Localized retinal morphology and differential light sensitivity in diabetic retinopathy. Methodology and clinical results

Abstract

The thesis presents a technique designed to allow a comparison of retinal function as assessed by computerized perimetry with retinal morphology as seen on photographs of the ocular fundus, including results from the practical application of this technique in the study of diabetic retinopathy. The basis of the technique is an optical algorithm that allows angular distances in the visual field to be transformed to match linear distances on fundus photographs and fluorescein angiograms. The visual field data is superimposed onto the corresponding retinal morphology on the photograph on the basis of two points of reference. The fixation point in the visual field is superimposed onto the foveola on the photograph of the ocular fundus, and the blind spot in the visual field is superimposed onto the optic disc on the photograph. In the practical application of this technique for the study of diabetic retinopathy, visual field scotomata were found corresponding to areas displaying signs of retinal vascular impairment in the form of vascular occlusion, while no relation was found between visual field scotomata and breakdown of the blood-retina barrier as studied on fluorescein angiograms. Furthermore, visual field scotomata were found to correspond to areas peripheral from retinal neovascularizations, a finding supporting the hypothesis that the neovascularizations develop because of stimulation from vasogenic factors released from ischaemic and hypoxic retinal tissue. Visual field scotomata were also found in relation to retinal cotton wool spots. These scotomata were localized, and not accurately extended, which could be expected if the retinal nerve fiber layer had been damaged. Finally, some visual field scotomata could not be related to any visible funduscopic or angiographical morphology. It is concluded that pathological changes in the inner retinal vascular supply may lead to impairment of visual function in diabetic retinopathy, but that also other mechanisms not manifested in a morphologically visible way, are involved. A further investigation of the pathophysiology leading to visual impairment in diabetes mellitus should focus on these unknown factors. A possible approach could be the development of new techniques for studying pathophysiological mechanisms in specific retinal layers, and especially the layers supplied by the external vascular supply to the retina from the choroidal circulation.