Evaluating Refractive Astigmatism, Corneal Astigmatism and Internal Astigmatism in Chinese Myopic Adults

Abstract

Purpose : There is a clear benefit to take cornea as a whole when analyzing astigmatism prior to surgical and other refractive interventions. This requires an adequate understanding of the corneal (CA) and internal astigmatism (IA). This study offers a detailed characterisations of refractive astigmatism (RA), CA and IA in a group of Chinese myopic adults and evaluates the role of IA in compensating for total astigmatism.

Methods : Right eyes of 898 adults were included in this population-based retrospective study. RA was obtained from manifest refraction. CA was obtained using corneal topographer (Sirius, CSO, Italy). IA was the vectorial difference between RA and CA. Relations between astigmatism components were assessed using Spearman correlation. Astigmatism compensation factor (CF) defined as the minus ratio of IA and CA.

Results : Eight-hundred and ninety-eight adult participants were eligible for inclusion in the study (mean age: 25.99±7.01 years, 446 male and 452 female). Average spherical and cylindrical component powers were -4.98±1.84D (range -11.00 to 0.00D) and -0.80±0.67D (range -4.75 to 0.00D) respectively. Average CA and IA were -0.92±0.60D (range -4.25 to -0.04D) and 0.47±0.27 D (range 0.01 to +1.98D). Mean total RA, CA, and IA were 0.25±0.34D, 0.34±0.37D and -0.09±0.20D respectively for J₀ and 0.03±0.18D, -0.05±0.21D, and 0.07±0.14D for J₄₅. Percentage of over 0.75D for RA, CA and IA were 34.9%, 52.9% and 15.9%. With-the-rule (WTR) was the most common type in RA (65.4%) and in CA (57.6%) while against-the-rule (ATR) was predominant in IA (60.1%). CA was the major component of RA. The proportion of IA in RA gradually decreased with the increase of RA. Significant correlation was found between total J₀ and corneal J₀ (r=-0. 807, P < 0.001) as well as between total J₄₅ and corneal J₄₅ (r=-0.633, P < 0.001). A negative correlation was found between corneal J₄₅ and internal J₄₅ (r=-0.587, P < 0.001). Based on CF, internal J₀ compensated for total J₀ in varying degrees (CF: 0.1–2) in 63.6% cases, while that percentage for J₄₅ component was 65.5%.

Conclusions : For myopes aged 18 to 50 years old, WTR was most common type for both RA and CA while it was ATR for IA. CA was the major component of RA while the role of IA cannot be ignored especially in lower-level astigmatism. The compensatory role of IA on reducing CA was prominent in adults.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.