Corneal dehydration

A passive flux of water continually flows across the endothelial layer toward the stroma, which has a tendency to swell. An active pump mechanism pulls an aqueous flux in the opposite direction which controls corneal turgescence [13]. Corneal deturgescence is an ATP-dependent process of the endothelial cells and as such any disruption of the endothelium may result in corneal oedema, thereby affecting corneal transparency. The specific distribution of different proteoglycans across the cornea has recently been implicated in water gradients across the cornea. This water gradient serves to diminish dehydration of the front of the cornea, which is exposed to the atmosphere. 

Glucose solutions ranging from 30% to 50% have been used topically on the eye to treat corneal edema. The dehydrating action of a 30-minute glucose bath eliminates corneal epithelial edema and reduces corneal thickness. The effect lasts 3 to 4 hours. 

The hydrophobicity of the surface prevents the wetting by tear and tends to expose dry surface of a contact lens. Therefore, rapid dehydration of the corneal tissues could occur, which could cause the damage of corneal epithelium. However, this explanation seems to be oversimplified in light of the adsorption of protein, which makes a hydrophobic surface wettable by tear fluid, as described in Chapter 26. Moreover, the highly hydrophobic surface characteristic of silicone rubber tends to encourage the deposition of protein and mucus of the tear on the surface of the lens. Lipids and lipid-soluble materials follow the same track and eventually penetrate into the bulk phase of the contact lens. Because of these undesirable factors, the use of silicone contact lenses of various chemical compositions and with surface treatments has not been successful but rather disastrous because of the interfacial characteristics of silicone contact lens on the cornea, which cannot be oflfset by these efforts. It indicates that more profound surface modification to cope with the problems rather than mere surface treatment is needed in capitalizing on the advantageous bulk properties of silicone polymers. 

Corneal edema is a clinical sign of corneal endothelial dysfunction, and topical osmotic agents may effectively dehydrate the cornea. Identifying the cause of corneal... 

A change in refraction at high altitude, and thus transient myopia, can occur through osmoticaUy altered vitreous volume or altered curvature of the lens secondary to edema or cihary muscle spasm. The authors pointed out that altitude-related hypoxia (causing corneal swelling) and dehydration (associated with both exercise and inadequate fluid intake) could have contributed in this case. Drugs that can cause transient myopia include aspirin [4 ], chlortalidone [5 ], co-trimoxazole [6, 7 ], dapsone [8 ], hydrochlorothiazide metronidazole [10, ... 

---