## 3 Optical Nerve Diseases  
### Glaucoma  
Glaucoma is a main cause of irreversible blindness worldwide [144]. Glaucoma is a multifactorial optic neuropathy of unknown etiology [145]. Elevated intraocular pressure (IOP) is one of the risk factors for developing glaucoma, but it is not a sine qua non of the disease, as many patients diagnosed with glaucoma are found to have normal IOP levels at the time of diagnosis. One theory regarding the pathogenesis of glaucoma is that it is vascular in origin [146]. Studying ocular microcirculation in glaucoma is challenging because of the difficulty in observing the microvasculature directly inside the retina's discrete layers and the optic nerve head (ONH) [147]. Fortunately, OCTA could potentially help in understanding the microvascular circulation role in the ONH and retina [6,148]. The OCTA is able to provide a 3-D visualization of the vasculature of the ONH from the disc surface to the lamina cribrosa, so it can be used as a diagnosis and evaluation of glaucoma or glaucoma suspects [149]. [PERSON] et al. [150] have presented a study to compare OCTA performance versus spectral-domain (SD)-OCT's in differentiating between glaucoma suspects, glaucoma patients, and healthy subjects. The results showed that both ONH density of vessels (OCTA) and the RNFL thickness (SD-OCT) have comparable diagnostic accuracy in differentiating between glaucomatous eyes and healthy eyes. Evaluation of optic disc perfusion by OCTA allows calculation of the optic disc flow index, which has high sensitivity and specificity in diagnosing glaucomatous versus normal eyes. The value of the optic disc flow index has significant correlation to visual field changes but is not correlated to changes in the thickness of the retinal nerve fiber layer (RNFL). Additionally, values of the flow index can guide the decision to treat glaucoma suspects or ocular hypertensive patients [149]. OCTA studies found reduction of microvasculature at the optic disc ([PERSON] et al. [149], [PERSON] et al. [151], [PERSON] et al. [152], [PERSON] et al. [153]) in the peripapillary region ([PERSON] et al. [148], [PERSON] et al. [152], [PERSON] et al. [154], [PERSON] et al. [155], [PERSON] et al. [156], [PERSON] et al. [157], [PERSON] et al. [158], [PERSON] et al. [159]) and in the macula ([PERSON] et al. [152], [PERSON] et al. [153], [PERSON] et al. [160], [PERSON] et al. [161], [PERSON] et al. [162]) in glaucoma patients. [PERSON] et al. [149] used OCTA to assess optic disc perfusion in 35 subjects (11 glaucomatous, 24 controls). They found a reduction in optic disc flow index from \(0.16\pm 0.01\) in normal healthy controls to \(0.12\pm 0.02\) in glaucomatous patients. Moreover, this reduction in optic disc perfusion is correlated with visual field defect. This conforms with the findings of [PERSON] et al. [151], who revealed a reduction in optic disc perfusion correlated with RNFL thickness, ganglion cell complex thickness, severity of glaucoma stage, and visual field mean deviation. [PERSON] et al. [152] noted a reduction of vessel density in the optic nerve head, peripapillary capillaries in 24 glaucomatous eyes versus 29 normal eyes using OCTA. They also found that the vessel density is reduced at the macular region at the SCP, DCP, outer retina, and choroid in glaucomatous patients versus controls. [PERSON] et al. [153] found progressive reduction of macular VD and optic nerve head VD in 83 glaucoma patients using OCTA with a 2-year follow-up period. This progressive reduction of VD is correlated with thinning of the RNFL and VF loss. [PERSON] et al. [154] studied the relation between density of vessels and visual field loss in glaucoma using OCTA. They revealed a positive correlation between reduction of peripapillary density of vessels and progression of visual field loss. However, there is no significant correlation between reduction of retinal nerve fiber thickness and visual field loss. They concluded that vascular-functional correlation is more important than structural-functional correlation.