Selected Antimicrobial Activity of Topical Ophthalmic Anesthetics

Endophthalmitis is a rare complication of intravitreal injection (IVI). It is recommended that povidone-iodine be the last agent applied before IVI. Patients have reported povidone-iodine application to be the most bothersome part of IVIs. Topical anesthetics have been demonstrated to have antibacterial effects. This study compared the minimum inhibitory concentration (MIC) of topical anesthetic eye drops (proparacaine 0.5%, tetracaine 0.5%, lidocaine 2.0%) and the antiseptic, 5.0% povidone-iodine, against two organisms causing endophthalmitis after IVI.

Minimum inhibitory concentration values of topical anesthetics, povidone-iodine, preservative benzalkonium chloride (0.01%), and saline control were determined using five isolates of each Staphylococcus epidermidis and viridans group Streptococcus species (VGS). A broth microdilution technique was used with serial dilutions.

Lidocaine (8.53 × 10^-5mol/mL) had MICs of 4.27 to 8.53 × 10^-5 mol/mL, and tetracaine (1.89 × 10^-5 mol/mL) had MICs of 9.45 × 10^-6 mol/mL for all isolates. Proparacaine (1.7 × 10^-5 mol/mL) had MICs of 1.32 to 5.3 × 10^-7 and 4.25 × 10^-6 mol/mL for S. epidermidis and VGS, respectively). Benzalkonium chloride (3.52 × 10^-7 mol/mL) had MICs of 1.86 × 10^-9 to 1.1 × 10^-8 and 4.40 × 10^-8 mol/mL for S. epidermidis and VGS, respectively. Povidone-iodine (1.37 × 10^-4 mol/mL) had MICs of 2.14 to 4.28 × 10^-6 and 8.56 × 10^-6 mol/mL for S. epidermidis and VGS, respectively.

Proparacaine was the anesthetic with the lowest MICs, lower than that of povidone-iodine. Benzalkonium chloride had lower MICs than proparacaine. All tested anesthetics and povidone-iodine inhibited growth of S. epidermidis and VGS at commercially available concentrations.

For certain patients, it could be possible to use topical anesthetic after povidone-iodine for comfort without inhibiting and perhaps contributing additional antimicrobial benefit.