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A. J. Sit, M. Malihi, J. W. McLaren; Image Analysis Techniques for Episcleral Venous Pressure Measurement Using a Computerized Episcleral Venomanometer. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3498.
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Episcleral venous pressure (EVP) is critical in understanding aqueous humor dynamics. EVP can be measured by applying an external force to an episcleral vein, observing the vein collapse, and inferring venous pressure based on the force required to partially compress the vein to a pre-defined endpoint. We recently developed a method of measuring of EVP by using a computerized venomanometer that allows objective assessment of venous collapse, potentially producing much more reliable EVP measurements. With this system, video images of the vein were synchronized with increasing known pressure throughout the venous compression. In this study, we compared EVP measurements that used several metrics and endpoints of vessel compression.
EVP was measured in 22 eyes of 11 healthy subjects by using the computerized venomanometer. Image intensity profiles were calculated for an episcleral vein in each video frame by using custom software. Four metrics from the cross-sectional vessel profile were analyzed for the degree of vessel compression: 1) Peak - difference between background and minimum brightness; 2) Width of the profile - width at half of the peak; 3) Total Area - sum of differences between intensity and background across vessel; and 4) Bounded Area - area bounded by the Width. Three endpoints were assessed for each metric: 100%, 90%, and 50% of the baseline. Metrics and endpoints were compared by using paired t-tests with Bonferroni correction.
Results are shown in the Table. EVP determined by Peak was different from EVP determined by Bounded Area for all endpoints, and EVP determined by Peak was different from EVP by Total Area for the 50% endpoint (P<0.001). EVP was significantly different between the 3 endpoints for all metrics (P<0.001).
Measurement of EVP varies significantly depending on the metric and endpoint used. Since EVP is an important component of aqueous humor dynamics, it will be vital to determine the metric and endpoint that reflects the true steady state EVP.
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