Evaluation of a retinal oximetry image quality indicator in patients with cataract.

PURPOSE: To evaluate a new automated retinal oximetry image quality indicator with cataract as a clinical model. METHODS: Sixty-one eyes in 61 patients were imaged by the Oxymap T1 Retinal Oximeter at baseline and 25 eyes were also examined 3 weeks after cataract surgery. Image quality (0-10 on a continuous scale) was compared with standardized AREDS cataract grading and Pentacam lens densitometry. Associations with retinal oximetry measurements and visual acuity were examined. RESULTS: Image quality correlated with total, nuclear and posterior subcapsular cataract grades (ANOVA, p < 0.05), tended to be associated with lens densitometry and it improved from 4.3 ± 1.4 to 5.7 ± 1.0 (p < 0.05) after cataract surgery. Very low image quality, below 3, led to vessel detection failure in retinal oximetry images. Higher image qualities were linearly associated with higher measured retinal oxygen saturations (r = 0.52 in arteries and r = 0.46 in veins; p < 0.001). CONCLUSION: Retinal oximetry image quality deteriorated with increasing cataract density and improved after cataract surgery, supporting its use as a measure of optical clarity. The numerical quality indicator demonstrated a threshold below which images of poor optical quality should be discarded. Image quality affects the estimates of retinal oximetry parameters and should therefore be included in future analyses.

Artery vein classification in fundus images using serially connected U-Nets.

BACKGROUND AND OBJECTIVE: Retinal vessels provide valuable information when diagnosing or monitoring various diseases affecting the retina and disorders affecting the cardiovascular or central nervous systems. Automated retinal vessel segmentation can assist clinicians and researchers when interpreting retinal images. As there are differences in both the structure and function of retinal arteries and veins, separating these two vessel types is essential. As manual segmentation of retinal images is impractical, an accurate automated method is required. METHODS: In this paper, we propose a convolutional neural network based on serially connected U-nets that simultaneously segment the retinal vessels and classify them as arteries or veins. Detailed ablation experiments are performed to understand how the major components contribute to the overall system's performance. The proposed method is trained and tested on the public DRIVE and HRF datasets and a proprietary dataset. RESULTS: The proposed convolutional neural network achieves an F1 score of 0.829 for vessel segmentation on the DRIVE dataset and an F1 score of 0.814 on the HRF dataset, consistent with the state-of-the-art methods on the former and outperforming the state-of-the-art on the latter. On the task of classifying the vessels into arteries and veins, the method achieves an F1 score of 0.952 for the DRIVE dataset exceeding the state-of-the-art performance. On the HRF dataset, the method achieves an F1 score of 0.966, which is consistent with the state-of-the-art. CONCLUSIONS: The proposed method demonstrates competitive performance on both vessel segmentation and artery vein classification compared with state-of-the-art methods. The method outperforms human experts on the DRIVE dataset when classifying retinal images into arteries, veins, and background simultaneously. The method segments the vasculature on the proprietary dataset and classifies the retinal vessels accurately, even on challenging pathological images. The ablation experiments which utilize repeated runs for each configuration provide statistical evidence for the appropriateness of the proposed solution. Connecting several simple U-nets significantly improved artery vein classification performance. The proposed way of serially connecting base networks is not limited to the proposed base network or segmenting the retinal vessels and could be applied to other tasks.

Automation improves repeatability of retinal oximetry measurements.

PURPOSE: Retinal oximetry is a technique based on spectrophotometry where images are analyzed with software capable of calculating vessel oxygen saturation and vessel diameter. In this study, the effect of automation of measurements of retinal vessel oxygen saturation and vessel diameter is explored. METHODS: Until now, operators have had to choose each vessel segment to be measured explicitly. A new, automatic version of the software automatically selects the vessels once the operator defines a measurement area. Five operators analyzed image pairs from the right eye of 23 healthy subjects with semiautomated retinal oximetry analysis software, Oxymap Analyzer (v2.5.1), and an automated version (v3.0). Inter- and intra-operator variability was investigated using the intraclass correlation coefficient (ICC) between oxygen saturation measurements of vessel segments in the same area of the retina. RESULTS: For semiautomated saturation measurements, the inter-rater ICC was 0.80 for arterioles and venules. For automated saturation measurements, the inter-rater ICC was 0.97 for arterioles and 0.96 for venules. For semiautomated diameter measurements, the inter-rater ICC was 0.71 for arterioles and venules. For automated diameter measurements the inter-rater ICC was 0.97 for arterioles and 0.95 for venules. The inter-rater ICCs were different (p < 0.01) between the semiautomated and automated version in all instances. CONCLUSION: Automated measurements of retinal oximetry values are more repeatable compared to measurements where vessels are selected manually.

