How Tear Hyperosmolarity and Other Factors Cause Dry Eye
One of the primary causes of chronic dry eye disease is tear hyperosmolarity. The Definition and Classification Subcommittee of the 2007 International Dry Eye Workshop defines dry eye disease as “a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation of the ocular surface.”
Given its mention in the official definition of dry eye, tear hyperosmolarity is clearly an underlying factor in the development of the disease. In basic terms, tear osmolarity can be defined as the saltiness of the tears. According to Tearlab.com, “as osmolarity in your tears increases ocular surface cells become damaged,” leading to an increased likelihood of developing dry eye disease. How does this happen?
Normal tear production is controlled by the lacrimal functional unit, which uses a number of biological mechanisms to secrete “the three major components of the tear film (mucin, aqueous, and lipid)...onto the ocular surface in a coordinated fashion.” Tear hyperosmolarity results from a reduction in the lacrimal functional unit’s normal production of tears.
Tear hyperosmolarity “arises as a result of water evaporation from the exposed ocular surface, in situations of a low aqueous tear flow and/or as a result of excessive evaporation.” When this occurs, the surface of the epithelium (the outer tissue layer of the eye’s surface) is damaged by a number of resulting factors, leading to cell death and tear film instability. As an unfortunately cyclical result, the tear film instability causes further hyperosmolarity, exacerbating ocular surface damage and potentially causing chronic dry eye disease.
Preventing tear hyperosmolarity as a means to reduce the risk of developing chronic dry eye is still a subject of ongoing research. According to this article found on theocularsurfacejournal.com, “recent attempts to counteract tear hyperosmolarity...have included osmoprotectants, small organic molecules that are used in many cell types throughout the natural world to restore cell volume and stabilize protein function.” This treatment may protect ocular cells from destruction due to hyperosmolarity, thus saving the eye from the brutal cycle of tear film instability described above.
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