Hypoxia in human BRVO improves after laser.
Studies with non-invasive automatic spectrophotometric retinal oximetry
Pool-lecture
Einar Stefánsson¹, S. H. Hardarson¹,
R. A. Karlsson¹, G. H. Halldorsson¹,
T. Eysteinsson¹, J. A. Benediktsson¹,
A. Thorsteinsson¹, P. Koch Jensen², J. Beach¹
(¹1Reykjavik, ²Copenhagen)

Purpose: We have developed a non-invasive automatic spectrophotometric retinal oximeter to measure hemoglobin oxygen saturation (SO2) in retinal vessels in health and disease. The oximeter is based on a fundus camera and is as easy and safe to use as a regular fundus camera. We test the reproducibility and sensitivity of the oximeter and study SO2 of retinal venules
in eyes with branch retinal vein occlusion (BRVO) before and after laser photocoagulation.
Material and methods: The oximeter is a fundus camera coupled with a beam splitter to a digital camera. In each channel there is a narrow band-pass filter, through which only light of specific wavelenghts (586 and 605 nm) can pass. Optical density ratio (ODR) between wavelengths can be calculated for every vessel. The ODR has an inverse and linear relationship to hemoglobin SO2. The computer automatically locates vessels on the fundus image and selects measurement points inside and outside vessels and calculates the mean ODR for each segment. The automatic software displays a color coded map of the vessels, where the colors indicate SO2.
Results: Reproducibility and sensitivity: The reproducibility was evaluated by analyzing 5 repeated measurements of the same vessels. The mean standard deviation for repeated measurements was 3.7% for arterioles and 5.3% venules in terms of %SO2 (5 repeats, 10 individuals). The arterial SO2 measured 96±9% (mean±SD) during normoxia and 101±8% during hyperoxia (n=16, p=0.0027, paired t-test). Corresponding numbers for venules were 55±14% and 78±15% (p<0.0001). Branch retinal vein occlusion: Seven patients with BRVO have had oximetry to date. The venular SO2 was 59±6% in healthy
fellow eyes (mean±SD; n=5), 60±12% in non-occluded areas of BRVO eyes (n=4), 39±23% before laser in occluded venules (n=5) and 56±4% in occluded venules following laser treatment (n=5). Before laser the occluded venules had a significantly lower SO2 than healthy eyes or non-occluded areas of BRVO eyes (p<0.05) and the SO2 was normalized by laser treatment.
Conclusions: The retinal oximeter is reliable, safe and easy to use and is sensitive to changes in SO2 when concentration of O2 in inhaled air is changed. The automatic non-invasive retinal oximeter demonstrates decreased SO2 in occluded retinal venules compared with healthy areas of the BRVO eyes and healthy eyes. Laser treatment in BRVO brings the SO2 back to
normal values.

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