Dose-response effects of corneal anesthetics

With double-masking procedures, the dose-response curves for 0.1, 0.2, and 0.4% benoxinate and 0.125, 0.25, and 0.50% proparacaine hydrochloride were determined by monitoring changes in corneal touch threshold after applying each anesthetic. The level of corneal anesthesia necessary for applanation tonometry was also determined. The maximum increase in threshold that could be measured following instillation of 50 microliter of the drug was 200 mg/mm2 All 6 anesthetic solutions produced this amount of decreased corneal sensitivity. Recovery from the anesthetic was exponential for all concentrations; however, the lower doses had the shortest duration. For applanation tonometry, the corneal threshold for touch must be 75 mg/mm2 or higher. We conclude that a quarter to a half of the commonly used anesthetic dose is sufficient for routine tonometric evaluation.