Understanding the role of hydration and hydration gradient in corneal disease

Maintaining proper corneal hydration is critical for vision. Yet, a quantitative understanding of hydration throughout the corneal stroma is lacking. The purpose of this work is to to better understand the corneal hydration gradient including its origins, diagnostic utility, and role in different diseases. A theoretical model was developed for fluid flow in the cornea, which is driven largely by the stromal swelling pressure. The model employs a novel, first principles equation of state for stromal swelling pressure that captures the strong dependence on hydration that is observed experimentally. The model is then used to evaluate steady-state corneal hydration profiles. The results are used to analyze clinical observations in dry eye disease, Fuchs' dystrophy, and macular corneal dystrophy. The model is also applied to keratoconus where the authors hypothesize previously undetected hydration-related changes may occur secondary to structural and biomechanical changes. The model results show typical changes in water volume fraction across the cornea range from 1 to 5%, and are driven primarily by the osmolarity difference between the aqueous humour and tear film. The model also accurately captures changes in corneal thickness and hydration in several diseases, and illustrates the diagnostic utility of the corneal hydration gradient. The model accurately relates microscopic physical processes in the stroma to observable macroscopic changes in corneal thickness and hydration that is consistent with clinical observation. Use of corneal hydration gradient as a diagnostic could help distinguish factors influencing corneal hydration.