Hyperosmolarity and tear film stability : are they related?

Purpose: Dry-eye is characterised by inability of tears to form a well-spread and stable film at the ocular surface. Osmolarity of tears also increases in dry-eye due to evaporation. Although interfacial films are known to get affected by increased salt concentrations of the subphase, it is not clear whether hyperosmolarity causes tear film instability in the pathology of dry-eye. Our previous research has shown that hyperosmolarity has no effect on the biophysical properties and stability of the lipid layer, a part of the tear film. The purpose of this study was to determine whether hyperosmolarity affects the biophysical properties and stability of the overall tear film. Methods: Unstimulated normal human tears were spread on an artificial tear (AT) solution in a Langmuir trough. The tear film was compressed and expanded to record surface pressure-area (Π-A) isocycles which provide information on the biophysical properties and stability of the film. Isocycles were recorded on AT solutions of 305 (normal tear osmolarity) and 633 (hyperosmolar) mOsm/kg at 20°C and 35°C (ocular temperature). Results: Π-A isocycles were unaffected by the osmolarity of AT solutions. The maximum surface pressure (~27 mN/m) and Π-A profiles were similar for different osmolarity. Isocycles obtained on solutions of different osmolarity at 20°C and 35°C were similar at the respective temperatures reaching the same maximum pressure of 27 mN/m indicating no adverse effect of increased salt concentration on Π-A isocycles. Conclusions: Hyperosmolarity has no adverse effect on the biophysical properties and stability of the tear film. This is in-line with our previous results of human meibomian lipids. Non-polar or hydrophobic constituents of tears particularly lipids (as opposed to hydrophilic ones which are likely to interact more with the changed ionic concentrations) might be the possible reason for these observations. These findings increase our understanding of the pathology of dry-eye - hyperosmolarity does not affect the lipid layer or whole tears and their stability as thin films but it may have other undesirable effects such as ocular irritation and decreased microbial defence at the ocular surface as reported in literature