Purpose

Non-invasive, detailed measurement of the dynamics of human aqueous outflow is difficult to achieve with currently available clinical tools. Here we used haemoglobin video imaging (HVI) to develop a technique to image and quantify human aqueous outflow non-invasively and in real time. Design

A prospective observational study to describe characteristics of aqueous veins and a pilot prospective interventional feasibility study to develop quantification parameters. Subjects

Participants and/or Controls: Patients were recruited from the Addenbrookes Hospital Glaucoma clinic. The observational study included 30 eyes and the pilot interventional feasibility study was performed on 8 eyes undergoing selective laser trabeculoplasty (SLT). Our SLT protocol also included the installation of pilocarpine and apraclonidine eye drops. Methods, Intervention, or Testing

Participants underwent HVI alongside their usual clinic visit. Main Outcome Measures

The change in cross sectional area (CSA) of the aqueous column (AQC) within episcleral veins was correlated with IOP reduction and change in visual field mean deviation before and after intervention. Fluctuations in contrast and pixel intensity of red blood cells in an aqueous vein were calculated to compare the flow rate before and after intervention using autocorrelation analysis. Results

HVI enables the direct observation of aqueous flow into the vascular system. Aqueous is seen to centralise within a laminar venous column. Flow is pulsatile, and fluctuations of flow through globe pressure or compression of the aqueous vein are observed.

There was a significant increase in the AQC following the administration of our SLT protocol (n=13; p<0.05). This correlated with the degree of IOP reduction (n=13; Pearson’s correlation coefficient 0.7; p=0.007) and the improvement in mean deviation (MD) observed post intervention (n=8; Pearson’s correlation coefficient 0.75; p=0.03). Autocorrelation analysis demonstrated a faster rate of decay in an aqueous vein following intervention 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 aqueous outflow in real time. It has the potential to be used to help target therapeutic interventions to improve aqueous outflow and further advance our understanding of aqueous outflow dysregulation in the pathogenesis of glaucoma.