The toxic effects of intravitreal injection of netilmicin on the rabbit retina at different concentrations were studied with transmission electron microscope.
One tenth milliliter of solution containing 50, 100, 200, or 1,000 pig of netilmicin as experimental group and 0.1 ml of balanced salt solution as control group was injected into the rabbit vitreous cavity through pars plana respectively. The eyeballs of each concentration were enucleated at 1, 3, 5, 7 and 14 days after injection for observation under transmission electron microscope.
One hundred micrograms of netilmicin or below produced mild changes in the retinal pigment epithelium (RPE). Mild swelling of the mitochondria, irregularity in the basal infolding, and the presence of a few residual bodies were observed in the basal infolding and the presence of a few residual bodies were observed in the RPE. Those changes are probably considered reversible.
Following injection of 200 g of netilmicin more advanced changes were observed in the retinal" pigment epithelial and photoreceptor cell layer. The changes were as follows: numerous vacuoles and residual bodies in the RPE, prominent basal infolding and irregularity of the Bruch´s membrane and fragmentation of the inner and outer segments of the photoreceptor cells. Although those changes were stationary, occasional loss of the retinal pigment epithelium was observed at 14th day after injection. With the increased dose of netilmicin up to 1,000 g, severe destructive change took place in the RPE and outer layer of the sensory retina.
Retinal degeneration produced by toxic dose of intravitreal netilmicin may be localized primarily in the RPE and safe dosage in the rabbit retina may be considered 100 g.
According to the above experimental result, it is concluded that the safe dosage of netilmicin into the human vitreous would be considered under 300 g.
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