The role of tear film osmolarity in contact lens discomfort

Abstract

Contact lens discomfort (CLD) remains a leading cause of discontinuation among wearers, with tear film (TF) and ocular surface changes increasingly implicated. Although numerous TF parameters have been studied, the effects of osmotic stress on these metrics and their association with discomfort are not fully understood. This PhD thesis examines the repeatability of TF metrics in habitual soft contact lens wearers and explores their relationships with CLD and eyelid margin staining under normal and experimentally elevated hyperosmolarity conditions. Hyperosmolarity, a key factor in dry eye pathophysiology, contributes to TF instability, inflammation, and discomfort—common issues in contact lens wear. By assessing TF responses under osmolar stress induced by a hyperosmolar ointment, this work provides insight into how osmolar challenges affect lipid layer thickness (LLT), tear evaporation, and ocular surface behaviour, thereby clarifying mechanisms underlying CLD and lid wiper epitheliopathy (LWE). Two clinical studies were undertaken. The first assessed the repeatability and correlations of key TF metrics in fifteen habitual daily lens wearers. Results indicated moderate to good repeatability, with thicker LLT and lower evaporation potentially linked to greater comfort. Repeated measurements appeared to improve reliability, informing the design of the second, larger study. The second was a subject-masked, randomised, cross-over study involving twenty-one habitual wearers, investigating TF metrics and eyelid staining under hyperosmolar conditions with and without lenses. Osmolarity increased significantly and remained elevated longer with lenses in situ. Comfort and vision were temporarily reduced, likely due to the ointment’s viscosity. LLT was maintained, but tear evaporation did not decrease as expected, questioning LLT’s role in this process. Lower eyelid staining increased compared to the upper, possibly due to mechanical or chemical influences. Lissamine green (LG) and sodium fluorescein (NaFl) revealed differing, rather than complementary, staining patterns, while semi-automated analysis showed promise for improving objectivity. Clinically, repeated measurements may enhance the reliability of TF assessments. The hyperosmolar ointment effectively elevated ocular surface osmolarity, with a significant negative correlation observed between osmolarity and comfort.