Retinal oxygen saturation changes progressively over time in diabetic retinopathy.

PURPOSE: According to cross-sectional studies, oxygen saturation is elevated in retinal vessels in diabetic patients. We evaluated how retinal oxygenation (metabolic marker), vessel diameters and retinopathy grade (structural markers) change over time in diabetic patients. DESIGN: Prospective cohort study following screening in a hospital setting. METHODS: Retinal oximetry images were acquired in 214 patients with the Oxymap T1 oximeter. Imaging was repeated after a median of 3.0 years (range 0.76-6.8 years). Oxygen saturation and vessel diameters were measured in the right eye. Semiquantitative grading of retinopathy according to international guidelines and red lesion count were performed on fundus photographs. RESULTS: Retinopathy grade according to the international semiquantitative grading system was unchanged. Arteriolar saturation increased by 0.75±0.15 percentage points per year of follow-up (p<0.0001). Venular saturation increased by 1.74±0.26 percentage points per year (p<0.0001) and arteriovenous difference decreased by 0.99±0.20 percentage points per year (p<0.0001). Arteriolar diameters decreased by 2.7±8.5µm (p<0.0001) between visits and venular diameters decreased by 2.4±9.1µm (p = 0.0002). Median increase in red lesion count between visits was 2 lesions (range -128 to 212 lesions, p<0.0001). The change in red lesion count and change in diameters did not correlate with the length of follow-up (p>0.44). CONCLUSIONS: Oxygen saturation in larger retinal vessels can increase and arteriovenous difference can decrease over time in diabetic patients without any observable changes in retinopathy grade. The results suggest that changes in retinal oxygen saturation may precede progression of diabetic retinopathy or that oxygen saturation is more sensitive to disease progression than retinopathy grade.

[Glaucomatous visual field loss in eyes undergoing first trabeculectomy in Iceland]. / Sjónsviðsskerðing við fyrstu hjáveituaðgerð (trabeculectomiu) vegna gláku

INTRODUCTION: Glaucoma is a degenerative disease in the optic nerve with associated visual field defects (VFD). Trabeculectomy is the most common glaucoma surgery. Surgery is indicated if glaucomatous optic neuropathy progresses despite tolerated medical treatment or in patients with severe VFD. The purpose of this paper is to describe the severity of visual field damage in patients undergoing their first trabeculectomy in Iceland. METHODS: A retrospective review of medical records of all patients with open angle glaucoma that underwent first trabeculectomy at Landspítali University Hospital, from June 2013 to March 2016. Visual fields were examined by Octopus automated perimetry and the severity of glaucoma damage was staged according to the mean defect (MD). RESULTS: 86 eyes were included in the study, mean age 75 ± 11 years, 57% men. Patients used on average three IOP lowering medications. Mean MD at referral to surgery was 13.4 ± 7.7dB (min 0.8dB, max 26.2 dB), 21% had early glaucomatous damage (MD <6dB), 23% moderate (MD 6-12 dB) and 56% severe (MD > 12). CONCLUSION: VFD at referral to surgery varied from mild VFD to severe damage. Like clinical guidelines recommend, treatment seems to be individualized and the most common indication for surgery was increased VFD despite medical treatment. Mean MD at referral to surgery was high compared to other studies. Eyes with severe VFD had on average lower IOP and thinner cornea. This might indicate that great emphasis is placed on high IOP and perhaps too little emphasis on VFD and cornea thickness.

