Einar Stefánsson

Panretinal Photocoagulation and Retinal Oxygenation in Normal and Diabetic Cats

We measured preretinal oxygen tension over laser-treated and intact retina in normal and diabetic cats. The oxygen tension was significantly higher over the area given panretinal photocoagulation than over the intact retina when the cats breathed 100% oxygen and the same trend was seen when the cats breathed 21% oxygen. There was no difference in preretinal oxygen tension between the normal and diabetic retinas. Light and electron microscopy showed laser damage to the mitochondria-rich photoreceptors in the outer retina as well as the retinal pigment epithelium. The inner retina showed no photocoagulation damage. Because panretinal photocoagulation destroys parts of the outer retina, thus reducing its oxygen consumption and allowing oxygen to diffuse from the choroid into the inner retina, it improves the oxygen supply to the inner retina.

Oxygenation and vasodilatation in relation to diabetic and other proliferative retinopathies.

The features of proliferative retinopathies are reviewed with reference to the vasodilatation that is a common stage in their pathogenesis. Vasodilatation in the retina is discussed in terms of a model in which vessel wall stretch in the retinal venules leads to leakage and proliferation. The beneficial effects of treatments such as panretinal photocoagulation and vitrectomy on the retinal hemodynamics are shown to agree with the hypothetical model.

Results of Silicone Oil Removal in Advanced Proliferative Vitreoretinopathy

We reviewed the results of silicone oil removal after vitreous surgery for recurrent retinal detachment with proliferative vitreoretinopathy in 55 eyes. The minimum follow-up period was six months. Retinal detachment recurred in five eyes (9%). Evidence of recurrent epiretinal membrane formation at the time of or after silicone oil removal was noted in 21 eyes (38%). Penetrating keratoplasty combined with silicone oil removal was successful in five of six patients. Mild corneal changes noted in ten eyes (18%) remained the same or cleared in seven eyes after silicone oil removal. Overall, postoperative visual acuity improved in 24 eyes (44%), remained the same in 18 eyes (33%), and decreased in 13 (24%). In 22 eyes without clinical evidence of silicone-associated complications at the time of silicone oil removal, postoperative visual acuity remained unchanged or improved in 20 (91%).

Oxygen and diabetic eye disease

In 1956, Wise suggested that retinal hypoxia stimulated retinal neovascularization in the ischemic proliferative retinopathies. Although not directly proven, this theory is strongly supported by a wealth of circumstantial information. Two treatment modalities, vitrectomy and panretinal photocoagulation, have been shown to be effective against retinal neovascularization in diabetics. Both of these treatment modalities improve retinal oxygenation, and we propose that this is the mechanism through which they halt retinal neovascularization. The mechanism for improving retinal oxygenation is different for the two treatment modalities. In the case of panretinal photocoagulation, the new oxygen supply comes from the choroid through the laser scar in the outer retina. In the case of vitrectomy, it comes from the vitreous cavity itself, but the end result is the same. We have expanded Wises hypothesis to include these two treatment modalities, which were not known at the time of Wises original paper.

Classification of Proliferative Vitreoretinopathy Used in the Silicone Study

The Silicone Study is a multicenter randomized clinical trial that compares a long-acting gas with silicone oil for the surgical treatment of proliferative vitreoretinopathy (PVR). As part of the study, a topographic classification of PVR has been developed that is based on the characteristic patterns of retinal distortion produced by the contraction of proliferative membranes on the retina or within the vitreous base. This classification is used to document the extent and anatomic distribution of PVR present preoperatively and to help standardize the surgical treatment. Experience has shown that this classification facilitates the identification of these membranes and their systematic dissection, and the authors therefore suggest that it be used to augment the Retina Society classification of PVR.

Panretinal photocoagulation and retinal oxygenation.

Panretinal photocoagulation destroys some of the photoreceptors and thereby reduces the oxygen consumption in the outer retina and allows more O2 to flow from the choroid to the inner retina. Measurements of the preretinal oxygen tension were made in rhesus monkeys that had portions of their retinas photocoagulated. With the animal breathing one atmosphere O2, the oxygen tension over photocoagulated areas of retina was significantly higher than the PO2 over normal, untreated areas of retina. It is proposed that the therapeutic effect of panretinal photocoagulation in diabetic retinopathy lies in the oxygenation of the inner retina from the choroid.

In vivo O2 consumption in rhesus monkeys in light and dark.

The rate of oxygen consumption in retinas in vitro has been measured in a great number of species, but in vitro estimates have varied partly because the rate of O2 uptake depends heavily upon the composition of the buffered solution used to maintain the retina. The in vitro experiments on frog retinas show a greater O2 consumption in dark than in light conditions, and indicate marked differences in metabolism between light and dark conditions. A method was developed to estimate retinal oxygen consumption in vivo. We evaluated the oxygen consumption of the in vivo Rhesus monkey retina with an intraocular oxygen microelectrode. The intact monkey retina consumes more oxygen in darkness than in light.

Vitrectomy, lensectomy, and ocular oxygenation.

The effect of vitrectomy and lensectomy upon the anterior chamber oxygen tension of the cat eye was measured polarographically. After vitrectomy and lensectomy alone, a reduction of 12 mm Hg (35%) was observed and when retinal veins were also partially occluded, a reduction of 17 mm Hg (50%) was measured as compared with the normal oxygen tension (PO2) of 34 mm Hg. If vitrectomy and lensectomy allow aqueous to deliver oxygen from the anterior part of the eye to the retina, the poorer oxygenation of the iris and the improved oxygenation of the retina might explain both the development of rubeosis iridis and stabilization of proliferative diabetic retinopathy so commonly seen following vitrectomy.

The relationship between retinal vessel tortuosity, diameter, and transmural pressure

Increases in retinal vein tortuosity are thought to be caused by increases in vascular transmural pressure. We have attempted to determine the relationship between retinal vessel tortuosity, diameter, and transmural pressure by examining the effects of changes in transmural pressure on latex tubes with fixed ends. As the transmural pressure is raised, tube diameter increases, but tortuosity does not begin increasing until a critical pressure is reached. Above the critical pressure, tortuosity increases more rapidly than diameter. Our results support the above hypothesis and also suggest that at high transmural pressures, retinal vessel tortuosity is a more sensitive indicator than is the diameter of changes in retinal venous transmural pressure, but diameter is more sensitive than tortuosity at lower pressures.

Optic disk neovascularization and retinal vessel diameter in diabetic retinopathy

We measured retinal vessel diameter before and after panretinal photocoagulation in 59 eyes with diabetic retinopathy and moderate to severe optic disk neovascularization. Treatment significantly reduced mean arteriolar and venular diameter. The diameter of the retinal arterioles after treatment correlated significantly with the amount of regression in disk neovascularization. Eyes with large diameter vessels after treatment usually had little or no regression of proliferative retinopathy, whereas regression was more frequently seen in eyes with smaller diameter vessels after treatment.