Contact lens care solutions need to provide rapid and effective disinfection against a wide variety of pathogenic microorganisms, be compatible with all lens materials, enhance wettability, and maintain wearer comfort without inducing toxic effects in the ocular structures (Jones and Senchyna, 2007; Willcox et al, 2021). The challenge is balancing those needs (Kuc and Lebow, 2018; Jones and Christie, 2008).

Electrolytes are the main drivers of tear osmolarity and contribute to a healthy ocular surface and to epithelial integrity (Craig and Tomlinson, 1995). The main electrolytes present in the tears are sodium, potassium, chloride, and bicarbonate (Stahl et al, 2012). Tear osmolarity is a key indicator of tear film homeostasis (Craig et al, 2017). Tear hyperosmolarity is an increase in electrolyte concentration as a result of a loss of tear film integrity (Craig et al, 2017). An osmolarity value of 308 mOsm/L in either eye is often used as the threshold to differentiate normal and early stages of dry eye disease (Thulasi and Djalilian, 2017). An osmolarity value of 316 mOsm/L is a cutoff for more advanced disease (Bron et al, 2014). Tear osmolarity is variable, both inter-eye and with repeat measurements in the same eye. An interocular difference greater than eight is significant and is harmonious with an unstable tear film (Thulasi and Djalilian, 2017).

The Studies

One study directly measured tear pH in 44 normal subjects by immersing the tip of a microcombination glass pH probe in the inferior cul-de-sac tear fluid. The normal mean pH was 7.0, with a range from 6.5 to 7.6 (Abelson et al, 1981).

In another study that examined the osmolality and pH of commercially available contact lens care solutions and eye drops (Pena-Verdeal et al, 2020), osmolality ranged from 192.6 ± 2.17 to 364.6 ± 2.88 mOsm/Kg, and pH ranged from 6.35 (6.35 to 6.26) to 7.99 (7.99 to 8.00). When classified by type or viscosity/lubrication-enhancing agent (ANOVA and Kruskal-Wallis, respectively; p < 0.001 for both), there was a large difference in the osmolality and pH values of care solutions and eye drops.

The physical properties of some care solutions and eye drops are not readily shared by manufacturers. Because the osmolality and pH values of various commercially available products vary significantly, changing lens solutions and/or eye drops may be needed if symptoms and/or ocular surface staining are discovered.

The pH, osmolality, surface tension, and viscosity of various multipurpose (MPS) and hydrogen peroxide soft contact lens solutions were evaluated, while controlling for room temperature (Dalton et al, 2008). Peroxide solutions were evaluated prior to and after neutralization. Most solutions had a pH close to neutral (range 7.00 to 7.36), except for one MPS and two neutralized peroxide solutions. Solution osmolality ranged from 275 to 310 mOsm/kg. The exceptions were one MPS with a significantly lower osmolality (225 mOsm/kg; p < 0.001) and one peroxide solution that was significantly higher (329 mOsm/kg; p < 0.001). The majority of solutions had an average viscosity at room temperature between 0.95 and 1.26 cP. At 34º C, the average viscosity of most solutions was between 0.70 and 0.83 cP. The exception was one MPS that had an average viscosity of 3.02 cP at room temperature (p < 0.001) and 1.92 cP (p < 0.001) at 34º C. As expected, non-neutralized peroxide solutions exhibited significantly different pH and osmolality values from all solutions that would directly contact the eye (p < 0.001). However, their viscosity and surface tension values were similar.

Specific properties vary significantly among products. Thus, a different type of lens care solution or eye drop type may be necessary for different individuals. CLS

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