Introduction
High myopia (HM) is a major threat to public eye health. Epidemiological surveys in 2020 showed that there were about 193 million patients with HM worldwide,1 and it is estimated to reach 938 million by 2050.2 The prevalence of HM is higher in Asian populations, such as in China, Singapore and Japan,3 and it is showing an upward trend.4 In adulthood, HM can cause a series of ocular complications, including glaucoma, cataracts, macular degeneration and retinal detachment,5 leading to progressive loss of visual quality and, in severe cases, blindness, which imposes a heavy economic burden on individuals and society.
The pathological mechanism underlying HM remains poorly understood but is widely believed to be a combination of environmental and genetic factors.6 The eye is an oxygen-demanding organ, and maintaining the integrity and function of ocular structures relies heavily on a sufficient blood supply. Recently, several studies have revealed that hypoxia may play an important role in the development of HM and proposed a novel hypothesis.7–10 These studies postulate that long-term near-distance use of the eye may cause insufficient choroidal blood perfusion; subsequent hypoxia leads to scleral remodelling and causes elongation of the axial length (AL), and the change in scleral tensile strength causes morphological and haemodynamic changes in the vascular network of the fundus, which causes further progression of myopia, and they have found hypoxia evidence based on a cellular study conducted in a wet laboratory11 12; however, there is still insufficient clinical evidence. Oxygen supply to the retina is primarily provided by the retinal and choroidal vessels; the retinal vessels have been extensively reported to change in myopia.10 13–16 In contrast, because of masking by the retinal pigment epithelium layer, observation of choroidal vessels requires invasive examination using indocyanine green angiography, which can only be used on a few patients and is not suitable for large population screening. With the advancement of optical coherence tomography angiography (OCTA) technology, non-invasive quantitative and in-depth analyses of choroidal blood flow are possible, which may provide clinical evidence for the hypoxia hypothesis.
Several studies have analysed eyes with myopia on 3×3 mm or up to 12×12 mm OCTA scans. The results showed that as AL increased, the perfusion area and vascular density in the macular region significantly decreased.17–22 Similar changes were observed in the peripapillary capillaries and deep parafoveal regions of the retina.23 However, restricted by the scanning area of the machine, information regarding the peripheral area is insufficient. With the introduction of ultrawidefield swept-source OCTA (SS-OCTA), which offers a faster scanning speed and clearer 24×20 mm images, two studies have reported a negative correlation between choroidal vascular density (ChVD) and thickness (ChVT) and AL in most peripheral grids of the fundus,24 and choroidal thinning was most evident in the macular region.25 Although these two studies provided some peripheral information on eyes with myopia, they had some prominent limitations. First, though both studies used the latest SS-OCTA, the built-in software algorithms were not the latest versions, resulting in inaccurate measurements. Additionally, the sample size was small, which did not represent all myopia groups and may have affected the statistical significance of the results. Therefore, this study aimed to compare the influence of myopia on ChVD and ChVT in a cohort study with a larger sample size using SS-OCTA with an updated version of the built-in software, which may provide a more accurate measurement of choroidal blood vessel parameters. This approach contributes to a broader understanding of the role of choroidal vascular changes in myopia progression.