Hemoglobin Video Imaging Provides Novel In Vivo High-Resolution Imaging and Quantification of Human Aqueous Outflow in Patients with Glaucoma

Abstract

Purpose: Noninvasive, detailed measurement of the dynamics of human aqueous outflow is difficult to achieve with currently available clinical tools. We used hemoglobin video imaging (HVI) to develop a technique to image and quantify human aqueous outflow noninvasively 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.

Participants: Patients were recruited from the Cambridge University Hospitals NHS Foundation Trust 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: Participants underwent HVI alongside their usual clinic visit.

Main outcome measures: The change in cross-sectional area (CSA) of the aqueous column within episcleral veins was correlated with intraocular pressure (IOP) reduction and change in visual field mean deviation (MD) 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: Hemoglobin video imaging enables the direct observation of aqueous flow into the vascular system. Aqueous is seen to centralize 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 aqueous column after 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 MD observed postintervention (n = 8; Pearson's correlation coefficient 0.75; P = 0.03). Autocorrelation analysis demonstrated a faster rate of decay in an aqueous vein after intervention, indicating an increase in flow rate.

Conclusions: Hemoglobin video imaging 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.

Keywords: CSA, cross-sectional area; HVI, hemoglobin video imaging; IOP, intraocular pressure; MD, mean deviation; MIGS, minimally invasive glaucoma surgery; SLT, selective laser trabeculoplasty.

Figure 1

Aqueous vein (arrow) captured using conventional techniques (A and B) and hemoglobin video imaging (HVI) (C).

Figure 2

Examples of aqueous veins obtained using hemoglobin video imaging (HVI) (white arrows). Aqueous is seen as a centralized erythrocyte void.

Figure 3

Displacement of aqueous after digital pressure on the inferior globe. A, Aqueous vein (black arrow) before digital manipulation. B and C, Aqueous is redirected into an episcleral blood filled vessel after digital pressure on the globe (white arrow). D, Immediate resumption of usual aqueous and blood flow after release of pressure.

Figure 4

Compression of an aqueous vein (white arrow) using a 10/0 Vicryl loop redirects aqueous to a nearby episcleral blood vessel (black arrow).

Figure 5

A, Schematic representation of the intensity profiles of transmitted light in an aqueous vein using hemoglobin video imaging (HVI). B and C, Aqueous vein transept with corresponding density profile and δ measurement. Scale bar = 0.5 mm. D, Bland–Altman plot of the difference in paired δ measurements using HVI against the mean δ measurement.

Figure 6

Aqueous column as a tool for quantifying aqueous outflow. A, Fold change in aqueous column cross-sectional area (CSA) after intervention (n = 13; P < 0.05; Student ratio paired t test). B, Correlation between IOP reduction and aqueous column CSA after intervention (n = 13; Pearson’s correlation coefficient 0.7; P = 0.007). C, Correlation between change in mean deviation (MD) and aqueous column CSA after intervention (n = 8; Pearson’s correlation coefficient 0.75; P = 0.03). AQC = aqueous column; IOP = intraocular pressure; SLT = selective laser trabeculoplasty; VF = visual field.

Figure 7

Flow rate using autocorrelation analysis before and after selective laser trabeculoplasty (SLT). A, Faster rate of decay is seen postintervention indicating an increase in flow rate. B, Similar autocorrelation decay rates seen in nonaqueous vein or background areas of the hemoglobin video imaging (HVI) images.