Rapid anterior segment and divergent corneal shape remodeling drive astigmatic compensation in the chick model.

This study investigated early temporal dynamics of ocular compensation to imposed astigmatic blur, specifically anterior segment alterations in a chick model. Fifty-four chickens were equally randomized to treatment or control groups at post-hatching day 5 (baseline, T0). Crossed-cylindrical lens (+4.00 DS/-8.00 DC) was used to induce astigmatic blur on the right eye for two weeks: With-the-rule (WTR, axis 90°) and Against-the-rule (ATR, axis 180°) astigmatism. Ocular axial dimensions and objective refraction were measured daily for the first four days (T0-T3), and at one (T7) and two weeks (T14). Corneal topography was measured at T3, T7 and T14. Our results showed that chicks treated with crossed-cylindrical lenses developed significantly higher refractive astigmatism compared to the control group (all p < 0.01), plateauing within two days. Corneal astigmatism diverged over two weeks, increasing in the WTR group but decreasing in the ATR groups, while internal astigmatism showed opposite trends. Critically, anterior segment changes emerged by T3: the ATR group exhibited a significantly deeper anterior chamber (T3: p = 0.034), and both treatment groups showed significantly thinner crystalline lenses (ATR: T2, p = 0.008; T3, p = 0.007; WTR: T3, p = 0.043) compared to controls. These changes persisted throughout the treatment. To conclude, refractive astigmatism compensated rapidly (plateauing within two days) in response to astigmatic blur, while corneal astigmatism exhibited divergent changes over two weeks. The persistent anterior segment alterations-lens thinning and anterior chamber deepening-demonstrate a key compensatory role for anterior ocular structures beyond corneal plasticity alone.