Optical coherence tomography angiography, or OCT-A, is the (relatively) new kid on the OCT block, and it’s starting to attract buzz in surprising places.
OCT-A tracks blood flow in various layers of the retina to produce three dimensional scans of the microvasculature in the back of the eye. Unsurprisingly, a cascade of research has followed its debut, showcasing the technology’s significant promise in screening for, managing, understanding and treating a host of retinal diseases.
Glaucoma, then, might seem like a surprising place for OCT-A to show up. But Dr. Jacqueline Chua, an associate professor at Duke-NUS Medical School and clinician scientist specialized in ocular imaging at the prestigious Singapore Eye Research Institute, believes that OCT-A has breakthrough potential for the disease.
Dr. Chua has authored a recent review1 summarizing just why this might be the case, and she commented for this publication on the body of evidence for glaucoma and its implications for the future of the disease.
“Currently, OCT-A is positioned as a promising and valuable tool in the diagnosis and monitoring of glaucoma,” she wrote.
“It offers reliable differentiation between glaucomatous and healthy eyes, with good repeatability and reproducibility. The technology has advanced to the point where it provides valuable insights, particularly in advanced glaucoma cases.”
Providing access to powerful biomarkers
Summarizing the recent findings across the eye care world, Dr. Chua explained, “OCT-A plays a crucial role in the glaucoma treatment journey by offering a non-invasive and high-resolution assessment of retinal vasculature.
“It aids in early detection and provides insights for treatment planning and extended monitoring, particularly in advanced stages where traditional methods face limitations.”
At the root of OCT-A’s utility in glaucoma is its ability to distinguish a number of powerful biomarkers. Among these are optic nerve head and peripapillary vessel density, macular vessel density, and a variety of choroidal parameters, like choroidal microvascular dropout.
Notably, OCT-A scans of glaucoma patients show reductions of blood vessel profusion around the optic disc and macula in the superficial layers and capillary loss at the choroidal level – biomarkers that line up well with OCT-A structural scans and visual field maps.2
Dr. Chua also believes that OCT-A has particular advantages for advanced glaucoma cases. Unlike currently-tracked changes in cup-to-disc ratio and nerve fiber layer thinning, density loss in retinal microvasculature correlates with disease progression through moderate and even advanced stages.
This all contributes to OCT-A overcoming what Dr. Chua terms the ‘floor effect’ with current structural measurements. “Unlike OCT metrics, vessel density does not exhibit a measurable floor value, providing a potential avenue for continued monitoring in advanced cases.”
Machines of the future
Dr. Chua sees such research revelations about OCT-A as a glaucoma game changer. “OCT-A is likely to be seamlessly integrated with structural measurements, providing a comprehensive understanding of both vascular and structural changes in the retina.
“This holistic approach can offer a more complete picture of glaucoma pathology and aid in treatment decision-making,” she said.
For Dr. Chua, the future of OCT-A in glaucoma is all about breaking through limitations in current standard-of-care imaging. “The ability of OCT-A to overcome the ‘floor effect’ in advanced glaucoma, particularly in monitoring perifoveal vessel density, could enhance the monitoring of disease progression.
“This continuous monitoring may provide valuable data for assessing the effectiveness of treatments and making timely adjustments.”
The process of integration is already under way. Doctors can now analyze structural and microvascular changes side-by-side to make a surer diagnosis. It’s not an easy art to master, however.
“[Eye care practitioners] should be familiar with the capabilities and limitations of OCT-A in glaucoma assessment. Proficiency in interpreting vessel density metrics and considering the impact of various factors on OCT-A measurements is essential for their effective use in future clinical practice.”
No matter how adept the clinician, manual comparison is a laborious process. The solution, says Dr. Chua, lies in the promise of artificial intelligence and machine learning.3
“Artificial Intelligence (AI) is anticipated to play a significant role in enhancing the capabilities of OCT-A in glaucoma assessment. AI’s expected role is the analysis of OCT-A multilayer images, enabling efficient processing of large datasets,” she explained.
“This is especially beneficial in clinical settings where time is a critical factor. Automation reduces the workload on clinicians, allowing for quicker and more standardized evaluations, leading to improved efficiency in glaucoma assessment,” added Dr. Chua.
Present challenges
Despite the promise of OCT-A in glaucoma, however, it’s not a plug-and-play solution – yet. Obstacles slowing full implementation of OCT-A fall under two headings: technical and regulatory.
Visual artifacts are one common tech complaint. Onboard software is designed to compensate for routine imaging villains like patient motion. However, eye blinks, vitreous floaters and overlying blood vessels can still cast shadows on the final scan and the reliability of the resulting images.
Segmentation errors – discrepancies in measured versus observed scan depth – are another common problem. According to Dr. Chua, “Providing comprehensive training to operators and clinicians on recognizing artifacts and emphasizing the importance of obtaining high-quality scans can minimize their impact.”
Segmentation errors are not always the fault of the machine or the operator, however. OCT manufacturers each have their own scheme for demarcating and labeling the various retinal layers–thus there is no standard roadmap to the retina.4
Confidentiality concerns present another regulatory hurdle. The inability to create large-scale scan databases for research purposes hampers both demographic studies and effective AI training.
Recent innovations in imaging and diagnostics (i.e. retinal fluid monitors, polygenic risk assessment for glaucoma) have shown that scientists and regulators can develop diagnostic software algorithms based on large sets of patient data without compromising privacy and security. Given the immense potential for OCT-A, it’s only a matter of time before these retinal scans will be similarly accessible.
Dr. Chua argues that cooperation and standardization are key on these issues: “One potential strategy is establishing consensus guidelines and standards for OCT-A imaging protocols, including acquisition parameters, scan patterns and device-specific settings. Collaboration among manufacturers, researchers and regulatory bodies can contribute to standardization efforts.”
References:
- Hong J, Tan SS, Chua J. Optical coherence tomography angiography in glaucoma. Clin Exp Optom. 2024:1-12.
- Rao HL, Pradhan ZS, Suh MH, Moghimi S, Mansouri K, Weinreb RN. Optical Coherence Tomography Angiography in Glaucoma. J Glaucoma. 2020;29(4):312-321.
- Li C, Chua J, Schwarzhans F, et al. Assessing the external validity of machine learning-based detection of glaucoma. Sci Rep. 2023;13(1):558.
- Patel PJ, Chen FK, da Cruz L, Tufail A. Segmentation error in Stratus optical coherence tomography for neovascular age-related macular degeneration. Invest Ophthalmol Vis Sci. 2009;50(1):399-404.