by Dr Marcus Ang
Optical coherence tomography (OCT) is now a common imaging technique used in our ophthalmology practice, especially to evaluate anterior segment and corneal conditions.1 Recently, we were the first to describe the adaptation of OCT technology to provide static angiography images for the cornea and anterior segment.2 Angiography for the anterior segment has a variety of clinical applications, such as evaluation of corneoscleral inflammatory disorders,3, 4 to the assessment of abnormal corneal vascularisation from limbal stem cell deficiency.5 However, current assessment of the anterior segment vasculature is constrained to angiography techniques using fluorescein or indocyanine green (ICG).6 These invasive angiography techniques expose patients to potential and serious adverse reactions;6 while significant time and preparation is required before each ICGA or FA imaging session.7, 8 Thus, imaging and evaluation of corneal vascularisation has been limited despite its prevalence and potential sight-threatening effects.9, 10
Therefore, there may be an increasing role for non-contact optical coherence tomography angiography (OCTA) for the anterior segment, which is rapidly obtained and easy to perform.11 While the split-spectrum amplitude-decorrelation angiography system has been most commonly described for the anterior segment, we have also described the use of other spectral domain and swept source OCTA systems successfully adapted for the anterior segment.12 It is important to note that current OCTA systems are not specifically designed for the anterior segment but may be adapted to assess the cornea or iris vasculature.2 Thus, there are several limitations such as the inability to demonstrate vessel leakage, and a limited field of view compared to the FA and ICGA.13 Nonetheless, we have reported that the OCTA adapted for the cornea was comparable to ICGA for measurement of the area of corneal vascularisation in our pilot clinical study.14
There are a wide variety of potential clinical applications for delineating the vasculature of the cornea and anterior segment.15 These include potential roles in the assessment of corneal transplant vascularisation for risk of graft rejection,16 evaluation of anti-angiogenic treatments for corneal vascularisation,17 studying limbal stem cell deficiency,18 or even glaucoma drainage bleb vascularity.19 Similar to the OCTA for the retina and posterior segment, there are several points to note when interpreting OCTA scans for the anterior segment. Image distortions may occur due to micro-saccadic movements of the eye, patient movement, or variations of the OCTA scanning plane relative due to the corneal surface. Fortunately, as each non-contact scan only requires 3-4 seconds to complete, patients are usually able to tolerate multiple scans to ensure a good quality image is achieved. Moreover, image artifacts may occur in areas of dense scarring, and further affected by the coronal 3-D reconstruction of images. Future improvements to the software and optimization for the anterior segment may further reduced such artifacts and improve the image resolution.20 Lastly, the OCTA systems for the anterior segment used do not come with an in-built motion correction and has no eye-tracking system with registration, which is required for comparisons with repeated scans. Nonetheless, with the help of adjunct image analysis software, it has been found to be potentially useful for serial scans and follow-up in various clinical indications.21
- Ang M, Chong W, Tay WT, et al. Anterior segment optical coherence tomography study of the cornea and anterior segment in adult ethnic South Asian Indian eyes. Invest Ophthalmol Vis Sci 2012;53:120-5.
- Ang M, Sim DA, Keane PA, et al. Optical Coherence Tomography Angiography for Anterior Segment Vasculature Imaging. Ophthalmology 2015;122:1740-7.
- Watson PG, Bovey E. Anterior segment fluorescein angiography in the diagnosis of scleral inflammation. Ophthalmology 1985;92:1-11.
- Watson PG. Anterior segment fluorescein angiography in the surgery of immunologically induced corneal and scleral destructive disorders. Ophthalmology 1987;94:1452-64.
- Spiteri N, Romano V, Zheng Y, et al. Corneal angiography for guiding and evaluating fine-needle diathermy treatment of corneal neovascularization. Ophthalmology 2015;122:1079-84.
- Kirwan RP, Zheng Y, Tey A, et al. Quantifying changes in corneal neovascularization using fluorescein and indocyanine green angiography. Am J Ophthalmol 2012;154:850-8 e2.
