Studying aqueous humour outflow with aqueous angiography

Dr Alex HuangA pioneering dye-based technique that enables aqueous humour outflow to be studied has the potential to improve glaucoma angle-based procedures and surgery outcomes.

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 years^2-4. These beads deposit in the trabecular meshwork in a segmental fashion with greater accumulation implying increased local aqueous humour outflow.

In aqueous angiography^5-10, 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.

A family of anterior segment angiographic methods

There are currently only a handful of anterior segment angiographic methods besides aqueous angiography. Dr Huang explained that scleral angiography and iris angiography are similar to retinal IFA in that tracers are placed into the peripheral vein to observe blood flow, in this case for the anterior segment. 

More specifically, iris angiography is useful for visualising neovascular membranes in the anterior segment, which are often associated with neovascular diseases such as diabetes. Scleral angiography has been reported to be useful in evaluating scleral ischaemia in scleritis¹².  Alternatively, Dr Huang’s group has used scleral angiography to visualise abnormal blood vessels in anterior segment ocular tumours¹³.

For anterior segment aqueous humour outflow, canalography is another angiographic method¹⁴. In canalograms, an external surgical approach is taken to access Schlemm’s canal similar to canaloplasty glaucoma surgery. Tracers are then directly delivered into the canal via a cannula or a canaloplasty fiberoptic probe.

“Unlike aqueous angiography, canalograms bypass the trabecular meshwork,” Dr Huang noted. “Therefore, the final angiographic image shows the surgeon what aqueous humour outflow looks like without the trabecular meshwork filter.”

A mathematical model including canalograms and aqueous angiography would be: canalogram – (aqueous angiography) = trabecular meshwork. In this way, studying outflow with both techniques will be important to reveal insights into the function of the trabecular meshwork in both normal and diseased eyes. 

Separately, a new and popular angiographic method is OCT angiography (OCTA)¹⁵. OCTA is advantageous as it is noninvasive and does not require tracer introduction. However, as Dr Huang stated, OCTA requires particulate motion to generate a signal.   

The blood corpuscle motion in the blood vessels is what is being visualised in retinal OCTA, and in the anterior chamber there is no equivalent structure that can be detected for aqueous humour outflow. Therefore, Dr Huang believes that OCTA is currently not yet relevant for visualising aqueous humour outflow.

Glaucoma surgery

For the glaucoma patient, trabecular-targeted minimally invasive glaucoma surgeries (MIGS) are commonly used to help lower IOP. Trabecular MIGS can undoubtedly lower IOP, but variable results hinder wider adoption. Dr Huang explained that many hypotheses exist, and one is that the surgery has to be placed in the correct location in the eye.

“Previously suspected, aqueous angiography has confirmed that aqueous humour outflow is segmental,” Dr Huang said. “In other words, outflow through the trabecular meshwork and Schlemm’s Canal is not 360° uniform radiating away from the limbus. Instead, outflow is usually (but not always) heavier in the nasal part of each eye, with every individual and every eye showing potentially different patterns.

“Early work with aqueous angiography in the lab has demonstrated that trabecular bypass in regions with lesser initial angiographic flow can be improved or rescued,” he continued. “Therefore, it is possible that targeting trabecular-MIGS toward these locations might provide greater and more predictable IOP lowering. Alternatively, placing the trabecular-MIGS next to regions of greater flow (also as assessed by aqueous angiography) in order to access these known flow pathways might be better as well. Future experiments will tell.”

Therefore, with aqueous angiographic information, individualised MIGS placement may become possible.

Future perspectives

Dr Huang notes some limitations in aqueous angiography. For example, currently, aqueous angiography with the Spectralis requires the FLEX module, which is a prototype stand only available to a handful of investigators in the world.

Furthermore, the steps involved in aqueous angiography-arranging the camera, accessing the eye, delivering the tracer and taking the images-require time. Thus, while not insurmountable, continued evolution of the method with future iterations should “streamline the process beyond current constraints and make it accessible to all ophthalmologists”.

Ultimately, future efforts should determine if aqueous angiography-guided MIGS provides better and more consistent results. If this is the case, “one could envision customised and targeted surgical placement of MIGS in each eye of every individual” for, potentially, better final IOP-lowering outcomes in the future, Dr Huang said.

Additionally, angiography may allow us to better understand both normal and diseased aqueous humour outflow. Pharmacologically, aqueous angiography may help identify new drugs for IOP lowering as well. 

Imagined to its greatest extent, aqueous angiography and individualised imaging-determined therapies supports the National Institute of Health’s goal to develop more customised patient care and personalised medicine.

References

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8.     Huang AS, et al.Ophthalmology. 2017.

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10.  Huang AS, Mohindroo C, Weinreb RN. J Clin Exp Ophthalmol. 2016;7(4).

11.  Jacobs DS, et al.Ophthalmology. 2006;113(4):707-13.

12.  Watson PG, Bovey E. Ophthalmology. 1985;92(1):1-11.

13.  Marvasti AH, et al. Case Rep Ophthalmol. 2016;7(1):30-8.

14.  Grieshaber MC, Pienaar A, Olivier J, Stegmann R. Invest Ophthalmol Vis Sci. 2010;51(3):1498-504.

15.  Akil H, et al. PLoS One 2017;12(2):e0170476.

E: AHuang@doheny.org

Dr Huang is an assistant professor, clinician-scientist and one of the founding members of the Doheny Eye Institute and University of California, Los Angeles, United States. His aqueous angiographic work is supported by Heidelberg Engineering and Glaukos Corporation.