Erythrocyte-Mediated Angiography: Quantifying Absolute Episcleral Blood Flow in Humans

The episcleral vasculature drains aqueous humor after the conventional pathway and has reported value in assessing the function of the aqueous humor outflow pathway. The visibility of episcleral vessels on the surface of the eye together with the pulsatile nature of aqueous humor synchronous with the cardiac cycle permits a unique opportunity for evaluating episcleral distal outflow. 1 Johnstone M. Martin E. Jamil A. Pulsatile flow into the aqueous veins: manifestations in normal and glaucomatous eyes. Exp Eye Res. 2011; 92: 318-327 Crossref PubMed Scopus (61) Google Scholar ,2 Carreon T. Van der merwe E. Fellman R.L. et al. Aqueous outflow—a continuum from trabecular meshwork to episcleral veins. Prog Retin Eye Res. 2017; 57: 108-133 Crossref PubMed Scopus (97) Google Scholar Over the last 70 years, numerous approaches have been pursued for measuring episcleral vessel flow rates. However, prior techniques either have been too invasive for human use or have been noninvasive but unable to provide objective and reproducible measurements. Consequently, episcleral distal outflow with respect to normal range, physiologic change, and abnormal variation in glaucoma is unclear; its purported clinical usefulness for guiding glaucoma management remains inconclusive. Accurate and precise measurements of episcleral vessel flow rates potentially can provide valuable insight into the physiologic features of aqueous humor outflow. 1 Johnstone M. Martin E. Jamil A. Pulsatile flow into the aqueous veins: manifestations in normal and glaucomatous eyes. Exp Eye Res. 2011; 92: 318-327 Crossref PubMed Scopus (61) Google Scholar ,2 Carreon T. Van der merwe E. Fellman R.L. et al. Aqueous outflow—a continuum from trabecular meshwork to episcleral veins. Prog Retin Eye Res. 2017; 57: 108-133 Crossref PubMed Scopus (97) Google Scholar