Retinal oximetry: Metabolic imaging for diseases of the retina and brain.

Retinal oximetry imaging of retinal blood vessels measures oxygen saturation of hemoglobin. The imaging technology is non-invasive and reproducible with remarkably low variability on test-retest studies and in healthy cohorts. Pathophysiological principles and novel biomarkers in several retinal diseases have been discovered, as well as possible applications for systemic and brain disease. In diabetic retinopathy, retinal venous oxygen saturation is elevated and arteriovenous difference progressively reduced in advanced stages of retinopathy compared with healthy persons. This correlates with pathophysiology of diabetic retinopathy where hypoxia stimulates VEGF production. Laser treatment and vitrectomy both improve retinal oximetry values, which correlate with clinical outcome. The oximetry biomarker may allow automatic measurement of severity of diabetic retinopathy and predict its response to treatment. Central retinal vein occlusion is characterized by retinal hypoxia, which is evident in retinal oximetry. The retinal hypoxia seen on oximetry correlates with the extent of peripheral ischemia, visual acuity and thickness of macular edema. This biomarker may help diagnose and measure severity of vein occlusion and degree of retinal ischemia. Glaucomatous retinal atrophy is associated with reduced oxygen consumption resulting in reduced arteriovenous difference and higher retinal venous saturation. The oximetry findings correlate with worse visual field, thinner nerve fiber layer and smaller optic disc rim. This provides an objective biomarker for glaucomatous damage. In retinitis pigmentosa, an association exists between advanced atrophy, worse visual field and higher retinal venous oxygen saturation, lower arteriovenous difference. This biomarker may allow measurement of severity and progression of retinitis pigmentosa and other atrophic retinal diseases. Retinal oximetry offers visible light imaging of systemic and central nervous system vessels. It senses hypoxia in cardiac and pulmonary diseases. Oximetry biomarkers have been discovered in Alzheimer's disease and multiple sclerosis and oxygen levels in the retina correspond well with brain.

Retinal vessel oxygen saturation is affected in uveitis associated with Vogt-Koyanagi-Harada disease.

AIMS: To discover whether retinal vessel oxygen metabolism is affected in uveitis associated with Vogt-Koyanagi-Harada (VKH) disease. METHODS: 41 patients with VKH disease (82 eyes) and 12 healthy subjects (24 eyes) matched in age and gender were prospectively evaluated. Retinal oxygen saturation and vessel calibre were measured with a non-invasive spectrophotometric retinal oximeter (Oxymap T1). RESULTS: In healthy controls, mean arteriolar oxygen saturation (%) was 93.8±5.9 and venular saturation was 60.1±5.8. In acute VKH uveitic phase associated with exudative retinal detachment (n=12), arteriolar and venular oxygen saturation values were 104.7±7.8 and 67.9±7.7, respectively, and both are significantly higher than the healthy group (p<0.001; p=0.001, respectively). In patients with VKH disease who recovered after immunosuppressive therapy and restored normal anatomy without 'sunset glow fundus' (n=13), oximetry values were 96.4±9.6 and 61.6±7.5, respectively, similar to healthy controls. In patients with 'sunset glow fundus' and chorioretinal atrophy (n=16), saturation levels were 88.6±7.8 and 50.0±13.1, respectively, significantly lower than healthy controls (p=0.02; p=0.003, respectively). These patients also had significantly smaller diameter of retinal arterioles and venules compared with controls (p=0.035; p=0.001, respectively). CONCLUSIONS: Retinal oxygen metabolism is altered in uveitis associated with VKH disease. Oxygen saturation profile is abnormal in acute uveitic phase of the disease and returns to normal in those who recover with normal fundus appearance, but not in eyes that suffer permanent anatomical damage with 'sunset glow fundus' and chorioretinal atrophy. Retinal oximetry may be of value in evaluating vascular and metabolic aspects of posterior uveitis.

Retinal oxygen saturation is an independent risk factor for the severity of diabetic retinopathy.

