THE ROLE OF GRAVITY IN GENTAMICIN-INDUCED TOXIC EFFECTS IN A RABBIT MODEL

Objective: To define the role of gravity in gentamicin sulfate-induced retinal toxic effects by injecting the drug into vitrectomized rabbit eyes oriented in one of two positions. Methods: Ten right eyes of New Zealand white rabbits underwent a two-port pars plana vitrectomy and were positioned with traction scleral sutures to rotate the medullary ray superiorly (five eyes) or nasally (five eyes). Six eyes received a pars plana intravitreal injection of 400 mu g (0.1 mL) of gentamicin, and four eyes received 0.1 mt of balanced salt solution only. All eyes were kept in position for 30 minutes after the injection. Unoperated-on fellow eyes served as controls. Three or 5 days later, the eyes were enucleated and histopathologic examination was performed. Results: Light and electron microscopy of gentamicin-treated eyes showed inner-retinal edema and disruption of the nerve-fiber and ganglion-cell layers within the more dependent retina vs sparing of the less dependent half of the retina. Eyes that received balanced salt solution showed rare inflammatory cells along the inner-retinal surface but essentially no inner-layer disruption. Examination of unoperated-on control eyes showed no inflammatory cells, edema, or necrosis. Conclusion: Gravitational effects and positioning contribute to the location of gentamicin-induced retinal toxic effects in vitrectomized eyes.