**3. Optical coherence tomography angiography in glaucoma**

#### **3.1 Optical coherence tomography angiography metrics for glaucoma evaluation**

Early OCTA work in glaucoma by Jia et al. in 2012, was limited to the observation and quantification of the optic disk alone (disk flow index and disk perfusion) [29]. As this technology has progressed, a broad range of OCTA metrics were introduced to quantify microvasculature and aid in the diagnosis of glaucoma. Specifically, density parameters are used to track disease progression [18, 30–33] and more recent work has explored how changes in the macular regions, foveal avascular zone (FAZ), and deep retinal layer microvasculature contribute to disease severity [9].

Current clinical evaluations of glaucoma using OCTA rely predominantly on eight parameters: RNFL (μm), cup/disk ratio (CDR), macular ganglion cell complex (mGCC) in μm, focal loss volume (FLV, %), global loss volume (GLV, %), whole image ONH vessel density (VD, %), whole image macular VD, (%), and FAZ area (mm2 ). Recently, a key study by AlSalem et al. explored the efficacy of each of these metrics for classifying the severity of glaucoma progression into three categories: mild, moderate, severe [9]. Their parameter classifications are summarized in **Table 1**. Of note, the authors described how severe cases of POAG showed a


#### **Table 1.**

*Structural and vessel density characteristics of healthy controls and glaucomatous eyes, adapted from AlSalem et al. [9].*

significant decrease in signal strength intensity (SSI) for RNFL and ONH VD scans, whereas four parameters (whole image ONH VD, whole image macular VD, average RNFL thickness, and average mGCC thickness) decreased in a stepwise fashion as glaucoma progressed. These findings enable researchers to use both structural and VD parameters as indicators of glaucoma severity, adding new prognostic metrics that could not be obtained through VF tests alone.
