OXYGEN-INDUCED RETINOPATHY IN THE NEWBORN RAT - EFFECTS OF HYPERBARISM AND TOPICAL ADMINISTRATION OF TIMOLOL MALEATE

Background: Lesions resembling those of human retinopathy of prematuri ty can be provoked in newborn Wistar rats by exposure to an FiO(2) of 80% for the first 5 days of life followed by 5 days recovery under roo m-air conditions. Methods: We evaluated the effects of moderate hyperb arism (+60.75 kPa, i.e. 455 mmHg or 0.6 atm) and topical administratio n of 0.25% timolol maleate on oxygen-induced retinopathy (OIR) in this experimental model. Results: OIR (including neovascularization in mos t cases) was observed in 100% of the retinas of normobaric oxygen-rear ed ratlings that did not receive timolol. OIR was less frequent in oxy gen-reared ratlings treated with hyperbarism (60%) or timolol (65%). H yperbaric oxygen supplementation combined with timolol treatment durin g both the hyperoxic and room-air phases reduced the incidence of OIR to 30%. There was no sign of vasoproliferation in any of the retinas f rom the latter three groups. Conclusions: The highly significant prote ctive effects of hyperbarism and timolol observed in this study are no t fully understood. We speculate that vasoconstriction induced by the hyperbarism reduces the amount of oxygen that reaches the retina from the choroid during O-2 supplementation, while an increased ocular perf usion pressure caused by timolol-induced reduction of the intraocular pressure might decrease the stimulus to vasoproliferation that normall y occurs with room-air recovery.