Quantitative analysis of intravitreal injections in the rat

Intravitreal injections are currently used in the rat to introduce a therap eutic factor in the eye, especially for experimental treatments of retinal degenerations. The injected volume and its location can influence the quant ification of results. We have investigated the quantitative effect of a sin gle intravitreal injection in rats at different ages and for different volu mes. Albinos rats aged three weeks or two months received intravitreal inje ctions of 1, 3, 5 or 10 mul China ink. Animals were sacrificed immediately after injection, eyes were enucleated, fixated, embedded in paraffin and mi crotomy was performed in a sagittal plane. Regularly spaced sections were a nalyzed to reconstruct the vitreous and injected dye volumes. The measured vitreous volume was 6.76 +/- 0.37 mm(3) in three weeks old rats and 13.36 /- 0.64 mm(3) in two months old rats. Mean intravitreal ink volumes immedia tely after injection were 0.8 mm(3) for 1 mul injections, 2 mm(3) for 3 mul , 2.3 to 2.6 mm(3) for 5 mul and 3.2 mm 3 for 10 mul. The percentage of vit reous volume involved by the injection ranged from 4.4% to 33.2%. The injec ted volume is limited by the large lens size of the rat. Extraocular loss o f injected solution increases for higher injected volumes, with larger stan dard deviations. In this model, the dye tends to localize behind the lens. A 3 or 5 mul volume appears to have the best reproducibility with minimum l oss of solution.