Purpose:
Local mechanisms involved in the formation and development of areas of tear film breakup (TBU) are not well understood. The purpose of this project was to develop mathematical theory based on experimental images of TBU to better understand conditions during and following TBU.
Methods:
Tear films of 5 subjects with a range of fluorescein (FL) tear break-up times (3-45 sec) were simultaneously recorded after the initial TBU using FL and retroillumination (RI) methods and aligned with the center of the pupil by custom MATLAB programs. Using these images as initial conditions, as well as others, math models were solved in the most complex case for the tear film thickness (h), insoluble surfactant concentration (representing the lipid layer and affecting evaporation), as well as osmolarity and fluorescein concentrations inside the tear film.
Results:
Theoretical results show that elevated surfactant concentration or evaporation rate may lead to thinner regions where TBU first occurs. The first figure shows results for h(x,t) with weak osmosis from the cornea; there are three possible breakup regions (blue) with different spacings in each panel. The model predicts merging of TBU regions if they are close enough; the minimum separation to prevent merging depends on the osmotic permeability of the cornea. The model also predicts locally elevated concentrations of osmolarity in TBU (red in the second figure). Further results show that the fluorescein and osmolarity distributions differ signficantly for moderate diffusivity differences.
Conclusions:
This model, which was developed using experimental data from subjects with a range of tear film instability, explains growth and merging of areas of TBU and predicts local increases in osmolarity in areas of TBU.
Keywords: cornea: tears/tear film/dry eye