Studies have reported that macular GCC loss is involved in early glaucomatous damage.14 Since the macula area is one of the most metabolically active tissues in humans and its oxygen requirement is derived from multiple capillary supplies, the changes in the macular VD after APAC episodes are thought to be related to the pathophysiology of RGC loss in APAC.15, 16 In the present study, the macular vessel densities were significantly reduced in the indices of wiVD-SL, and pfVD-SL in the APAC eyes, and strong correlations were found between the wiVD-SL, pfVD-SL and traditional glaucoma diagnostic parameters.
The macula has the highest density of RGCs, and reduced macular VDs were firstly reported in POAG eyes when compared with normal subjects.17, 18 The changes in macular VD were then also observed in PACG, that there were pfVD and wiVD reductions in the superficial vascular plexus when compared with healthy eyes.19 In our study, we further provide the information of macular VD reductions in APAC eyes, and these suggest that although in different glaucoma types, the macular vessel density changes may share some similarities.
The superficial vascular complex supplies the inner retina from the nerve fiber layer to the inner part of the IPL. The deep vascular layers supply part of the IPL and the middle retinal layers without retinal ganglion cells.20, 21 In our study, we scanned the deeper macular vessels and observed that the pfVD-DLs were not significantly different among the APAC eyes, fellow eyes and normal controls. Therefore, only superficial macular vessel supplies were greatly affected in APAC eyes. This finding of unchanged pfVD-DL in APAC suggests that deep vascular plexus is not susceptible in acute IOP elevation. This is similar to the findings in the POAG eyes.17 Hence, the macular vessel densities in superficial layers provide more important information for glaucoma than in the deep layers.
FAZ, another macular microcirculation parameter, has been regarded as a representative indicator of retinal capillary non-perfusion. Specifically, the enlargement of FAZ was correlated with decreased visual acuity, and the irregular shape of FAZ could contribute to capillary occlusion, and hemodynamic disturbance.22–24 In our study, no statistical differences were found in FAZ area, perimeter and AI among the APAC eyes, fellow eyes and normal controls. In the study of Liu et al., they also demonstrated no differences of FAZ area and perimeter among the three groups, while they found that circularity was lowest in APAC eyes, followed by the fellow eyes and normal control eyes.11 The reason may be due to the retinal edema caused by the acute attack of angle closure, since they investigated the macular microcirculation immediately after acute attack had been resolved. The irregularity of FAZ may also contribute to the reduced visual acuity of APAC eyes in acute phase apart from corneal edema induced by IOP elevation.
The correlations of the vessel density parameters with traditional glaucoma parameters in APAC eyes depends on the disease course specifically the time interval between the OCTA examination and acute attack was resolved. The results in our study were consistent with those in the study of Zhang et al.2 In the study of APAC conducted by Zhang et al, cpVD was related to RNFL, GCC, and VF MD.2 Nonetheless, Wang et al. illustrated that the ONH vessel densities in APAC eyes were only correlated with VF PSD and MD but not with RNFL and GCC thicknesses.3 This is due to the different time intervals between the OCTA examination and acute attack of angle closure, which determines whether the retinal edema was eliminated and the structural parameters were reduced in the APAC eyes. We performed the OCT-A examination at least 3 months after acute attack, and retinal edema was disappeared. The time interval in the study of Zhang et al. ranged from 7 days to 2 years, although retinal edema was still present in some cases, the mean RNFL and GCC thicknesses in his study were significantly reduced compared with the fellow eyes.2 However, Wang et al. conducted it immediately after acute attack was resolved, and the RNFL and GCC thicknesses were not significantly different between APAC eyes and fellow eyes when the OCT-A examination was performed.3
Comparing the diagnostic abilities of macular VD indices, the diagnostic abilities were consistent with Rao’s report, which showed that the diagnostic abilities of macular vessel densities were lower than those of the structural parameters such as RNFL and GCC.19 Nevertheless, it is not clear why the diagnostic ability of the structural measures, especially GCC, is better than that of the macular OCT-A. Takusagawa et al. reported comparable diagnostic abilities of the macular superficial vascular complex and the GCC thickness.17 They accounted this to the larger scan area of the macular VD (6 mm × 6 mm) than in the traditional small scan (3 mm × 3 mm).17 Since approximately half of RGC soma are located in the macula and the metabolic requirements from the regional capillary plexuses are high, RGCs are prone to be more sensitive to microcirculation dropout in the macula. A larger scan area of the macula may be more accurate in reflecting the diagnostic ability of macular VD.
This study has a few limitations. First, the current scan was limited to an area of 3 mm × 3 mm. A larger macula area of 6 mm × 6 mm which should yield a higher diagnostic accuracy for macular VD measurements, should be used in future studies. Second, this is a cross-sectional study and the macular VD was examined 3 months after APAC to eliminate the influence of retinal edema. Hence, the early stage information of macular VD after APAC episode was lacking. Further research is necessary to determine the sequence of VD and the structural changes to elucidate the role of vascular perfusion in the disease mechanism of APAC.
In summary, statistically significant attenuations of macular wiVD and pfVD in the superficial layer were observed in APAC eyes. These parameters were correlated with RNFL, GCC, VF MD and PSD. The diagnostic abilities of the macular VD indices were lower than those of the structural measures of RNFL and GCC thicknesses. Nevertheless, they may still be useful for monitoring the progression of APAC.