BACKGROUND: The oxygen saturation in larger retinal vessels has been shown to increase with increasing diabetic retinopathy (DR) grade and to help predict the effect of antivascular endothelial growth factor treatment in patients with diabetic maculopathy. However, it is unknown to what extent the increased oxygen saturation co-varies with other risk factors and whether it is an independent risk factor for the severity of DR. METHODS: Seven hundred and twenty-two successive patients referred for specialist evaluation of diabetic retinopathy including retinal oximetry were studied. Multiple regression analysis was used to investigate whether oxygen saturation in the larger retinal arterioles and venules contributed to the severity of diabetic retinopathy, independently of gender, age, diabetes duration, diabetes type, body mass index, blood pressure, haemoglobin A1c, visual acuity and central retinal thickness. RESULTS: The included parameters could explain less than 15% of the variation in retinopathy grade. Approximately, one-third of the explained variation was related to the retinal oxygen saturation. CONCLUSIONS: Prospective studies are needed to evaluate whether retinal oxygen saturation is predictive for the development of diabetic retinopathy and how it interacts with other biomarkers and risk factors over time.

Handheld Retinal Oximetry in Healthy Young Adults.

PURPOSE: The objective of this study was to measure the relative retinal oxygen saturation with a prototype, mobile handheld oximeter in upright and supine position and to compare these measurements to the gold standard Oxymap T1 retinal oximeter in upright position. A handheld oximeter is needed for measurements of infants with retinopathy of prematurity as well as acutely injured and bedridden adults. METHODS: Healthy volunteers (age 18-35) were recruited at the Leiden University Medical Center. Retinal images were acquired with the handheld oximeter and the Oxymap T1. Both cameras are dual-wavelength oximeters and acquire images with wavelengths of 570 and 600 nm. Retinal oxygen saturation values were determined for both the handheld camera and the Oxymap T1. RESULTS: Twenty-one subjects (age 25 ± 2 years) were included. In upright position, the oxygen saturation for the arterioles was 92.2% to 4.9% vs. 95.5% ± 4.2% and for the venules 57.9% ± 10.2% vs. 57.7% ± 6.4% for the handheld camera and Oxymap T1, respectively. The oxygen saturation was higher in the arterioles than the venules for both cameras (P < 0.05). In supine position, measured with the handheld oximeter, the oxygen saturation in the arterioles was 92.3% ± 5.8% and 59.2% ± 6.1% in the venules. CONCLUSIONS: Performance of the prototype, mobile handheld oximeter Corimap camera compares well with the Oxymap T1, with a slightly larger standard deviation in oxygen saturation measurements, both in upright and supine patients. TRANSLATION RELEVANCE: To date, to our knowledge, no oximeters are available for handheld use and for measurement in supine position in infants and bedridden adults. Here we tested such an oximeter and show that its performance compares well with that of the gold standard Oxymap T1 in healthy adults.

Retinal oxygen metabolism in patients with mild cognitive impairment.

INTRODUCTION: We have previously reported that retinal vessel oxygen saturation is increased in mild-to-moderate dementia of Alzheimer's type when compared with healthy individuals. Mild cognitive impairment (MCI) is the predementia stage of the disease. The main purpose was to investigate if these changes are seen in MCI. METHODS: Retinal vessel oxygen saturation was measured in 42 patients with MCI and 42 healthy individuals with a noninvasive retinal oximeter, Oxymap T1. The groups were paired according to age. RESULTS: Arteriolar and venular oxygen saturation was increased in MCI patients compared to healthy individuals (arterioles: 93.1 ± 3.7% vs. 91.1 ± 3.4%, P = .01; venules: 59.6 ± 6.1% vs. 54.9 ± 6.4%, P = .001). Arteriovenous difference was decreased in MCI compared to healthy individuals (33.5 ± 4.5% vs. 36.2 ± 5.2%, P = .01). DISCUSSION: Increased retinal vessel oxygen saturation and decreased arteriovenous difference in MCI could reflect less oxygen extraction by retinal tissue. This indicates that retinal oxygen metabolism may be affected in patients with MCI.

Retinal oximetry is affected in multiple sclerosis.

