Functions of the corneal endothelium as measured by swelling and deswelling dynamics in response to contact lens-induced transient hypoxia

Abstract

Purpose : Ohguro et al. (Jpn J Oph 2000; 44: 325–333) developed a protocol for assessment of the barrier and pump functions of the corneal endothelium based on stromal swelling and deswelling in response to 1-2 hours exposure to N₂ stream on the ocular surface. In this paradigm, semi-quantitatively, the barrier function of the endothelium determines the initial swelling rate while the initial deswelling rate reflects the pump function. The objective of this study is to adapt the protocol with modifications so that the duration of the protocol is reduced and the measurements are accomplished in the absence of large changes in stromal pH or pO₂ at the endothelium.

Methods : Thick RGP contact lenses (Dk = 60 x 10^-9 [cm/sec]/[mL O₂/mL x mm Hg]); 430 µm; fluorosilicone acrylate) were fitted to young healthy volunteers (20 ± 2 yrs) having a corneal thickness of 532 ± 45 µm and endothelial cell density of 2959 ± 177 cells/mm². To record the swelling-deswelling dynamics, the eyes were patched after wearing the contact lenses. The patches were briefly removed (< 60 sec) every 30 min to record OCT images of the cornea (Tomey Casia SS-1000). After 2 hours of swelling, contact lenses were removed, and deswelling was followed every 30 min until baseline thickness was attained.

Results : During the two hours of contact lens wear, swelling dynamics was linear. The rate of swelling was 24 ± 6 µm/hr (14 eyes). The mean swelling of 45 ± 10 µm over 2 hrs of hypoxia is ~5-fold greater than the axial resolution of the instrument (10 µm ). Removal of the contact lens led to prompt deswelling and the stromal thickness reached baseline in most eyes at a linear rate of 23 ± 5 µm/hr (14 eyes). Furthermore, there was no significant difference between the swelling and deswelling rates (paired t-test; p ~ 0.25).

Conclusions : Induction of transient corneal hypoxia by thick contact lens wear led to linear swelling and deswelling. The magnitude of the swelling was 5x greater than the axial resolution of the instrument. Since high-resolution OCT images can be acquired in < 5 sec, the protocol can potentially be abridged to less than an hour, maintaining all the measurements at a high precision, sensitivity, and temporal resolution. Overall, both the barrier and pump functions of the endothelium can be assessed by OCT-based enhancements of the hypoxia stress test.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.