Retinal oximetry is a non-invasive imaging technique that allows for the calculation of the relative oxygen saturation of retinal blood vessels.
The first use of oximetry in a clinical setting was in 1935, when a German doctor produced an ear oximeter. Early versions of retinal oximetry arose in the 1950s, but the imaging method has been greatly influenced by advances in digital technology. There are currently two main systems: the Oxymap Retinal Oximeter (Fig. 1-2) and the Vesselmap system (Fig. 3).   
Figure 1. Image from the Oxymap system showing the oxygen saturation in the retinal vessels; Retrieved from http://www.oxymap.com/ on 12/18/14.
Figure 2. Picture of the Oxymap device installed and mounted on top of the Topcon TRC-50DX/IX fundus cameras; Retrieved from http://www.oxymap.com/?page_id=1196 on 12/18/14.
Figure 3. Image from the Vesselmap system showing the oxygen saturation in the retinal vessels; Retrieved from http://www.imedos.de/Oxygen-Module.147.0.html?&L=1 on 12/18/14.
Although different retinal oximeter systems have slightly different constructions, the basic retinal oximeter consists of a fundus camera with an attached image splitter. The image splitter contains mirrors that split the beam from the camera into smaller beams based on wavelengths of light, as well as filters that further filter the beam. The apparatus also includes a digital camera to record the fundus images. Analysis of the images has been shown to be most effective with comparison of the images at two distinct wavelengths, at about 600 nm (sensitive to oxyhemoglobin) and about 570 nm (not sensitive to oxyhemoglobin). Computer software detects retinal vessels and uses relative light intensities to calculate relative vessel oxygenation. Multivariate regression analyses of studies on normative values in healthy subjects has shown no significant associations between retinal oxygen saturation (SO2) or vessel width and iris pigmentation, ethnicity, gender, and smoking status. Aging, however, is a factor that has been shown to decrease retinal venous (SvO2) and arterial oxygen saturation (SaO2) and affects most retinal SO2 parameters and therefore must be accounted for in interpreting and comparing measurements.
Retinal Oximetry in Ocular Diseases
Retinal Vessel Occlusions
Retinal vessel occlusions  are blockages of the blood vessels in the retina, often due to atherosclerosis and blood clot formation. There are both central vessel occlusions, near the optic nerve, and branch vessel occlusions, at branches in arterioles or venules. The subsequent blockage of blood flow can lead to blood supply deprivation (ischemia) in areas of the retina.
Retinal oximetry studies have indicated various findings on the relationship between retinal vessel occlusions and oxygenation. In central retinal vein occlusions, oxygen saturation has been found to be lower in venules. In central retinal arterial occlusions, oxygen saturation has been found to be lower in arterioles. However, oxygenation data on branch vessel occlusions has been variable and necessitates further study for conclusive results. These associations show promise for retinal oximetry as a clinical tool for addressing retinal vessel occlusions.
Diabetic retinopathy  is a disease caused by damage to the blood vessels in the retina as a result of chronic hyperglycemia. The disease has four stages, three of which are nonproliferative (NPDR) and the most severe of which is proliferative (PDR), or involves growth of new, fragile blood vessels. Microaneurysms and blood vessel blockage typical of diabetic retinopathy can lead to ischemia in certain retinal areas. A deficit of oxygen in the inner retina as a result of blood supply deprivation has been linked to diabetic retinopathy in various studies. It is further suggested that poor oxygen distribution in the retina leads to high oxygen saturation in retinal blood vessels. Early retinal oximetry analysis indicates that patients with retinopathy in general have higher arterial and venous oxygen saturation levels than healthy subjects. As such, retinal oximetry has promising clinical applications for the diagnosis and management of the disease. (Fig 4A & B)
Figures 4A & B. Comparison of oximetry for a healthy patient in top image (A) and a patient with moderate NPDR in image directly below (B); Retrieved from the UNC Kittner Eye Center.
Glaucoma  is a disease characterized by damage to the optic nerve, typically caused by increase intraocular pressure. While various factors can play a role in the onset of glaucoma, retinal ischemia to the area of the optic nerve is thought to make the optic nerve susceptible to further insults.
While little research has been done relating retinal oximetry and glaucoma, oximetry studies have shown a mild difference in oxygenation pre- and post-glaucoma treatment. It is suggested that oxygen metabolism is affected by glaucoma, and research is being undertaken to find the potential of retinal oximetry as a diagnostic tool for glaucoma.
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