PURPOSE. The purpose of this study was to investigate the topographic relationship between the decreased parapapillary retinal microvasculature as assessed by optical coherence tomography angiography (OCTA) and retinal nerve fiber layer (RNFL) defect in eyes with primary open-angle glaucoma (POAG) and a localized RNFL defect.METHODS. The peripapillary retinal circulation was evaluated using the OCTA centered on the optic nerve head in 98 POAG eyes having a localized RNFL defect and 45 healthy control eyes. A vascular impairment (VI) was identified in OCTA by the presence of a sign indicating decreased microvasculature. The frequencies of VI were compared between the POAG and control groups, and the topographic correlation between the VI and the RNFL defect identified in red-free fundus photographs was determined in the POAG group.RESULTS. The VI was observed as an area of decreased density of the microvascular network of the retina in 100% of the POAG eyes. The VI exactly coincided with the RNFL defect evident in red-free fundus photographs in terms of both the location and extent (Pearson's correlation coefficient ¼ 0.997 and 0.988, respectively, all P < 0.001). None of the control eyes exhibited VI in OCTA.CONCLUSIONS. Decreased parapapillary microvasculature of the retina determined by OCTA was found at the location of RNFL defect in POAG patients. This finding suggests that the decreased retinal microvasculature is likely secondary loss or closure of capillaries at the area of glaucomatous RNFL atrophy.Keywords: primary open-angle glaucoma, optical coherence tomography angiography, retinal microvasculature S tudies have shown that peripapillary retinal blood flow 1,2 and retinal vessel caliber 3,4 are reduced in glaucoma patients compared with healthy subjects, using laser Doppler flowmetry, 1 Doppler optical coherence tomography (OCT), 2 and measurements of retinal vessel caliber. [3][4][5][6] Decreased retinal perfusion has also been demonstrated angiographically in glaucoma patients using fluorescein angiography.7-10 These findings has raised the interest in the potential role of decreased ocular perfusion as an etiopathogenic factor for the glaucomatous optic neuropathy (GON), together with epidemiologic or clinical data that demonstrated the association of low blood pressure [11][12][13] or nocturnal blood pressure dips 14-16 with glaucoma. In contrast, Quigley et al. 17 demonstrated that the density of capillaries remained constant across a wide range of neural tissue losses within the optic nerve head (ONH) in both experimental and human glaucoma eyes. In addition, Cull et al. 18 showed that the ONH blood flow measured by laser speckle flowgraphy increased during the earliest stage of glaucoma followed by a linear decline that was strongly correlated with thickness reduction of the retinal nerve fiber layer (RNFL) thickness. These findings suggest that the reduced retinal perfusion could simply result from ONH degeneration and a consequently diminished metabolic demand.1,2,4-6 It therefore remains unc...