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Tasneem Z Khatib, Paul AR Meyer, Jed Lusthaus, Keith R Martin; In vivo imaging of human aqueous outflow and calculation of aqueous column diameter. Invest. Ophthalmol. Vis. Sci. 2017;58(8):2097.
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© ARVO (1962-2015); The Authors (2016-present)
Aqueous drains from the eye into the episcleral venous circulation, giving rise to aqueous veins. The importance of this relationship is evidenced by multiple disease processes in which raised episcleral venous pressure and glaucoma are associated. In this study, we used haemoglobin video imaging (HVI), a non-invasive technique that enables detailed dynamic examination of microvascular rheology in the episcleral circulation, to study aqueous outflow in vivo.
We used HVI to perform an observational study on 30 eyes. Images were captured using a monochromatic Prosilica GC1380H camera attached to a Zeiss slit lamp with a bandpass filter of wavelength 520-600nm. Quantification was undertaken on 9 of these eyes (4-10 images per eye). The images were stabilised in MATLAB using both rigid and affine image registration to offset saccadic eye movement. We developed a mathematic model of the relationship between density profiles of vein transepts (figure 1) and the diameter (hence, cross-sectional area - CSA) of a central aqueous stream (AQC). This corresponded with the separation of intensity troughs (δ), measured immediately up-stream of a vessel confluence (figure 2).
The HVI technique demonstrates aqueous as an erythrocyte void in episcleral venous blood. Aqueous tended to centralise within a laminar venous column, regardless of its point of entry into the episcleral circulation. The length of aqueous streams varied, but some continued beyond the conjunctival reflection. Fluctuations arose in the aqueous stream, corresponding with cardiac rhythm, eye movements and digital pressure on the globe.The CSA calculations arising from δ were consistent and repeatable for each eye measured. The mean CSA varied widely in different aqueous veins: from 53.1px2 to 507.4px2. The standard error of δ ranged from 0.16 to 0.45.
We have used HVI to develop a method for the detailed observation and quantification of aqueous columns in episcleral venous blood. We are currently using this method to explore the relationship between AQC area, episcleral venous pressure and IOP, and the effect of glaucoma medications and selective laser trabeculoplasty on these variables.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Figure 1: Pixel transept across aqueous vein.
Figure 2: Measurement of δ during stable laminar flow
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