(522f) Can Soft Contact Lenses Protect the Cornea Against Tear Hyperosmolarity?

Under fluorescein instillation, growing black spots/streaks appear in the human tear film upon eye opening. Black-spot formation correlates strongly with dry-eye disease and is now part of the diagnosis. Because the tear film strongly wets the cornea, how tear ruptures to form black spots remains an active area of research. Peng et al. successfully analyzed a proposed breakup mechanism of pre-rupture of the evaporation-protective thin lipid film covering the tear followed by local enhanced tear evaporation.¹ The resulting evaporative-driven holes/trenches increase the underlying salt concentration exposing the cornea to local hyperosmolarity spikes. Figure 1 illustrates the dramatic rise in salt-concentration spikes predicted in a black trench where the osmolarity increases to well over 1000 mOsM (milliOsmolar) in 30 s compared to normal tear osmolarity of about 310 mOsM. Human corneal nerve endings are exquisitely sensitive to osmolarity: when salt instilled in the eye reaches five to seven hundred mOsM, it is painful to keep the eye open.² Repeated black hole/streak formation upon blinking continually irritates the eye consistent with dry-eye symptoms.

A soft contact lens (SCL) places a 100-µm thick hydrogel between the pre-lens tear film exposed to evaporative salinity spikes and the post-lens tear film covering corneal nerve fibers. Thus, SCL wear might protect the cornea from large salinity spikes and, accordingly, prevent corneal inflammation due to chronic hyperosmolarity exposure. We address this question by analyzing 2D-transient Fickian salt transport during interblinks from the pre-lens tear film to the cornea through a SCL exposed to a pre-lens salinity spike. Initial condition of the post-lens tear film and contact lens is set from the periodic steady analysis of the tear system by Cerretani et al.³ The boundary condition at the anterior surface of the SCL depends on time and is obtained from the black-spot/streak analysis of Peng et al.¹ Zero salt flux is imposed at the interface of the post-lens tear film and the cornea. Remaining lens/tear-film boundaries require continuity of flux and Nernst-law phase equilibria. Numerical solution is by finite-element discretization.

Figure 2 provides example osmolarity profiles at the corneal surface while wearing a commercially available SCL that exhibits a measured salt diffusivity of D_S = and a partition coefficient of k_S = 0.28 ⁴ (lens salt partition coefficients range between 0.15 ~ 0.70 and have little influence on post-lens tear osmolarity within this range). Clearly, the SCL significantly attenuates the inflammatory salt spike of the pre-lens tear film in Figure 1. Lower lens diffusivities prevent corneal hyperosmolarity altogether.

For the first time, we provide quantitative assessment of the post-lens tear-film osmolarity during SCL wear. We conclude that the human cornea can be protected from experiencing hyperosmolarity by wearing a SCL designed with low salt diffusivity.

References

1. C. Peng, C. Cerretani, R.J. Braun, and C.J. Radke, “Evaporation-Driven Instability of the Precorneal Tear Film,” Advances in Colloid and Interface Science, 206, 250-264 (2014).
2. Liu H, Begley C, Chen M, Bradley A, Bonanno J, McNamara NA, et al. A link between tear instability and hyperosmolarity in dry eye. Invest Ophthalmol Vis Sci., 50:3671–9 (2009).
3. Cerretani and C. J. Radke, “Periodic-Steady Tear Dynamics in Healthy and Dry Eyes,” Current Eye Research, 39(6), 580-595 (2014).
4. Guan, M. E. González Jiménez, C. Walowski, A. Boushehri, J. M. Prausnitz, and
   C. J. Radke, “Permeability and Partition Coefficient of Aqueous Sodium Chloride in
   Soft Contact Lenses,” Journal of Applied Polymer Science, 122(3), 1457–1471 (2011)