Tear Exchange and Midday Fogging

Purpose: Approximately 30% of scleral gas-permeable lens (SGP) wearing patients are affected by midday fogging (MDF). The purpose of this study was to determine the influence of post-lens tear exchange and other fitting characteristics on the presence or absence of MDF.Methods: 33 subjects were recruited for this study. 23 subjects were habitual SGP wearers (11 MDF, 12 non-MDF subjects), and 10 were non-SGP wearing normal controls. At the study visit, dry eye symptoms were quantified (TERTC DryEye Questionnaire), and lens-fitting characteristics were evaluated using ocular coherence tomography and biomicroscopy. Tear exchange rates were measured using the Fluorotron fluorophotometer. The procedure was to instill high molecular weight fluorescein(FITC) Dextran into the tear film reservoir beneath the SGP, and measure the tear fluid fluorescein concentration every 5-30 minutes over a period of 4 hours. The tear reservoir fluorescein concentrations were plotted to measure the fluorescein decay from within the tear fluid, which was used to calculate the tear exchange rate. Statistical analysis was done using student t-test and ANOVA.Results: In this study, there was less tear exchange in the MDF group (mean: 0.111%) when compared to the nonMDF group (mean: 0.417%), although statistical significance was not reached due to the high variability of the exchange rates (p = 0.26). There was no significant difference between the tear film reservoir thickness in the MDF (283um) and nonMDF (326um)groups (p = 0.53), or with dry eye scores (mean of 29.5 in MDF, 30.4 in nonMDF) (p = 0.91).Conclusions: In this study, there is no clear relationship between the amount of tear exchange during SGP wear and the incidence of MDF. Tear exchange may indeed be a factor, although additional studies are needed to clarify its role, and to further explore other contributing factors that may be involved in modulating the occurrence of MDF.