Tear fluid plays a vital role in protecting the ocular surface and maintaining conditions optimal for ocular health and vision. Tear fluid may also potentially have a role as a diagnostic fluid, in a similar manner to the way in which saliva has increasingly been utilized for diagnostic tests. This study aimed to determine if tear fluid analysis could distinguish subjects classed as stressed, based on their scores on a validated psychological questionnaire, from non-stressed subjects. The levels of a number of molecules known to be involved in the stress response were investigated in tear fluid. Two of these stress mediators, cortisol and dehydroepiandrosterone (DHEA), were reported for the first time in tear fluid, while the remaining molecules, serotonin, secretory immunoglobulin A (sIgA) and the cytokines interleukin (IL)-1β, IL-6, IL-8, IL-10, IL- 12p70 and TNF-α, had already been reported in tears but not in relation to stress levels. After preliminary investigation of chromatographic methods, enzyme immunoassays were used to detect and quantitate all the molecules except for the cytokines, which were analysed on a flow cytometer using a cytometric bead array assay. The levels of tear cortisol, DHEA and the molar ratio of these two, [C/D], were significantly different between stressed and non-stressed groups, with cortisol being lower, DHEA higher and [C/D] lower in the stressed group. Further extensive investigation is required before the reliability of these potential biomarkers is confirmed. However, preliminary studies showed that tear cortisol and DHEA concentrations exhibited a diurnal variation, and while they showed considerable variation between individuals, the tear [C/D] showed a lower ratio of within-subject variance to between-subject variance than the saliva [C/D] or serum [C/D]. Of the other molecules investigated, tear sIgA concentration normalised for tear protein concentration, as well as the levels of the cytokines IL-12p70 and IL-8, had some relationship to the stress level of the study’s subjects. However, further studies with larger subject groups are required before making firm conclusions on these markers.
Copyright LK Banbury 2009