Layer-Specific Associations of Axial Length With Retinal and Choroidal Thickness: A Nationally Representative Population-Based Study

Purpose: To characterize layer-specific, potentially non-linear associations between axial length (AL) and macular ganglion cell-inner plexiform layer (GCIPL), peripapillary retinal nerve fiber layer (RNFL), outer-retinal, and subfoveal choroidal thickness (SFCT) in a nationally representative sample of Korean adults. Methods: Cross-sectional analysis of the Korea National Health and Nutrition Examination Survey 2017-2021. Right eyes of adults ≥19 years with AL and optical coherence tomography measurements were included; eyes with major retinal or optic-nerve disease or implausible measurements were excluded. Survey-weighted spline regression tested non-linearity by a design-based nested comparison adjusted for age and sex. Ocular magnification, refractive error, and intraocular pressure were addressed in sensitivity analyses. Results: Among 9867 participants (mean age 42.6 years; AL 24.4 mm), AL was associated with all four layers (all P < 0.001). GCIPL was nonlinear (P < 0.001), steepening at longer AL, and remained so after adjustment for magnification and refractive error. The RNFL association was not robust to magnification correction: a keratometry-based correction moved the slope from -2.2 to +0.2 µm/mm (P = 0.48), and an axial-length-only factor reversed it, indicating over-correction. Outer retina was essentially preserved (≈-0.4 µm/mm), whereas SFCT showed the largest, approximately linear association (≈-31.8 µm/mm). Excluding ocular hypertension did not change the findings. Conclusions: The apparent RNFL thinning with longer AL was largely attributable to scan-circle magnification rather than tissue thinning. The other layers differed in form: a non-linear GCIPL association and a large, approximately linear choroidal reduction, with outer-retinal preservation. Interpretation should account for AL, magnification, and functional form across layers.