Optical Coherence Tomography Angiography
The potential role of the microvasculature and blood flow in the pathophysiology of glaucoma has been extensively investigated and debated. Studies have demonstrated reduced ocular blood flow in the ONH, retina, choroid, and retrobulbar circulations in eyes with glaucoma. These changes may simply be a consequence of glaucomatous damage to the RGCs and may have no predictive value. The lack of a reproducible and relevant in vivo quantitative assessment method has limited the study of both ocular perfusion and microvascular networks as potential predictors of future vision loss. Optical coherence tomography angiography (OCTA) is a new imaging modality that can be used to characterize vasculature in various retinal layers, providing quantitative assessment of the microcirculation in the ONH, peripapillary region, and macula.
The principle of imaging in OCTA is to contrast blood vessels from static tissue by assessing the change in the OCT signal caused by flowing blood cells. OCTA compares the differences in the backscattered OCT signal intensity or amplitude between sequential OCT B scans taken at precisely the same cross section in order to construct a map of blood flow. Axial bulk motion from patient movement is eliminated; sites of motion between repeated OCT B scans are thought to represent erythrocyte movement in retinal blood vessels.
The different OCTA parameters include vessel density, corresponding to the percentage of detected vessel area over the imaged area, and flow index, corresponding to a dimensionless parameter between 0 and 1 representing the average signal. Note that these indices are surrogate measures and that their validity for measurement of blood flow remains to be investigated. Studies have shown significant differences in vessel density and flow index in the peripapillary and macular regions in eyes of glaucoma patients versus those of healthy subjects. Also, these indices have shown significant association with measures of visual field loss. Figure 5-20 shows OCTA scans of a healthy eye and eyes of glaucoma patients with different levels of damage.
Figure 5-20 Optical coherence tomography angiography (OCTA) scans of a healthy eye and glaucomatous eyes with mild, moderate, and severe damage. The OCTA scans show a dense peripapillary vessel network in the healthy eye, illustrated in red and yellow in the color-coded map. Glaucomatous damage causes a decrease in vessel damage and replacement of the red/yellow areas with blue, indicating either absence of vessels or no flow.
(Courtesy of Yarmohammadi A, Zangwill LM, Diniz-Filho A, et al. Relationship between optical coherence tomography angiography vessel density and severity of visual field loss in glaucoma. Ophthalmology. 2016;123(12):2498–2508.)
Motion artifacts and projection artifacts are common in OCTA imaging, and a considerable proportion of OCTA images remain suboptimal in quality for interpretation. Poor-quality OCTA scans are more common than poor-quality OCT scans. It remains to be determined whether OCTA will be able to provide additional information that can assist in diagnosis and detection of glaucoma progression beyond what can currently be obtained with OCT.
Jia Y, Wei E, Wang X, et al. Optical coherence tomography angiography of optic disc perfusion in glaucoma. Ophthalmology. 2014;121(7):1322–1332.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.