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D. S. Gallagher, T. R. Friberg, R. E. Coffee, S. D. Schwartz; Ultrawide Field Angiographic Imaging (Optos P200A) and the Development of an Ischemic Index (ISI). Invest. Ophthalmol. Vis. Sci. 2009;50(13):5270.
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Diabetic retinopathy has historically been classified using morphological features such as the presence of macular edema, neovascularization of the disc (NVD) or neovascularization elsewhere (NVE). With the availability of ultrawide angle angiography, we measure the extent of retinal capillary drop out (ischemia) in the fundi of patients with proliferative diabetic retinopathy. The extent of ischemia might be predictive in a prospective trial of which eyes are at the highest risk of developing neovascularization. Such knowledge could alter our treatment strategies using either laser photocoagulation or anti-VEGF therapy.
We retrospectively reviewed 10 consecutive digital angiograms that were obtained during ultrawide field angiography (Optos P200A, Edinburgh, Scotland) in patients who had clinically evident diabetic retinopathy. If both eyes were affected, only one eye was chosen, and that was the eye that had been imaged in the early phase. We calculated the ischemic index (ISI) of each eye by dividing the total area of ischemia found on their arteriovenous phase images by the total image area. This was quantified using drawing tools and calculating the total pixels showing dropout by the total number of pixels in the image. We made no correction for the curvature of the fundus. The ischemic index then represents grossly the percent of the fundus that has normal capillary perfusion.
In this sample of patients, we found that the ischemic index or percent of non-perfusion ranged from 46% to 84%, mean ± standard deviation of 74.1 ± 11.5. Patients with more extensive capillary dropout had qualitatively more extensive retinal neovascularization, possibly related to the chronicity of their disease. The use of this index prospectively might help identify eyes that are likely to convert to neovascularization. Furthermore, targeted retinal photocoagulation or the use of anti-VEGF therapy might be indicated depending on the results of the larger scale prospective study using such a measure.
Quantification of the degree of ischemia on fluorescein angiography (ischemic index) allows for a better description of the severity of diabetic retinopathy than morphological characteristics alone. This index can be used to develop risk factors for the development of proliferative retinopathy and perhaps for the development of macular edema.
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