Discussion
In the current study, we explored the longitudinal changes in ChT over 2 years and identified associated factors. Our findings revealed a significantly greater reduction in average macular ChT in the HM group compared with the non-HM group. Older age, larger PPA area and greater axial elongation were identified as risk factors for greater ChT reduction. Moreover, the combination of the PPA area and the change in AL could predict the decrease in ChT.
Axial elongation is a recognised phenomenon predominantly occurring during childhood and adolescence, potentially leading to myopia if elongation exceeds the eyes’ focal point.19 The prevalence of myopia reaches its peak, while that of hyperopia reaches its minimum at the age of 25, suggesting a possible age at which axial elongation ceases.20 21 In the current study, the mean AL elongation was 0.06 mm for non-HM eyes and 0.10 mm for HM eyes over 2 years, corroborating findings from previous studies on adult populations with HM in Japanese and Chinese demographics.22–24 For example, Saka et al22 observed significant AL increases of 0.13 mm over a 2-year period in adults aged 22–84 years, while Ohsugi et al23 reported an annual AL increase of 0.04 mm in HM eyes without macular complications among individuals aged 34–82 years. Recently, Lee et al24 reported an annual axial elongation rate of 0.05–0.07 mm in HM patients over 20 years old. Given that our study’s participants were in their early adulthood (aged 18–30 years) and exhibited no obvious myopia-related complications, there exists a potential oversight in healthcare engagement and a tendency to overuse their eyes in this demographic. The continuous myopia progression in young adults underscores a critical need for heightened surveillance and the implementation of preventative measures to curb the advancement of myopia and mitigate the risk of myopic maculopathy.25
Choroidal thinning has been established as an important structural change in myopia in numerous animal studies and cross-sectional research of adults.26–29 Furthermore, longitudinal investigations in children have also reported marked choroidal thinning concurrent with myopia development.30–32 In this study, the mean change in ChT over 2 years was −2.98 µm (range −83.53 to 43.90 µm). This change parallels findings from a 1-year longitudinal study in adolescents aged 14–18 years,32 highlighting the choroid’s pivotal role in the pathogenesis of HM and its association with myopic atrophy maculopathy, such as lacquer cracks and choroidal neovascularisation.33 Compared with those without HM, young adults with HM experienced ChT decreasing over 2 years, indicating early structural changes during HM development, although pathological maculopathy typically manifests in later stages, predominantly among middle-aged and elderly people. In addition, a greater decrease was observed in ChT in the perifoveal temporal region compared with the perifoveal nasal and subfoveal regions. This difference may be attributed to the fact that the choroid is thickest in the temporal sector and thinnest in the nasal sector of all nine ETDRS sectors for eyes with HM, limiting the degree of change.29 34 The minimal change observed subfoveally may be protected for individuals with HM since central fovea plays a crucial role in determining visual acuity.
Various factors correlated with ChT changes in myopia have been widely discussed. Age is an important factor influencing choroidal changes; both children and adults with myopia have shown decreased ChT with age.35 In this study, older age, greater baseline ChT, larger PPA area and greater AL elongation were correlated with larger choroidal thinning in the non-HM group, while the change in AL was the solitary factor influencing ChT dynamics in the HM group. Moreover, every 1 mm increase of the change in AL was associated with a more significant decrease in macular ChT (β=−36.216, p<0.001) in the non-HM group compared with that in the HM group (β=−30.044, p=0.016). Duan et al14 found greater AL was the exclusive contributor to choroidal thinning in college students with HM. However, previous studies have shown discrepancies regarding the relationship between AL and ChT. In myopic children, the degree of choroidal thinning was found to be associated with AL.9 35 36 Furthermore, the ChT could predict the rates of ocular growth in chick eyes.37 Troilo et al27 and Xiong et al32 36 suggested a potential compensatory mechanism underlying choroid modulation during early-stage myopia progression. The association between the rate of AL elongation and the rate of choroidal thinning in this study also hinted at a potential mechanism affecting both ChT and AL in HM. Conversely, a faction of research demonstrates that the increase of AL is not significantly related to the decrease of ChT, attributing the observed differences to variances across study demographics, the effect of circadian rhythm, and the presence of unknown mechanisms governing choroidal changes.31 32 38 39
The PPA area and changes in AL could predict whether the ChT decreased or not in the whole cohort. PPA is the temporal parapapillary region with uncovered Bruch’s membrane, associated with axial elongation-induced optic disc rotation in myopic eyes.40 The presence and enlargement of PPA are risk factors for pathological myopia.11 41 Our previous cross-sectional study10 discovered the PPA area was negatively correlated with the ChT. These observations are further validated by longitudinal analysis of the same cohort,42 indicating the potential of using artificial intelligence for PPA area quantification from fundus photographs, alongside routine AL measurement, to predict decreases in ChT, facilitating clinical application.
The study’s main strength is its comprehensive follow-up of young adults. Nevertheless, several limitations warrant attention. The relatively short follow-up period and limited number of visits may have insufficient power to fully elucidate the changing process of biometrics and subtle relationships between the changes in ChT and other biometrics. Additionally, while the predictive ability (AUC=0.741) for choroidal thinning using baseline PPA area combined with change in AL is considered acceptable, it falls short of excellence. Furthermore, it should be noted that the participants of the study solely consisted of college students, limiting generalisability to a wider demographic. Finally, the omission of peripapillary ChT change analysis, which might be related to the structural changes of the optic disc in myopic eyes, represents a further limitation. Future studies should consider extending the duration of follow-up and increasing the frequency of evaluations to more accurately delineate the progression of ChT changes and their correlation with other ocular metrics so as to address these limitations.