In this study, we evaluated the association of each macular layer thickness with axial length and the microstructure of PPA in myopic eyes. We found that central macular thickness positively correlated with axial length, whereas peripheral macular thickness and subfoveal choroidal thickness were negatively associated with axial length. Our study also revealed that PPA+BM width had no association with any macular layer thickness. However, PPA-BM width had strong correlations with many macular layer thicknesses. Moreover, both PPA+BM and PPA-BM widths were negatively correlated with subfoveal choroidal thickness. To our knowledge, this is the first study demonstrating the association between macular thickness and the microstructure of PPA in myopic eyes.
In terms of regional changes, our study showed that macular thickness increased in the central region, but was stable in the pericentral region and decreased in the peripheral region, with a more obvious change in the longer eye. This finding is in accordance with those of previous studies [3, 34-36]. The thickening of the central region might be due to the elongation of the eyeball, which was, in turn, caused by mechanical traction of the sclera. In such a condition, the extension of the sclera drives retinal thinning. Meanwhile, the tendency of the ILM to remain flat and the centripetal force of the posterior vitreous lead to the thickening of the central region .
In our study, the thickening of the central region was mainly attributed to the increase in OPL thickness, which was formed by dendrites . Our result was consistent with that of a recent SD-OCT study on retinal thickness . We speculate that the OPL comprises thousands of dendrites, and is more elastic than the ONL. As the axial length increases, the elevation of the central macular region would result in OPL deformation. However, some studies have reported different results about the correlations between macular thickness and axial length. The different study design and inclusion criteria might explain these conflicting results. For instance, in the study of Wakitani et al.  OCT scans were centered on the fovea with a scan length of 3 mm, while the scan length was 6mm in our study. High myopic eyes were excluded from the study by Ooto et al. , and hyperopic eyes were included in the study by Song et al. , while high myopia was included and hyperopia was excluded in our study. It is possible that the changes in retinal thickness in low-to-moderate myopia may be not enough to induce a morphological alteration.
In the peripheral region, the correlation between macular thickness and axial length was strong. All intraretinal layer thicknesses were correlated with axial length, and the thinning of the peripheral region was mainly because of the thinning of the GCL, IPL, and INL. This tendency was reported by Harb et al. , who stated that macular thinning in myopic eyes was more obvious in the peripheral region than in the other 2 regions.
In this study, RNFL thickness had a strong positive correlation with axial length in the pericentral and peripheral regions. Similar results had been described by Kim et al.  They proposed that with the expansion of the posterior pole, the retina could be pulled toward the temporal side, and in the RNFL, the compressed fiber bundles from the hemisphere on either side would be dragged to the horizontal raphe.
In terms of β-zone PPA, PPA+BM has been reported to be associated with older age or myopia [15-17]. But previous investigations have also revealed that PPA+BM was more significantly correlated with glaucoma [13, 14, 16, 43]. Teng et al.  described that high IOP in patients with glaucoma may cause obstruction of the parapapillary choriocapillaris and, in turn, may lead to the degeneration of the RPE and adjacent cells. Similarly, Sullivan-Mee et al.  reported a significant correlation between juxtapapillary choroidal volume and PPA+BM. The loss of RPE in the PPA+BM could be caused by disturbance of blood supply due to a thinned choroid, suggesting that PPA+BM may be associated with vascular compromise.
Conversely, PPA-BM has been known to be associated with mainly axial elongation. Recent studies showed that an increase in axial length could lead to remodeling of the parapapillary region, and the backward pull via the optic nerve may act on the posterior sclera, leading to the formation and development of PPA-BM [15, 46, 47]. Lee et al.  reported that the development of PPA-BM reflected scleral overgrowth when compared with the inner retinal structures in the growing eye. Hence, PPA-BM is strongly correlated with axial length, and this might account for the association between PPA-BM and myopia. Chui et al.  proved that retinal extension may not mirror scleral growth when measuring 2 parameters of the posterior pole, and the retina could slide to the temporal side during eye growth. Collectively, we suggest that PPA+BM is primarily involved in localized impairment of parapapillary choriocapillaris circulation and that PPA-BM mainly reflects the broad posterior pole change caused by optic nerve traction due to axial elongation.
Multiple linear regression analysis revealed that PPA+BM width had no association with macular layer thickness. However, the relationship between PPA-BM width and macular layer thickness was obvious. In addition, PPA-BM width had a stronger correlation with several macular intraretinal layer thicknesses than with axial length. This result led us to presume that as a monitoring marker, PPA-BM might reflect changes of macular microstructure.
The choroids can be thinner as the axial length increases [5, 6]. In our study, subfoveal choroidal thickness was significantly correlated with both PPA+BM and PPA-BM widths. Interestingly, PPA-BM width was correlated with both subfoveal choroidal thickness and macular thickness. Nevertheless, PPA+BM width was correlated with subfoveal choroidal thickness, but not with macular thickness. The development of PPA-BM reflects that axial elongation and broad posterior pole changes may be involved in the sclera, choroid, and retina. Hence, PPA-BM is associated with both subfoveal choroidal thickness and macular thickness. As mentioned above, PPA+BM could be associated with disturbance of blood supply due to a thinned choroid, not with retina. That might explain why PPA+BM was correlated with choroid, but not with macula.
Our study has several limitations. First, we could not account for optic disc tilt, which may be an important measurement indicator of the ONH. Previous studies have shown that optic disc tilt is correlated with the perfusion of the foveal zone  and RNFL thickness . A more important association between optic disc tilt and macular thickness may exist. Second, observing the changes of the equatorial retina in detail was difficult, but knowledge about this could let us further understand the progression trend of myopia. Finally, we did not measure the lamina cribrosa and sclera. For understanding the underlying mechanism of myopia, future studies should assess the lamina cribrosa and sclera in myopic eyes.
In conclusion, our study showed that macular intraretinal layer thicknesses have significant associations with axial length and PPA-BM width. With an increase in axial length, whole macular thickness in the peripheral region decreased, while whole macular thickness in the central region increased. PPA-BM width was correlated with several macular intraretinal layer thicknesses; however, no correlation was observed between PPA+BM width and macular intraretinal layer thickness. Given the clinical significance of macular evaluation in managing glaucoma, the microstructure of PPA, especially PPA-BM, should be considered when evaluating the macula in patient with myopia and glaucoma.