- Kwiterovich KA, Maguire MG, Murphy RP, et al. Frequency of adverse systemic reactions after fluorescein angiography. Results of a prospective study. Ophthalmology 1991;98:1139-42.
- Stanga PE, Lim JI, Hamilton P. Indocyanine green angiography in chorioretinal diseases: indications and interpretation: an evidence-based update. Ophthalmology 2003;110:15-21; quiz 2-3.
- Lee P, Wang CC, Adamis AP. Ocular neovascularization: an epidemiologic review. Surv Ophthalmol 1998;43:245-69.
- Resnikoff S, Pascolini D, Etya'ale D, et al. Global data on visual impairment in the year 2002. Bull World Health Organ 2004;82:844-51.
- Ang M, Cai Y, Shahipasand S, et al. En face optical coherence tomography angiography for corneal neovascularisation. Br J Ophthalmol 2015.
- Ang M, Cai Y, Tan AC. Swept Source Optical Coherence Tomography Angiography for Contact Lens-Related Corneal Vascularization. J Ophthalmol 2016;2016:9685297.
- Ang M, Cai Y, MacPhee B, et al. Optical coherence tomography angiography and indocyanine green angiography for corneal vascularisation. Br J Ophthalmol 2016.
- Ang M, Cai Y, Shahipasand S, et al. En face optical coherence tomography angiography for corneal neovascularisation. Br J Ophthalmol 2016;100:616-21.
- Ang M, Sng C, Milea D. Optical coherence tomography angiography in dural carotid-cavernous sinus fistula. BMC Ophthalmol 2016;16:93.
- Bachmann B, Taylor RS, Cursiefen C. Corneal neovascularization as a risk factor for graft failure and rejection after keratoplasty: an evidence-based meta-analysis. Ophthalmology 2010;117:1300-5 e7.
- Chang JH, Gabison EE, Kato T, Azar DT. Corneal neovascularization. Curr Opin Ophthalmol 2001;12:242-9.
- Kim YJ, Yoo SH, Chung JK. Reconstruction of the limbal vasculature after limbal-conjunctival autograft transplantation in pterygium surgery: an angiography study. Invest Ophthalmol Vis Sci 2014;55:7925-33.
- Sng CC, Singh M, Chew PT, et al. Quantitative assessment of changes in trabeculectomy blebs after laser suture lysis using anterior segment coherence tomography. J Glaucoma 2012;21:313-7.
- Girard MJ, Ang M, Chung CW, et al. Enhancement of Corneal Visibility in Optical Coherence Tomography Images Using Corneal Adaptive Compensation. Transl Vis Sci Technol 2015;4:3.
- Cai Y, Alio Del Barrio JL, Wilkins MR, Ang M. Serial optical coherence tomography angiography for corneal vascularization. Graefes Arch Clin Exp Ophthalmol 2017;255:135-9.
About the Author
Dr Marcus Ang, MBBS, MMED, MCI, FRCS is a Consultant, Cornea Service, SNEC and Asst. Professor at DUKE University - NUS. He has currently more than 90 peer-reviewed publications (H-index=21), majority of which are first or corresponding author with JIF>2.0, focusing on translational research, such as corneal transplantation studying prognostic factors to improve outcomes. He has published a few book chapters, including one in the prestigious CORNEA. He has filed for several patents for ophthalmic devices and awarded several national grants to study cornea imaging and innovation. His research work has also been recognized by international awards and is a regular invited speaker at international conferences. He is a Clinical Lecturer at the National University of Singapore School of Medicine, teaching and mentoring undergraduates, post-graduates, and Residents. He has several past and present leadership positions, including Chair of the Ophthalmology Pharmacy & Therapeutics at the Singapore National Eye Center and he has just graduated from the Leadership Development Program at the Asia Pacific Academy of Ophthalmology. He is also heavily committed to charity work in the Ophthalmology field, running community projects with the Singapore Society of Ophthalmology. As Director of Global Clinic (www.global-clinic.org), he regularly organizes missions and travels to provide free cataract surgery in countries such as Indonesia, Thailand, Cambodia, India and Myanmar.