Prevalence and Distribution of Axis Subtypes of Refractive Astigmatism in Mexican Outpatients: A Nationwide Multicenter Clinic-Based Cross-Sectional Study.

Background/Objectives: Astigmatism is one of the most common refractive errors and represents a risk factor for other ocular conditions, including myopia and amblyopia. In Mexico, comprehensive epidemiological data on refractive astigmatism, particularly its axis-related subtypes, remain limited. This study aimed to estimate the prevalence and distribution of axis subtypes of refractive astigmatism in a nationwide, clinic-based cohort of Mexican outpatients. Methods: We conducted a retrospective, cross-sectional, multicenter study using anonymized electronic health records (EHRs) from Salud Digna clinics across all 32 states of Mexico. Individuals aged 5-90 years who underwent routine non-cycloplegic eye examinations between 2021 and 2023 were included. Refractive astigmatism was defined as a cylinder power (Cyl) of ≤-0.75 D. Age-adjusted prevalence and distribution of axis subtypes (with-the-rule [WTR], against-the-rule [ATR], and oblique [OBL]) were determined according to sex, age groups, and geographic locations. Logistic regression models evaluated potential associations between demographic variables and refractive astigmatism. Results: Data from 8,622,191 individuals were analyzed (median age: 45 years; 63.4% females). The age-standardized prevalence of refractive astigmatism was 41.59% (95% CI: 41.54-41.64%) and was higher in males (45.43%) than females (39.32%). WTR astigmatism predominated among younger individuals, peaking at ages 15-19 (48.97%), whereas ATR astigmatism increased with age, reaching 35.36% among those aged 80-90 years. In multivariable analysis, refractive astigmatism was significantly associated with age, sex, and state of residence, showing a non-linear, non-monotonic age pattern with peaks in adolescence (aOR = 1.47; 95% CI, 1.456-1.484) and older (aOR = 1.177; 95% CI, 1.161-1.193) age, higher odds in males (aOR = 1.269; 95% CI, 1.265-1.272), and marked geographic variability, with the highest in Tlaxcala (aOR = 3.581; 95% CI, 3.491-3.675). Conclusions: Refractive astigmatism was more prevalent in males and showed a clear age-related shift in the axis subtype beginning in the early 40s. These findings provide large-scale, real-world evidence that enhances the understanding of astigmatism epidemiology in Mexico and may inform future research and public health planning.