PURPOSE: Structural and physiological abnormalities have been reported in the retina in patients with multiple sclerosis (MS). Retinal oximetry has recently detected changes in retinal oxygen metabolism in Alzheimer's disease and mild cognitive impairment. Our goal was to determine whether oxygen saturation in retinal blood vessels of patients with patients is different from that of a healthy population. METHODS: Oxygen saturation of haemoglobin was measured in retinal blood vessels, using imaging with spectrophotometric noninvasive retinal oximeter. Eight MS patients with history of optic neuritis were measured and compared to 22 healthy individuals matched in age and gender. RESULTS: Venular oxygen saturation was increased in patients with MS compared to healthy individuals (70.7 ± 3.4% versus 66.2 ± 4.7; p = 0.021, mean ± SD). The arteriovenous (AV) difference was lower in patients with MS compared to healthy (26.6 ± 3.6% versus 30.5 ± 4.8%; p = 0.049). There was no difference measured in arterioles when patients with MS (97.3 ± 1.7%) and healthy individuals (96.7 ± 2.8%) were compared. CONCLUSION: Increased venular oxygen saturation and lower AV difference in patients with MS may indicate reduced oxygen uptake. This may be due to less oxygen demand following atrophy and may be a useful objective biomarker for MS. Further studies are needed to confirm and expand these findings.

Retinal Oximetry Discovers Novel Biomarkers in Retinal and Brain Diseases.

Purpose: Biomarkers for several eye and brain diseases are reviewed, where retinal oximetry may help confirm diagnosis or measure severity of disease. These include diabetic retinopathy, central retinal vein occlusion (CRVO), retinitis pigmentosa, glaucoma, and Alzheimer's disease. Methods: Retinal oximetry is based on spectrophotometric fundus imaging and measures oxygen saturation in retinal arterioles and venules in a noninvasive, quick, safe manner. Retinal oximetry detects changes in oxygen metabolism, including those that result from ischemia or atrophy. Results: In diabetic retinopathy, venous oxygen saturation increases and arteriovenous difference decreases. Both correlate with diabetic retinopathy severity as conventionally classified on fundus photographs. In CRVO, vein occlusion causes hypoxia, which is measured directly by retinal oximetry to confirm the diagnosis and measure severity. In both diseases, the change in oxygen levels is a consequence of disturbed blood flow with resulting tissue hypoxia and vascular endothelial growth factor (VEGF) production. In atrophic diseases, such as retinitis pigmentosa and glaucoma, retinal oxygen consumption is reduced and this is detected by retinal oximetry. Retinal oximetry correlates with visual field damage and retinal atrophy. It is an objective metabolic measure of the degree of retinal atrophy. Finally, the retina is part of the central nervous system tissue and reflects central nervous system diseases. In Alzheimer's disease, a change in retinal oxygen metabolism has been discovered. Conclusions: Retinal oximetry is a novel, noninvasive technology that opens the field of metabolic imaging of the retina. Biomarkers in metabolic, ischemic, and atrophic diseases of the retina and central nervous system have been discovered.

Retinal oximetry measures systemic hypoxia in central nervous system vessels in chronic obstructive pulmonary disease.

