Colloid Osmotic Pressure of Artificial Tears

Abstract

While the total osmolality of the aqueous tears and tear substitutes has received much attention in the past few years, the colloidal osmolality or the oncotic pressure (which includes the Donnan effect), has received practically no attention except for one single foreign publication. The colloidal osmolality of tears is twentyfold less than that of the corneal stroma, which in turn is less than 1% of the total osmolality of an isotonic solution, i.e. the magnitude of the colloidal osmolality is only a few hundreths of a per cent of total osmolality. This may be the reason why its role was thought to be unimportant by many researchers. Despite its relatively small magnitude when compared to total osmotic pressure, the oncotic pressure has been shown to play a major role in the maintenance of the water balance of bodily tissues and has been used as a diagnostic parameter in alveolar edema. The same principle has been used to formulate a collyrium, Dehydrex, or dextran-containing storage media for excised corneas such as the Kaufman-McCarey medium that have a colloidal osmolality at least equal to that of deturgescent corneal stroma. Such formulations are able to dehydrate corneal stroma even in the total absence of epithelium. Dehydrex has been shown to have a beneficial effect on damaged epithelium and is thought to be the drug of choice for the treatment of recurrent epithelial erosion when other treatment modalities have failed. In the present study, the total osmolality and the oncotic pressure of several artificial tear preparations presently marketed was determined and compared with the oncotic pressure of tears and the corneal stroma. We have found that the oncotic pressure of HypoTears is nearly sixty times higher than that of the leading artificial tear, thus it is comparable to the oncotic pressure of Dehydrex. We believe that the favorable patient acceptance of HypoTears is more likely due to this unusually high oncotic pressure than to its hypoosmolality. Such an artificial tear formulation should be effective in ameliorating microcystic epithelial edema and in increasing impaired epithelial adhesion to the underlying tissue in the cornea.