Non-invasive in vivo imaging and quantification of human aqueous outflow

Abstract

Purpose : Non-invasive, detailed measurement of the dynamics of human aqueous outflow is difficult to achieve with currently available clinical tools. The advent of minimally invasive glaucoma surgery (MIGS) procedures has led to renewed interest in the dynamics of aqueous outflow. Targeting trabecular bypass stents to ocular quadrants with good aqueous outflow could improve the success rates of these procedures. Here we used haemoglobin video imaging (HVI) develop a technique to image and quantify human aqueous outflow non-invasively and in real time.

Methods : HVI utilises the haemoglobin absorption spectrum to enhance the contrast between red blood cells and aqueous in the episcleral circulation. We measured the cross sectional area (CSA) of the aqueous column (AQC) within episcleral veins in glaucoma patients who underwent selective laser trabeculoplasty (SLT) and correlated the change in CSA with IOP reduction and change in visual field mean deviation before and after intervention. We also calculated the fluctuations in contrast and pixel intensity of red blood cells in an aqueous vein to compare the rate of flow before and after SLT using autocorrelation analysis.

Results : There was a significant increase in the AQC immediately following SLT (n=13; p<0.05) and this correlated to the degree of IOP reduction observed (n=13; Pearson’s correlation coefficient 0.7; p=0.007) as well as the improvement in mean deviation (MD) observed following SLT (n=8; Pearson’s correlation coefficient 0.75; p=0.03). The autocorrelation analysis demonstrated a faster rate of decay in an aqueous vein following SLT indicating an increase in flow rate.

Conclusions : HVI can be incorporated into a routine clinic slit lamp examination to allow a detailed assessment and quantification of physiological and pathological aqueous outflow in real time. We suggest that HVI has the potential to be used as a tool to help target therapeutic interventions to improve aqueous outflow and to further advance our understanding of aqueous outflow dysregulation in the pathogenesis of glaucoma.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.