• Evaluation of retinal, optic nerve, and choroidal hemodynamics

Optical coherence tomography angiography (OCT-A) is a novel and noninvasive imaging modality that builds on existing OCT technology and offers the promise of highly specialized outcomes of optic nerve vascularity. OCT-A utilizes the differences in amplitude between subsequent beams of reflected infrared light to analyze both cross-sectional structural information and compute the quantity of retinal and choroidal blood flow (dimensionless units), providing vessel density percentage in the retina and optic nerve in 3 to 4 seconds. Two algorithms, split-spectrum amplitude-decorrelation angiography (SSADA) and optical microangiography (OMAG), have been widely used in research and are commercially available for clinical application. The SSADA algorithm utilizes the reflected amplitude of an infrared laser to find the decorrelation between consecutive B-scans (Fig. 9-2). OMAG utilizes a Hilbert transformation to assess scattering light reflections as either static or in motion. These calculations are used to generate high-resolution 3D (18 µm in coronal plane) images of the optic disc vasculature with segmentation of vascular layers (Fig. 9-3), including deep into the lamina cribrosa (2- to 3-mm penetration depth), and provide simultaneous structural and vascular assessments.

• Specialized angiography SSADA algorithm provides a reduction of artifacts from saccadic eye movements, high signal-to-noise ratio and vessel continuity, as well as vessel quantification in terms of density and flow index. OMAG allows for highly sensitive capillary images, and detection of blood flow as slow as 4 µm/s³.

• Vessel density (%) demonstrated similar diagnostic accuracy compared to RNFL thickness measurement for differentiating between healthy subjects and those with glaucoma (Fig. 9-4).

• Current cross-sectional studies in glaucoma patients show correlations between lower peripapillary vessel density and visual field defects, and correlation of visual field deficits to be stronger with vessel density deficits than with RNFL thinning (Fig. 9-5).

• Current limitations include difficulty imaging deep vasculature of the optic disc because of shadow interference from the larger and overlying central retinal vessels, flow index of the optic disc may include retinal microvasculature, difficulty distinguishing between decreased vascularity because of tissue loss versus ischemia, vascular leakage cannot be assessed, and no longitudinal studies of glaucoma progression have been performed, thus resulting in unknown prognostic ability.

• It does not provide a comprehensive assessment of all significant vascular beds in glaucoma; therefore, reductions in vascularity may represent a consequence of decreased upstream blood flow.