BACKGROUND: Determination of the blood oxyhemoglobin saturation in the retinal vessels of the eye can be achieved through spectrophotometric retinal oximetry which provides access to the state of oxyhemoglobin saturation in the central nervous system circulation. The purpose of this study was to test the capability of the Oxymap T1 oximeter to detect systemic hypoxemia and the effect of supplemental oxygen on retinal vessel oxyhemoglobin saturation. METHODS: Oxygen saturation of hemoglobin in retinal arterioles and venules was measured in 11 subjects with severe chronic obstructive pulmonary disease (COPD) on long term oxygen therapy. Measurements were made with and without their daily supplemental oxygen. Eleven healthy age and gender matched subjects were measured during ambient air breathing for comparison of oxyhemoglobin saturation in retinal arterioles and venules. Retinal arteriolar oxyhemoglobin saturation in COPD subjects inspiring ambient air was compared with finger pulse oximetry and blood samples from radial artery. RESULTS: COPD subjects had significantly lower oxyhemoglobin saturation during ambient air breathing than healthy controls in both retinal arterioles (87.2%±4.9% vs. 93.4%±4.3%, p = 0.02; n = 11) and venules (45.0%±10.3% vs. 55.2%±5.5%, p = 0.01). Administration of their prescribed supplemental oxygen increased oxyhemoglobin saturation in retinal arterioles (87.2%±4.9% to 89.5%±6.0%, p = 0.02) but not in venules (45.0%±10.3% to 46.7%±12.8%, p = 0.3). Retinal oximetry values were slightly lower than radial artery blood values (mean percentage points difference = -5.0±5.4, 95% CI: -15.68 to 5.67) and finger pulse oximetry values (-3.1±5.5, 95% CI: -14.05 to 7.84). CONCLUSIONS: The noninvasive Oxymap T1 retinal oximetry detects hypoxemia in central nervous system vessels in patients with severe COPD compared with healthy controls. The instrument is sensitive to changes in oxygen breathing but displays slightly lower measures than finger pulse oximetry or radial artery measures. With further technological improvement, retinal oximetry may offer noninvasive "on-line" measurement of oxygen levels in central circulation in general anesthesia and critically ill patients.

Retinal Oximetry with Scanning Laser Ophthalmoscope in Infants.

PURPOSE: Dual wavelength retinal oximetry has been developed for adults, but is not available for infants. Retinal oximetry may provide insight into the pathophysiology of oxygen-mediated diseases like retinopathy of prematurity. More insight in the oxygen metabolism of the retina in infants may provide valuable clues for better understanding and subsequent prevention or treatment of the disease. The measurements of oxygen saturation are obtained with two fundus images simultaneously captured in two different wavelengths of light. The comparison in light absorption of oxygenated and deoxygenated hemoglobin can be used to estimate the oxygen saturation within the retinal vessels by means of a software algorithm. This study aims to make retinal oximetry available for neonates. The first step towards estimating retinal oxygen saturation is determining the optical density ratio. Therefore, the purpose of this study is to image healthy newborn infants with a scanning laser ophthalmoscope and determine the optical density ratio for retinal oximetry analysis. METHODS: Images of the retina of full-term healthy infants were obtained with an SLO, Optomap 200Tx (Optos), with two laser wavelengths (532nm and 633nm). The infant lay face down on the lower arm of the parent, while the parent supported the chest and chin with one hand, and stabilized the back with the other hand. No mydriatics or eyelid specula were used during this study. The images were analyzed with modified Oxymap Analyzer software for calculation of the Optical Density Ratio (ODR) and vessel width. The ODR is inversely and approximately linearly related to the oxygen saturation. Measurements were included from the superotemporal vessel pair. A paired t-test was used for statistical analysis. RESULTS: Fifty-nine infants, (58% female), were included with mean gestational age of 40 ± 1.3 weeks (mean ± SD) and mean post-natal age of 16 ± 4.8 days. A total of 28 images were selected for retinal oximetry analysis. The ODR was 0.256 ± 0.041 for the arterioles and 0.421 ± 0.089 for the venules (n = 28, p < 0.001). The measured vessel-width for the arterioles was 14.1 ± 2.7 pixels and for the venules 19.7 ± 3.7 pixels (n = 28, p < 0.001). CONCLUSIONS: Retinal oximetry can be performed in newborn infants by combining an SLO and a dual-wavelength algorithm software. Sensitivity of the approach is indicated by the fact that the ODR measurements are significantly different between the arterioles and the venules. However, more variability in ODR is seen with the SLO approach in babies than is seen with conventional oximetry in adults. This approach is completely non-invasive, non-contact and even avoids the use of mydriatics or eyelid specula.

Retinal oximetry imaging in Alzheimer's disease.

