A novel method for computerized measurement of episcleral venous pressure in humans

Abstract

Episcleral venous pressure (EVP) is an important determinant of intraocular pressure (IOP) and can be estimated by the pressure required to compress an episcleral vein. However, the lack of objective measurement endpoints makes EVP measurements in humans uncertain. To address this issue, we developed a new method to measure EVP objectively and reproducibly, and demonstrated its utility on a group of normal subjects. Our system for pressure chamber based venomanometry included a computer-controlled motor drive to increase pressure automatically, a transducer to record pressure, and a high-definition video camera to record vein collapse. Pressure measurements were synchronized with the video stream to determine the pressure required to collapse the vein to a specific pre-determined degree. This system was used to measure EVP in 10 eyes from 5 young healthy volunteers. Episcleral veins were selected in each of 4 quadrants. EVP was calculated to be the pressure in the chamber that compressed the vein by 0% (by back-projection), 10% or 50% as determined by using image analysis of the video stream. For this group of subjects, mean EVP was 6.3 ± 2.8 mmHg (mean ± SD, n = 40 measurements), 7.0 ± 2.6 mmHg, and 9.6 ± 2.6 mmHg using the 0%, 10% and 50% reduction endpoints, respectively. Pressures and standard deviations determined from these endpoints were significantly different from each other (p < 0.001). Coefficients of variation between right and left eyes were 12.7%, 10.2%, and 6.8% using the 0%, 10% and 50% endpoints, respectively. Based on previous research and theoretical considerations, the 0% endpoint is assumed to provide the most accurate estimate of baseline EVP, and can only be estimated by analyzing the brightness profiles of the vessels in the video stream. Objective measurement of EVP is important for understanding normal aqueous humor dynamics and its changes in disease states and with therapies. EVP has typically been assumed to be constant because of the lack of a convenient means of its measurement. This new method provides a precise means to assess EVP based on specific endpoints of vessel collapse, and enables, for the first time, objective and non-invasive measurements of EVP changes.