Retinal toxicity of gentamicin after subconjunctival injection performed adjacent to thinned sclera

To investigate the potential toxicity to the retina of gentamicin injected near surgically thinned scleral areas in a rabbit model.

Experimental study.

Scleral scraping to half thickness was performed in the superotemporal scleral area in both eyes of adult rabbits (n = 10). Gentamicin sulfate was injected subconjunctivally to the right eye and saline to the left eye, which always served as a control eye. Four weeks after the procedure, electroretinography (ERG) was performed to assess retinal function. Then, the eyes were enucleated and prepared for histologic evaluation of structural damage. In four eyes of two additional rabbits, vitreous gentamicin concentrations were measured using a fluorescence polarization assay.

Dark- and light-adapted ERG responses and histopathologic damage.

Dark- and light-adapted ERG responses in all rabbits were similar in the experimental and control eyes. Gentamicin levels were more than 10 microg/ml after subconjunctival injection of gentamicin with scraping and 0.29 microg/ml after subconjunctival injection of gentamicin with no scraping. Histopathologic examination revealed significant local damage to the photoreceptors adjacent to the area of scraping and subconjunctival injection. A significantly lesser degree of damage was seen if gentamicin was injected in pigmented rabbits or in albino rabbits, but only 4 weeks after scleral scraping.

Increased penetration of gentamicin through thinned sclera may lead to toxic levels of the drug in a localized area adjacent to the site of injection. These toxic effects are also influenced by the degree of pigmentation and acute inflammation.