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

Abstract

PurposeNoninvasive, 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.DesignA prospective observational study to describe characteristics of aqueous veins and a pilot prospective interventional feasibility study to develop quantification parameters.ParticipantsPatients 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.MethodsParticipants underwent HVI alongside their usual clinic visit.Main Outcome MeasuresThe 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.ResultsHemoglobin 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.ConclusionsHemoglobin 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.

Effects of Schlemm’s Canal Expansion: Biomechanics and MIGS Implications

“…For analysis of the 2D images, the boundary between SC and CC was semi-automatically delineated. ImageJ software then provided delineation of the SC and CC area [ 6 ]. The total area of SC within a 2 mm length was automatically calculated from the 3D images.…”

“…Its morphology and behavior suggests that it is a component of a lymphatic-like pump that regulates IOP [ 5 ]. Many studies and multiple lines of evidence document that flow to the aqueous veins is pulsatile and that the origin of the aqueous inducing pulsatile flow is SC and the distal outflow channels [ 6 , 7 ]. Especially compelling evidence is that pulsatile aqueous in the aqueous veins increases when access to the episcleral veins is blocked in normal veins [ 8 , 9 ].…”

Aqueous outflow imaging techniques and what they tell us about intraocular pressure regulation

“…Until recently, imaging of conventional aqueous outflow pathways has required the introduction of a marker into the anterior chamber. However, Haemoglobin Video Imaging (HVI), a slit-lamp technique originally developed to investigate limbal microvascular blood flow 86 , can demonstrate these pathways without the need for more invasive methods 86,87 .…”

“…7B), a number of aqueous vein characteristics previously reported by Ascher 1 have been demonstrated. 87,90 These include: pulsatile flow corresponding with the heart rate, parallel stratification of flowing blood and aqueous, disruption of the aqueous stream during eye movements and gentle globe compression, and redirection of aqueous flow into adjacent episcleral vessels following localised aqueous vein compression.…”

“…Computer modelling of the intensity of light absorbed by transepts of aqueous veins demonstrates that the transition between blood and aqueous corresponds with the transmission minima. Therefore, from light intensity transepts (ImageJ -open source software), it is possible to calculate aqueous column cross-section area (AqCA), a statistic presumed to be proportional to bulk flow in the cylindrical aqueous column 87,90 (Fig. 9B,C).…”