Critique of current theories of corneal hydration control

Abstract

The abilities of several current theories of corneal hydration control to explain some aspects of in vivo corneal behavior are examined. The theories are those based on active endothelial transport of water, solute or isotonic solution from the stroma to the anterior chamber, and a theory in which no such pump is postulated but which relies heavily on the unsteady character of corneal dynamics to explain how the normal corneal thickness is attained. The corneal phenomena to be explained are selected because they differentiate among the theories and include the effects of artificial endothelia and epithelia, and of changes in tear tonicity. None of the hydration control theories definitively explain all the corneal responses examined. It is concluded that (i) transepithelial water flow and corneal unsteadiness are essential features of any realistic description of corneal behavior; (ii) additional computational capability must be developed before the theory which contains these features can be completely evaluated; and (iii) a synthesis of these theories, in which a fundamentally unsteady cornea possesses an endothelial pump, is not unreasonable.