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Editors Selection IGR 23-4

Animal Models: Pulsatile aqueous outflow in primates

Comment by Arthur Sit on:

71574 Aqueous Angiography in Living Nonhuman Primates Shows Segmental, Pulsatile, and Dynamic Angiographic Aqueous Humor Outflow, Huang AS; Li M; Yang D et al., Ophthalmology, 2017; 124: 793-803

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Aqueous humor outflow is typically modelled as a steady-state system using the modified Goldmann equation, which describes IOP as a function of aqueous humor production rate, outflow facility, and episcleral venous pressure and uveoscleral outflow rate. However, previous investigators have demonstrated a pulsatile nature to aqueous humor outflow, which may be problematic for the Goldmann model.¹ Huang et al. investigated this issue in a study to examine the dynamic nature of aqueous humor outflow in the aqueous and episcleral veins.

As with cadaver eyes, flow was found to be segmental with regions of flow and no-flow. Using a novel technique that they have called aqueous angiography, the authors infused fluorescein or indocyanine green (ICG) into the anterior chamber of non-human primates (NHP). Fluorescent images of the outflow tract were used to capture and document the presence of aqueous humor flow. Concurrent OCT imaging was used to match the angiography images to the structural location of aqueous and episcleral veins.

As with cadaver eyes, flow was found to be segmental with regions of flow and no-flow

This technique has previously been used by this group on cadaver eyes,² and was adapted to living NHP eyes for this study. As with cadaver eyes, flow was found to be segmental with regions of flow and no-flow. However, the authors also reported pulsatile flow, and dynamic shifts where flow would stop in one region and begin in another. These results demonstrate a previously undocumented dynamic and variable nature to distal outflow.

The authors also reported pulsatile flow, and dynamic shifts where flow would stop in one region and begin in another

While the results of this study are very interesting, it was limited by the short duration (three to nine seconds) of the videos obtained. Therefore, dynamic changes could be observed, but it is unknown if these were short transient changes or longer term shifts in flow patterns. As well, since the entire eye could not be imaged in a single video, it is not known if the overall flow rate remained constant during the shifts in pattern. Nevertheless, aqueous angiography appears to be a novel technique for investigating the physiology of the distal aqueous humor outflow pathway. The use of tracers compatible with living eyes makes use of the technique in humans a possibility. Adaptation of this technique to human patients may enable better understanding of the effect of glaucoma therapies, and improved targeting of devices that alter outflow patterns, such as minimally invasive glaucoma surgeries.

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