Osmolality and tear film dynamics

Abstract

The tear film is a nourishing, lubricating and protecting layer that bathes the ocular surface. It is continuously replenished through cycles of production and elimination via evaporation, absorption and drainage. These processes are often referred to as tear film dynamics. Osmolality is an objective clinical measurement that provides insight into the balance of these complex tear film dynamics. Balanced tear production and elimination is vital for tear film integrity, stability and normal osmolality. Imbalances cause alterations of the tear film structure and composition, ultimately leading to tear film instability and measurable tear film hyperosmolality. Elevated tear film osmolality is considered a core mechanism in dry eye, forming the basis of dry eye symptoms and leading to ocular surface damage. Despite its immense potential in the diagnosis of dry eye, tear film osmolality is not commonly assessed. This review will focus on the current knowledge of tear film dynamics and tear film osmolality. The pre-ocular tear film is the outermost layer of the eye and is of critical importance to the health of the ocular surface. A continuous cycle of production, evaporation, absorption and drainage leads to a dynamic equilibrium in the pre-ocular tear film. Tear film osmolality or tear saltiness can be considered a consequence of these various factors in tear film dynamics.1 Homeostatic balance leads to tear film stability, enabling the tear film to fulfil its vital functions such as lubrication, nourishment and protection of the ocular surface.2,3 Homeostatic imbalance causes alteration of the tear film structure and composition, ultimately leading to tear film instability and tear film hyperosmolality.2 Measurement of tear film osmolality has been suggested as a gold standard in the diagnosis of dry eyes as elevated tear film osmolality is considered a core mechanism in symptoms and ocular surface damage in dry eye. 2,4,5 Despite its immense potential in the diagnosis of dry eye, measurement of tear film osmolality has not been amenable to in-office use and is largely confined to use in research. Newly available technology might increase its uptake by clinicians. This review focuses on the current knowledge of tear film dynamics and the relevance and clinical implications of tear film osmolality. TEAR FILM STRUCTURE AND FUNCTIONThe classical description of the tear film is a three-layered structure, with a predominant aqueous phase, a superficial thin oily layer, which interfaces with the environment, and a deep mucous layer at the base. 6,7 The thickness of the tear film was originally estimated to be 4 to 8 mm, with the aqueous layer being the thickest

Reference values, intertest correlations, and test-retest repeatability of selected tear film tests in healthy cats

“…59 As a result, tear osmolarity has been recommended as an objective clinical measurement of the balance among these complex tear film dynamics. 57 In humans, tear film hyperosmolarity is considered a piv-SMALL ANIMALS/ EXOTIC otal pathophysiologic factor in dry eye disease, 46 with healthy subjects having low and stable tear osmolarity values and subjects with dry eyes having high and less stable values. 60,61 Healthy humans have a median tear osmolarity of 301 mOsm/L, with a 50% CR (25th to 75th percentile) of 298 to 304 mOsm/L.…”

“…56 Imbalances between tear production and elimination cause tear film instability and hyperosmolarity in humans 57,58 and rabbits. 59 As a result, tear osmolarity has been recommended as an objective clinical measurement of the balance among these complex tear film dynamics.…”

Human Amniotic Membrane Extracts have Anti-Inflammatory Effects on Damaged Human Corneal Epithelial Cells <i>In Vitro</i>

“…Tear hyperosmolality, which arises from excessive water evaporation at the exposed ocular surface, leads to ocular surface inflammation with a cascade of inflammatory events. This result in tear film instability is considered as one of the symptoms in DES because it causes alterations of the tear film 20) . Previous reports showed that tear concentrations of inflammatory cytokines IL-6 and IL-8 were significantly higher in dysfunctional tear syndrome.…”

Development of a cyclodextrin-based aqueous cyclosporin A eye drop formulations

“…The osmolality of F1 and F5 were similar, 129 and 127 mOsm/kg, respectively, while formulation F7 had lower osmolality of 67.4 mOsm/kg. Patients with dry eye disease have a higher tear film osmolality, 312 mOsm/kg or higher, than healthy individuals, 302 mOsm/kg (Stahl et al, 2012). Hypoosmotic artificial tears have been used to lower the tear film osmolarity in patient with dry eyes, and the optimum osmolarity of these tears is between 110 and 150 mOsm/L (Anon., 2007a;Gilbard and Kenyon, 1985).…”