Effect of humidity on the deswelling rate of the human cornea



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The human cornea maintains its normal thickness and consequently its transparency by the interaction of two mechanisms which control water that tends to enter the cornea. Of these two mechanisms, one active (endothelial pump) and the other passive (tear evaporation), the endothelial pump is generally believed to be more effective in maintaining corneal thickness and its recovery from swelling. However, the well known observation that corneal recovery from edema is faster with the eye open than closed raises the possibility of tear evaporation having a major effect on corneal deswelling. Indeed, based on their recent study, O"Neil and Poise stated that the principal contributing mechanism for open-eye corneal deswelling is passive tear evaporation. Moreover, they argue that the closed-eye deswelling rate is a result of nonevaporation of tears (as occurs with 100[percent] humidity) rather than on reduced oxygen with eyes closed. Their study only included an ambient humidity of 60[percent] and an additional humidity of 100[percent]. If open-eye corneal deswelling depends chiefly on tear evaporation, it would be important to know how deswelling varies as a function of tear evaporation occurring over a wide range of humidities. The present study was designed to determine the effect of the full range of humidities on the deswelling function of the human cornea. The open-eye deswelling responses for five different levels of humidity (0[percent], 25[percent], 60[percent], 85[percent], and 100[percent]) were assessed for 8 normal, young-adult subjects. The main result of this experiment is that corneal deswelling for the 8 subjects was unaffected by humidities from 0 to 100[percent]. A secondary result, for 4 subjects tested, was that with the eye open and exposed to 100[percent] humidity, but to only 7[percent] oxygen (which is close to the average closed eye condition), the deswelling was significantly faster than with the eye closed. Thus, neither reduced tear evaporation nor lowered oxygen concentration can substantially explain the slow corneal deswelling with eye closure. One needs to conclude that the endothelial pump is the main mechanism in maintaining normal corneal thickness and its recovery from swelling.



Cornea, Humidity--Physiological effect