Modulation of Total Corneal Astigmatism, Corneal Thickness, and Corneal Higher-Order Aberrations Following Tear-Enhancing Drop Instillation: A Retrospective Comparative Study

This study aimed to quantify changes in corneal astigmatism, thickness, and corneal higher-order aberrations after instilling a tear-enhancing drop (Systane Hydration Ultra, SHU) or 0.9% saline. This retrospective comparative study included 56 eyes (SHU, n = 38; 0.9% saline, n = 18). Corneal pachymetry and optical parameters were assessed using CASIA2 swept-source anterior segment optical coherence tomography. Measurements were acquired immediately after a natural blink (baseline, time 0). Subsequently, one drop of SHU or 0.9% saline was instilled, and measurements were repeated at 5 and 10 min postinstillation. No clinically relevant changes in mean total corneal astigmatism (TCA), apparent corneal thickness (CT), or corneal higher-order aberrations were observed following instillation of either SHU or 0.9% saline. On a case-by-case basis, changes observed in the 0.9% saline group correlated with preinstillation values. CT tended to reduce with saline and generally increase with SHU. The induced change in TCA ( y ) 10 min after instillation significantly correlated with preinstillation. CT ( t ) in SHU cases, where y = 0.317−0.003 t +0.006 t 2 ( r 2 = 0.361, n = 38, p < 0.001); and TCA ( x ) in 0.9% saline cases, where y = 0.68 x −0.30 ( r 2 = 0.562, n = 18, p < 0.001). Overall, the induced change in TCA exceeded 0.50 D in 18% of SHU and 17% of 0.9% saline 10 min after instillation. Tear supplementation induces short-term case-specific changes in astigmatism and optical parameters, particularly within the first 5 min after Systane Hydration Ultra instillation. These findings may be relevant when interpreting corneal measurements in patients with mild ocular surface dysfunction undergoing refractive or cataract assessment.