Oxygen-induced retinopathy in the newborn rat: effects of hyperbarism and topical administration of timolol maleate

• Background: Lesions resembling those of human retinopathy of prematurity can be provoked in newborn Wistar rats by exposure to an FiO2 of 80% for the first 5 days of life followed by 5 days recovery under room-air conditions. • Methods: We evaluated the effects of moderate hyperbarism (+60.75 kPa, i.e. 455 mmHg or 0.6 atm) and topical administration of 0.25% timolol maleate on oxygen-induced retinopathy (OIR) in this experimental model. • Results: OIR (including neovascularization in most cases) was observed in 100% of the retinas of normobaric oxygen-reared ratlings that did not receive timolol. OIR was less frequent in oxygen-reared ratlings treated with hyperbarism (60%) or timolol (65%). Hyperbaric oxygen supplementation combined with timolol treatment during 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 from the latter three groups. • Conclusions: The highly significant protective effects of hyperbarism and timolol observed in this study are not fully understood. We speculate that vasoconstriction induced by the hyperbarism reduces the amount of oxygen that reaches the retina from the choroid during O2 supplementation, while an increased ocular perfusion pressure caused by timolol-induced reduction of the intraocular pressure might decrease the stimulus to vasoproliferation that normally occurs with room-air recovery.