In vivo assessment of mechanisms controlling corneal hydration

The endothelial pump and evaporation components of corneal recovery were studied in the in vivo human cornea by inducing corneal swelling with the use of hypoxia and monitoring the subsequent decrease in corneal thickness. Corneal recovery follows a nonlinear time course with the rate of recovery decreasing as the cornea thins. Following 60 micron of induced edema, recovery with the eyes open required an average of 2.5 hr to reach baseline corneal thickness, while recovery with the eyes closed took an average of 4.0 hr to reach the normal physiologic corneal swelling (17 micron). Our analysis indicates that for open eye recovery from 60 micron of swelling, the endothelial pump provides 20%, while the osmotic thinning caused by tear evaporation contributes 80% of recovery. During recovery, the rate of water evaporation from the anterior corneal surface remained relatively steady at 2.5 microliter/cm2 X hr. Comparison of measured vs calculated recovery rates during recovery with the eyes closed suggests that the endothelial pump functions at one speed and that the "pump-leak" theory of corneal hydration control is applicable for the human cornea.