Influence of controlled environmental conditions in potential salivary ocular pain biomarkers for enhancing the assessment of ocular pain

Abstract

Purpose We endeavored to identify objective salivary biomarkers for pain, a subjective sensation with a biological basis, using molecules already described related to pain. The study aimed to analyze inter-individual differences and intersession variability in salivary potential ocular pain biomarkers on healthy subjects, in samples obtained under the influence of controlled environmental conditions. Methods Thirty-four healthy subjects, 20 male, 14 female, median age 35.44 years (range 30–40) were exposed for 30 minutes under standard environmental conditions (T: 22°C, 50% relative humidity) in the Controlled Environmental Research Laboratory (CE-Lab, Vision R&D, Valladolid Spain) in two separate visits (V1, V2) at least 24 hours apart. Saliva was collected after the exposure in each of the visits, and cortisol, α-amylase (sAA), secretory IgA (sIgA), testosterone, and soluble fraction of TNFα receptor II (sTNFαRII) were analyzed by ELISA. Repeatability of inter-subject inter-session measurements was assayed by intraclass correlation coefficient (ICC). Results There were no significant inter-session differences in testosterone (p = 0.2497), sTNFαRII (p = 0.6451) and sIgA (p = 0.9689) salivary levels. The reproducibility for salivary cortisol, sAA, testosterone, sTNFαRII and sIgA were 0.98 ng/ml, 20.58 U/ml, 21.07 μg/ml, 24.68 pg/ml and 0.19 pg/ml, respectively. Salivary cortisol, sAA, testosterone, sTNFαRII and sIgA yielded the following ICCs: 0.506, 0.569, 0.824, 0.870 and 0.4295, respectively; all these ICCs (except that for cortisol and sIgA) were found to be improved compared to those found previously by our group in a previous study in salivary samples obtained from healthy subjects under non-controlled environmental conditions; Cortisol´s ICC didn´t improve and was in both cases at the limit of acceptability. Conclusion Environmental factors such as temperature and relative humidity affect the reproducibility of measurement of some salivary molecules which have been proposed as potential pain biomarkers. The exposure of subjects to standard controlled environmental conditions before salivary sample obtention would improve the reproducibility of these molecule measures’ as potential biomarkers of chronic ocular pain.