<b>Regional Tear Film Instability in Dry Eye Disease: A Comparative Analysis of Central and Inferior Tear Breakup Time</b>

Background: Dry eye disease is a multifactorial ocular surface disorder characterized by tear film instability, ocular discomfort, and visual disturbance. Conventional tear breakup time assessment commonly emphasizes the central corneal zone, although regional tear film disruption may occur earlier in more vulnerable areas such as the inferior cornea. Objective: To compare central and inferior tear breakup time in patients assessed for dry eye disease and determine whether inferior-zone evaluation identifies greater regional tear film instability. Methods: This observational cross-sectional study was conducted at a tertiary care hospital in Lahore, Pakistan, from February to May 2026. A total of 113 clinical observations were included. Tear film stability was assessed using fluorescein-assisted tear breakup time under slit-lamp biomicroscopy with cobalt blue illumination. Three readings were obtained from the central 3–4 mm optical zone and inferior 4–8 mm corneal zone, and mean values were analyzed. Paired-samples t-test was used to compare regional tear breakup time, and correlation analysis assessed the relationship between age and inferior tear breakup time. Results: Mean age was 47.8 years. Central tear breakup time was 9.29 seconds, while inferior tear breakup time was 7.33 seconds, giving a mean paired reduction of 1.96 seconds in the inferior zone. The difference was statistically significant (t = 10.88, df = 112, p < 0.001), with a large effect size. Age showed a strong negative association with inferior tear breakup time (r = -0.715). Conclusion: Inferior corneal tear breakup time was significantly shorter than central tear breakup time, indicating greater regional tear film instability. Incorporating inferior-zone assessment may provide a more complete clinical evaluation of dry eye disease.