Practical Insights on OCTA in Glaucoma

OCT angiography (OCTA) is a valuable imaging modality in the diagnosis and management of chorioretinal pathology, optic neuropathy, and glaucoma. In 2025, 92137 will be a new Category I CPT code applicable for this imaging modality; thus, brushing up on its clinical utility is beneficial. In this article, I focus on the use of OCTA in the management of glaucoma.

OCTA: BACKGROUND

OCTA images are generated by motion contrast, in which multiple OCT scans are taken at the same location over a defined period of time. The OCT signal created by the movement of red blood cells serves as a marker for the presence of blood vessels, and the resulting image provides a structural map of the microscopic vasculature of the retina, optic nerve, and choriocapillaris.¹ Commercially available devices differ in their algorithms to detect the presence of red blood cell movement, and each incorporates noise reduction and other techniques to minimize motion artifacts. As algorithms differ between manufacturers, quantitative measurements are not interchangeable between devices.¹

In detecting glaucoma and measuring its progression, OCTA parameters serve as a complement to the OCT measures of retinal nerve fiber layer (RNFL) and retinal ganglion cell (RGC) analysis, rather than as a replacement.^1,2 OCTA provides qualitative and quantitative information about the microvasculature of the superficial peripapillary plexus and the parafoveal superficial capillary plexus. Loss of density of the superficial capillary plexi in the macular and parapapillary regions in glaucoma correlate with OCT findings of ganglion cell complex and RNFL loss, respectively, and with visual field damage in glaucoma.^1,2

Assessment of microvascular dropout at the level of the choriocapillaris in areas of parapapillary atrophy in eyes with glaucoma has been previously established; however, with improved imaging and segmentation techniques, peripapillary choriocapillaris changes in glaucoma have been a metric of recent research interest to evaluate for patients with glaucoma.^3,4

OCTA: CLINICAL CONSIDERATIONS

Age, systemic conditions (eg, hypertension, diabetes mellitus), certain medications, and myopia have all been described to affect vessel density measured by OCTA.¹ For patients with diabetes mellitus, even in the absence of clinical diabetic retinopathy, reduction in capillary density may be apparent, so the comparison of relative symmetry and assessment for additional capillary density reduction in regions where characteristic glaucomatous damage occurs requires particular attention (Figure 1).

OCTA images also have a higher degree of intra- and interscan variability in comparison with OCT images; imaging through a dilated pupil may maximize overall image quality and repeatability with OCTA.^1,2 Due to the potential variability in scans, once a baseline of three high-quality OCTA scans has been established, it is best to evaluate a patient for progressive change in vessel density at a location of interest over time, which may require repeated scans on the same day (Figure 2).

OCTA ADVANTAGES IN GLAUCOMA

Artifacts in OCTA imaging remain a practical clinical challenge, especially in eyes with media opacity or concomitant retinal pathology. In fact, in one study, one third of all OCTA scans were excluded due to poor quality.² Detection of the presence and progression of glaucoma in highly myopic eyes also presents a challenge. RNFL segmentation error and false positives are common in highly myopic eyes due to disc torsion, rotation, tilt, and presence of peripapillary atrophy.^5,6 OCTA has been proposed as an additional structural biomarker for evaluation of vascular density of the peripapillary and parafoveal regions in highly myopic eyes, which may help to differentiate glaucomatous optic neuropathy and detect disease progression.⁵

In advanced glaucoma, RNFL thickness as measured by OCT reaches a floor at which further disease progression is no longer reflected by a decrease in RNFL thickness. The functional threshold at which this RNFL floor exists is highly variable between individuals, and fluctuations in visual field parameters can make the detection of progression of advanced disease challenging.^7-9 Parafoveal vessel density as measured with OCTA, however, is not known to be limited by a detectable floor in advanced disease, which makes it a useful objective metric to incorporate for detection of disease progression, especially in eyes with mean deviation more severe than -14 dB.¹⁰

WHICH COMES FIRST?

Does RGC damage precede microvasculature loss, or does damage to retinal microvasculature lead to RGC damage? Considering the complex autoregulation of the retinal vasculature and neurovascular coupling, determining the order of this relationship is not so simple. Longitudinal studies are required to best understand the causal relationship between retinal microvasculature damage and RGC damage in glaucoma.¹¹

Despite the known complexity in establishing a temporal relationship, a recent retrospective cohort analysis identified that individuals who demonstrated rapid OCTA progression as recognized by reduction of the density of the parapillary capillary plexus over 2 years were more likely to demonstrate progressive visual field loss over 5.7 years in comparison with those who were considered to be slow OCTA progressors.² Considering the initial change of OCTA and the evaluation of future functional progression, in this particular study, it appeared that the rate of early microvasculature change predicted long-term visual field damage.

LOOKING AHEAD

OCTA has been of particular interest to individuals wanting to expand our understanding of the vascular component of glaucomatous neurodegeneration. Relative flow index has been proposed as an additional OCTA-based biomarker of the vasculopathic nature of glaucoma, with the controversial proposal that decreased perfusion of the central retinal artery system may be the driver of reduced radial peripapillary capillary plexus perfusion and resulting ganglion cell RNFL loss. This may be relevant in individuals with widespread vascular dysfunction and primary open-angle glaucoma (eg, those with Flammer syndrome or peripheral artery disease).⁴

The effects of peripapillary choroidal microvasculature density have also been investigated in primary open-angle glaucoma with demonstration of reduction in peripapillary choroidal microvasculature density as glaucoma severity increases.³ Although not ready for clinical application, these and other potential OCTA-based biomarkers may improve risk stratification and earlier detection of disease.

A USEFUL TOOL (AND GETTING BETTER)

OCTA has been proven to show unique biomarkers that correlate with established structural and functional metrics in the evaluation of glaucoma, offering advantages when compared with OCT in highly myopic eyes and eyes with advanced disease. Advances in technology, including shorter image acquisition time, improved motion artifact and projection artifact correction, and wider field of view, continue to develop with the goals of detecting more subtle pathology and determining disease progression earlier.