Contact lens dehydration is a complex material trait that has been studied by a number of investigators, likely because dehydration has been theorized to be associated with contact lens comfort (Dillehay, 2007; González-Méijome et al, 2013). The water content of a contact lens is typically defined as the percentage of water by weight within a contact lens, while dehydration is the loss of this water (González-Méijome et al, 2007). The literature contains a number of studies focused on gaining a better understanding of contact lens dehydration. The aim of this article is to summarize these studies for you, so that you can use this information to better mange your patients’ contact lens needs.
González-Méijome et al (2007) studied the general pattern of contact lens dehydration, and they found that dehydration typically happens in three different phases. The first phase is characterized by a short, uniform dehydration rate; the second phase is typified by a rapid, linear dehydration rate; and the final phase is marked by equilibrium (the total time was less than 100 minutes).
The literature contains conflicting results on the relationship between dehydration and contact lens comfort, though the preponderance of the data do suggest that ocular comfort is unrelated to dehydration (Ramamoorthy et al, 2010; Dillehay, 2007). With that said, the conflicting literature results may indicate that dehydration has a weak, complex association with contact lens comfort.
Many contact lens material associations with dehydration have been studied over the years, though differences between hydrogel and silicone hydrogel contact lenses have been of general interest (Jones et al, 2002; González-Méijome et al, 2007). One material difference of note is that hydrogel contact lenses that have higher water contents have higher transmissibility, while the reverse situation is true for silicone hydrogel contact lenses (Dillehay, 2007).
This information is pertinent to dehydration because as hydrogel contact lenses dehydrate, their oxygen transmissibility decreases; the opposite relationship is true for silicone hydrogel contact lenses (Dillehay, 2007). Furthermore, in comparing hydrogel and silicone hydrogel contact lenses that have the same water content, both can be expected to undergo a similar amount of dehydration (Jones et al, 2002; González-Méijome et al, 2007).
The literature contains a number of conflicting reports related to the effects of the environment on contact lens dehydration. In general, in vitro reports suggest that increased wind and decreased humidity result in increased contact lens dehydration, though on-eye human studies have frequently reported no association between these two factors and dehydration (Martín-Montañez et al, 2014; Morgan et al, 2004). A similar lack of association has been found with human studies that have analyzed the relationship between temperature and dehydration (Morgan et al, 2004). Although the human results may seem counterintuitive, investigators may have arrived at these results because the tears and blinking may protect against contact lens dehydration (Morgan et al, 2004).
Something to Keep in Mind
Even though contact lens dehydration may not be related to contact lens comfort (Dillehay, 2007), dehydration is still an important contact lens property because dehydrated contact lenses may have steeper base curves, smaller diameters, and reduced thicknesses; these changes may result in tighter-fitting contact lenses (Jones et al, 2002; Ramamoorthy et al, 2010). Thus, if you dispense a high-water-content trial contact lens that appears to fit well at dispense but has an unacceptable fit upon follow up, you may want to refit your patient into an alterative contact lens. CLS
For references, please visit www.clspectrum.com/references and click on document #256.