In vitro Improvement of the Quality of Sheep Tears after the Addition of Dilute Metal Chloride Solutions: An Observational Study

Abstract Background: The quality of tears is crucial for maintaining healthy vision in animals and humans. To address dry eye symptoms, artificial tears that can enhance tear quality must be developed. This observational study was conducted to investigate the in vitro effect of sodium chloride (NaCl) and potassium chloride (KCl) solutions on the quality of sheep tears. Materials and Methods: A sample (20 μL) of sheep tears was collected from 20 healthy sheep (11 males and 9 females). A sample (0.5 μL) of sheep tears was mixed with different volumes of NaCl and KCl solutions to produce homogeneous mixtures. A sample (1 μL) of each mixture was placed on a microscopic slide to dry. The ferns produced were observed and graded using a microscope. Results: Significant improvements (Wilcoxon signed-rank test, P < 0.001) were observed in the quality of sheep tears after adding NaCl or KCl solutions to sheep tears, regardless of their proportions in the mixtures. Significant differences (Wilcoxon test, P = 0.014) were seen in the tear ferning (TF) grades after adding NaCl and KCl solutions to sheep tears. The TF grades of sheep tears were improved from 3.1-1.7 to 1.7-0.4 after adding metal chloride solutions. For example, the TF grade of the third sheep tears was improved from 3.1 to 1.0 when NaCl (10 mg in 100 mL of water in a proportion of 1:1 of pure tears to NaCl by volume) was added. Conclusions: Both NaCl and KCl solutions significantly improved the TF grades of sheep tears. This current research addresses most of the limitations of an earlier report, proving an in vitro practical method of TF test for assessing the effect of metal chloride solutions on sheep tears.