Chronic Effects of Photocoagulation on PO2 in the Cat Retina

Abstract

Abstract: : Purpose: To test the hypothesis that panretinal photocoagulation damages photoreceptors, leading to a decrease in O₂ consumption that relieves inner retinal hypoxia. Methods: Lesions, similar to those employed in panretinal photocoaulation, were placed in retinas of normal adult cats with an argon laser using a slit lamp delivery system. These lesions damaged only the outer retinal layers. Four weeks after the photocoagulation, PO₂ depth profiles were collected from intact, dark–adapted cat retina using double–barreled microelectrodes. Profiles were collected from untreated and photocoagulated parts of the retina. At the end of the measurements, the eyes were removed for histological processing. Results: Histopathologic examination of the retinas confirmed that the photocoagulation destroyed the photoreceptors and reduced the thickness of the tapetum, but left the inner retina largely intact. As a result, the inner retina was closer to the choroid in lesioned compared to untreated retina. The average PO₂ across the retina was significantly higher in photocoagulated retina compared to untreated retina (p < 0.03, 6 cats). The minimum PO₂, which was close to zero in untreated retina, significantly increased in photocoagulated retina (p < 0.011, 6 cats), as expected from the loss of the large O₂ consumption of the photoreceptors. Choroidal PO₂ in the photocoagulated regions significantly decreased (p < 0.01, 6 cats). Conclusions: These measurements in normal animals provide the first direct evidence that photocoagulation leads to increased intraretinal PO₂ during air breathing. They support the hypothesis that panretinal photocoagulaion in diabetics relieves the hypoxia that is believed to underly neovascularization. They suggest that the benefit of panretinal photocoagulation could be limited, however, by damage to the choroid.

Keywords: laser • diabetic retinopathy • retina