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    Studying aqueous humour outflow with aqueous angiography

    With aqueous angiographic information, individualised MIGS placement may become possible

    Dr Alex Huang, a glaucoma clinician scientist at the Doheny Eye Institute and Department of Ophthalmology of the David Geffen School of Medicine at UCLA in Los Angeles, United States, described the new method, “aqueous angiography”, as a “combination of current clinical methods used daily in retina offices, synergised with prior basic science approaches to study aqueous humour outflow” (see Figure 1).Figure 1. Aqueous Angiography was performed with indocyanine green in the right eye of a 74 year-old Indian male undergoing cataract surgery. The AC maintainer delivering the tracer was placed temporal (asterisk). Most aqueous angiographic signal was seen nasal (green arrow) with less signal seen elsewhere (red arrow). S = superior, N = nasal, I = inferior and T = temporal.

    Currently, as a part of routine retina clinical care, patients undergo intravenous fluorescent angiography (IFA), where fluorescent tracers (either fluorescein or indocyanine green [ICG]) are introduced into a peripheral vein in the arm that are then distributed throughout the body via blood vessels1. Using angiographic cameras that visualise the dyes, normal and pathological retinal blood flow, such as blockage or leakage, can be observed. 

    In the laboratory, investigators have introduced fluorescent beads into anterior chambers of cultured human anterior segments or rodent eyes for years2-4. These beads deposit in the trabecular meshwork in a segmental fashion with greater accumulation implying increased local aqueous humour outflow.

    In aqueous angiography5-10, the goal was to visualise aqueous humour outflow using clinically compatible tools. 

    “We melded clinical IFA with glaucoma outflow laboratory techniques,” Dr Huang said. “The starting point was modifying the same angiographic camera and tracers used to evaluate retinal blood flow to assess aqueous humour outflow.”

    In this case, the Spectralis (Heidelberg Engineering) served as the angiographic camera, and a 55° lens was used for a greater angiographic field of view, or the anterior-segment optical coherence tomography (OCT) lens for OCT structural determination of aqueous humour outflow pathways.

    To image live animals or human patients in a supine position, a prototype FLEX module that situates this camera was developed, so that imaging could be undertaken in any body position (see Figure 2).Figure 2. The FLEX module from Heidelberg Engineering fully integrates the Spectralis HRA+OCT onto a surgical boom arm. Multiple pivot joints allow positioning of the camera head for imaging (OCT or angiography) in any body position (supine, prone or tilt/diagonal). Image provided courtesy of Heidelberg Engineering.

    To deliver the tracer, an anterior chamber maintainer (familiar to most anterior segment surgeons) was placed. Fluorescein or ICG was then delivered into the anterior chamber in a similar manner to how trypan blue is administered. Using the angiographic camera, aqueous angiographic images were acquired.    

    Like trypan blue, both fluorescein and ICG can also be used as capsular dyes11to facilitate the capsulorhexis of cataract surgery. Therefore, aqueous angiography could be conducted immediately prior to cataract surgery to simultaneously aid the cataract surgery in addition to providing angiographic information about individual flow patterns of patients’ eyes.

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