BACKGROUND: Structural and physiological abnormalities have been reported in the retina in Alzheimer's disease (AD). Retinal oximetry detects changes in retinal oxygen metabolism in many eye diseases, where structural changes are seen. OBJECTIVE: To compare oxygen saturation in retinal blood vessels in patients with AD and a healthy cohort. METHODS: Oxygen saturation of hemoglobin was measured in retinal blood vessels, using imaging with spectrophotometric noninvasive retinal oximeter. 18 individuals with mild to moderate dementia of the Alzheimer-type (stage 3-5 according to the Global Deterioration Scale) and 18 healthy subjects underwent retinal oximetry in a case control study. RESULTS: Retinal oxygen saturation in arterioles and venules in patients with moderate AD was significantly elevated compared to healthy individuals. Retinal arterioles have 94.2 ± 5.4% oxygen saturation in moderate AD compared with 90.5 ± 3.1% in healthy subjects (mean ± SD, n = 10, p = 0.028). Retinal venules were 51.9 ± 6.0% saturated in moderate AD compared with 49.7 ± 7.0% in healthy subjects (mean ± SD, n = 10, p = 0.02). CONCLUSION: This is the first study of retinal oxygen metabolism in any central nervous system disease. It discovers abnormalities in retinal oxygen metabolism in AD. The findings are similar to those seen in age-related macular degeneration and diabetic retinopathy. Noninvasive retinal oximetry may offer new insights into pathophysiology of AD. Further studies are needed to confirm and expand these findings.

Retinal Vessel Oxygen Saturation during 100% Oxygen Breathing in Healthy Individuals.

PURPOSE: To detect how systemic hyperoxia affects oxygen saturation in retinal arterioles and venules in healthy individuals. METHODS: Retinal vessel oxygen saturation was measured in 30 healthy individuals with a spectrophotometric retinal oximeter (Oxymap T1). Oximetry was performed during breathing of room air, 100% oxygen (10 minutes, 6L/min) and then again room air (10 minutes recovery). RESULTS: Mean oxygen saturation rises modestly in retinal arterioles during 100% oxygen breathing (94.5%±3.8 vs. 92.0%±3.7% at baseline, p<0.0001) and dramatically in retinal venules (76.2%±8.0% vs. 51.3%±5.6%, p<0.0001). The arteriovenous difference decreased during 100% oxygen breathing (18.3%±9.0% vs. 40.7%±5.7%, p<0.0001). The mean diameter of arterioles decreased during 100% oxygen breathing compared to baseline (9.7±1.4 pixels vs. 10.3±1.3 pixels, p<0.0001) and the same applies to the mean venular diameter (11.4±1.2 pixels vs. 13.3±1.5 pixels, p<0.0001). CONCLUSIONS: Breathing 100% oxygen increases oxygen saturation in retinal arterioles and more so in venules and constricts them compared to baseline levels. The dramatic increase in oxygen saturation in venules reflects oxygen flow from the choroid and the unusual vascular anatomy and oxygen physiology of the eye.

Venous oxygen saturation is reduced and variable in central retinal vein occlusion.

PURPOSE: To estimate the presence and variability of retinal hypoxia in patients with central retinal vein occlusion (CRVO). METHOD: Hemoglobin oxygen saturation was measured in retinal vessels of both eyes in 14 patients with unilateral CRVO. The noninvasive spectrophotometric retinal oximeter is based on a fundus camera and simultaneously captures two images at 570 nm and 600 nm wavelengths. Five of the patients were followed with repeated retinal oximetry images over time. RESULTS: The mean oxygen saturation in retinal venules was 31 % ±12 % in CRVO eyes and 52 % ±11 % in unaffected fellow eyes (mean ±SD, n = 14, p < 0.0001). The arteriovenous difference was 63 % ±11 % in eyes with CRVO and 43 % ±7 % in fellow eyes (p < 0.0001). The variability of retinal venous oxygen saturation was substantial within and between eyes affected by CRVO. Venular oxygen saturation improved with treatment and over time in all five patients that were followed. CONCLUSION: CRVO eyes are hypoxic compared to fellow eyes and arteriovenous difference in hemoglobin oxygen saturation is increased. This is consistent with tissue hypoxia resulting from reduced blood flow. Further studies are needed to understand the correlation between hypoxia, severity of disease and prognosis.