Rule similarity and symmetry patterns of astigmatism axes in refractive surgery candidate

To determine the status of rule similarity (isorule vs. anisorule) and symmetry pattern (direct versus mirror) in refractive surgery candidates

This population-based study was conducted on the available data of 2015-2016. The clinical records containing complete eye examinations including the measurement of visual acuity, objective and subjective refraction, biomicroscopy, and topographic images were evaluated. Clinical and demographic data including the amount of refractive error (diopter and cylinder) and astigmatism axis were extracted from the patients’ records. In this study, refraction was recorded as a negative cylinder value, and spherical values of more than 0.5 D for myopia and hyperopia and astigmatism less than 1 D were considered as the exclusion criteria.

After applying the exclusion criteria, the data of 2196 astigmatic patients were analyzed. In the study population, 95.9% of the patients had isorule and 4.1% had anisorule astigmatism. The prevalence of isorule and anisorule astigmatism was similar in terms of gender (p=0.204) and age (0.244). An increase in the amount of astigmatism was associated with an increase in the prevalence of isorule and a decrease in the prevalence of anisorule astigmatism (p=0.012). The prevalence of WW, AA, and OO was 75.8%, 19%, and 1.1% and the prevalence of WA, WO, and AO was 0.6%, 2.5%, and 1%, respectively. As for the symmetry of astigmatism axes, the median difference in the axis of astigmatism between the fellow eyes was 5˚ in mirror symmetry and 10˚ in direct symmetry. According to the results, ageing was associated with an increase in the axis difference between the fellow eyes in both direct (p=0.023) and mirror (p<0.001) symmetry, while an increase in the amount of astigmatism increased the axis difference between the of fellow eyes in direct (p=0.002) and mirror (p<0.001) symmetry.

Isorule astigmatism is a very common pattern in refractive surgery candidates and anisorule astigmatism is a rare condition. Symmetry was also common in this study and there was a higher tendency towards mirror symmetry compared to direct symmetry. Patients with WW astigmatism had the highest symmetry among patients.