned treatment (p < 0.0083). With tramadol, complete corneal anesthesia (CTT = 0) was achieved within 1-5 min in 18 eyes and ranged from 5 to 25 min. Co-administration of dexmedetomidine to tramadol resulted in significantly increased CTT values from 5 to 20 min following topical application, compared to tramadol alone (p < 0.0083), and complete corneal anesthesia was achieved in only 14 out of 20 treated eyes.

Conclusion: Tramadol might be a useful alternative to topical anesthetic agents, providing a dose-related corneal anesthetic effect. Co-administration of dexmedetomidine does not potentiate its anesthetic effect. The underlying mechanism(s) of drug antagonism between tramadol and dexmedetomidine remains to be determined.