Oximetry

Retinal oximetry is a noninvasive method to evaluate retinal oxygen consumption. By fundus photography at different wavelengths, the differences in light absorption between oxyhemoglobin and deoxyhemoglobin can be used to calculate the retinal arteriolar and venular oxygen saturation as well as the arteriovenous oxygen saturation difference, which can be considered as a marker of oxygen consumption by the retinal tissue. Retinal oximetry has been studied in various ocular and nonocular diseases. Most notably, retinal oximetry has demonstrated markedly alteration in ischemic retinal disease. In diabetic retinopathy, increased retinal venular oxygen saturation has consistently been reported, whereas some studies have also demonstrated increased retinal arteriolar oxygen saturation. In retinal vein occlusion, the retinal venular oxygen saturation is also universally decreased in eyes with central occlusion, in contrast to branch retinal vein occlusion, where the retinal oximetry findings seem to be more related to the degree of retinal ischemia. In age-related macular degeneration and glaucoma, studies indicate a lower retinal arteriovenous oxygen difference, as an indication of a lower retinal oxygen consumption. In cerebral neurodegenerative diseases like Alzheimer's disease and mild cognitive impairment, a higher retinal arteriolar oxygen saturation indicates that retinal oximetry may act as an ocular marker of systemic disease. In systemic inflammatory diseases like multiple sclerosis and sarcoidosis, studies indicate a higher retinal arteriovenous oxygen difference in inflammatory stages followed by a lower difference in the quiescent, atrophic stage. Finally, the retinal oxygen extraction metabolism reflects systemic hypoxia, as measured in chronic obstructive pulmonary disease and cardiovascular disease. In conclusion, retinal oximetry is a noninvasive method to evaluate retinal oxygen consumption and to reflect systemic disease. Nevertheless, more studies are needed to confirm data and to expand the current findings, which are mainly from cross-sectional studies. To validate retinal oximetry as a reliable marker of ocular and systemic disease, more prospective studies are needed and sensitivity, specificity, and area under the curve should be presented in comparison to present gold-standard diagnostic tests.