Fourier Analysis of Circular Pachymetry Profile: Associating Corneal Thickness Variation With Astigmatism Severity

To investigate the relationship between corneal thickness and corneal astigmatism by assessing the circular pachymetry profile using Fourier analysis in normal corneas with varying degrees of regular with-the-rule (WTR) astigmatism.

This cross-sectional study included 518 eyes from 259 participants with regular WTR corneal astigmatism (0-6 D). Eyes were stratified into 4 groups: <1 D (n = 176), 1 to 2 D (n = 182), 2 to 4 D (n = 108), and 4 to 6 D (n = 52). Corneal pachymetry was measured using Casia2 swept-source optical coherence tomography (SS-OCT) and analyzed via Fourier decomposition at six radii (0.5-3 mm). Harmonic amplitudes (H0-H4) represented mean thickness (H0), asymmetry (H1), toric (biaxial) variation (H2), and higher-order components (H ≥ 3).

The Fourier model fit the pachymetry data well (mean R2 = 0.93). Amplitudes of non-zero harmonics (H1-H4) increased with greater astigmatism severity (P < 0.01), more prominently at paracentral (3 mm) than central radii (P < 0.001), and most markedly for H1 (P < 0.001). For instance, H1 amplitude rose from 7.4 ± 2.4 to 9.7 ± 4.1 at 0.5 mm and from 26.2 ± 15.8 to 51.2 ± 27.3 at 3 mm in <1 D versus 4 to 6 D groups (both P < 0.001). Post-hoc tests confirmed significant differences mainly between 4 to 6 D and lower groups.

Corneal astigmatism showed a positive association with circular variation in corneal thickness, offering new insight into structural characteristics of astigmatic corneas.