Five-year structural changes in severe high myopia: the role of choroidal and retinal thickness

High myopia is associated with vision-threatening complications and progressive structural changes of the posterior segment. While choroidal thickness is a sensitive marker of myopia-related axial elongation, combining choroidal and retinal thickness into a choroidal-to-retinal thickness (CR) ratio may better capture clinically relevant chorioretinal alterations. This study aimed to evaluate 5-year changes in choroidal thickness, retinal thickness, and the CR ratio and their associations with axial elongation in severe high myopia.

This retrospective study included 62 eyes from 37 patients with high myopia (axial length ≥ 26.0 mm; spherical equivalent ≤ -6.0 D), excluding eyes with pathologic myopia, foveoschisis, or macular hole; eyes with posterior staphyloma were included. Optical coherence tomography was used to measure subfoveal and perifoveal choroidal thickness and retinal thickness. The CR ratio was analysed in relation to age, axial length, axial elongation rate, and posterior staphyloma. Multivariable regression identified predictors of axial elongation and staphyloma.

The cohort represented a severely myopic spectrum (mean axial length 30.80 ± 1.96 mm; mean spherical equivalent - 17.3 ± 5.10 D; mean age 46.5 ± 16.2 years), hereafter referred to as severe high myopia. Axial length increased from 30.80 to 31.17 mm over 5 years (P < 0.001), while subfoveal choroidal thickness (67.76 to 58.63 μm; P = 0.005), foveal retinal thickness (203.34 to 195.42 μm; P = 0.042), and the subfoveal CR ratio (0.33 to 0.30; P = 0.036) decreased. The baseline CR ratio inversely correlated with age, axial length, and refractive error (R² = 0.15-0.48, all P < 0.01). A higher subfoveal CR ratio was independently associated with a reduced risk of rapid axial elongation (OR 0.032; 95% CI, 0.003-0.395; P = 0.007), along with older age and lower baseline foveal retinal thickness. Axial length was the strongest predictor of staphyloma (OR 10.367; P = 0.005).

Subfoveal choroidal thickness declined progressively with age and axial elongation, serving as a sensitive marker of myopic progression. A higher subfoveal CR ratio independently predicted a lower risk of rapid axial elongation, suggesting its potential as a prognostic biomarker in severe high myopia.