Have Silicone Hydrogels Improved Contact Lens-Related Discomfort and Dryness?

Nancy Keir, BSc, OD, Centre for Contact Lens Research, University of Waterloo Nancy Keir is currently a Research Associate at the Centre for Contact Lens Research at the University of Waterloo in Ontario, Canada, where she is responsible for conducting clinical research in the areas of contact lenses and refractive surgery. She graduated with honours in Optometry from the University of Waterloo and is currently working towards her PhD Degree in Vision Science on a part-time basis.

Silicone hydrogel (SiH) contact lenses have proven to be superior to conventional hydrogels in terms of reducing signs of contact lens (CL)-related hypoxia for both overnight wear (including extended wear and continuous wear) and daily wear.  What has been less clear, however, is their impact on CL-related discomfort and dryness. 

As we know, CL-related dryness is common, especially end-of-day dryness, and can lead to CL drop-outs.[1-5]  CL-related dryness is a complex issue with many confounding factors.  Unlike dry eye in non-CL wearers, research suggests that CL-related dryness has less gender bias and occurs in a younger population.[4]  Attempts to improve comfort with CLs can include the use of artificial tears, a reduction in wearing time and/or a change in lens material. 

A recent and compelling review article,[6] has cited many clinical trials that have found:  fewer symptoms of dryness and discomfort,[7] less end-of-day dryness and discomfort,[8] reduced frequency and intensity of dryness symptoms[9, 10] and an overall preference for SiH lenses compared to conventional hydrogel lenses for both daily wear[8] and overnight wear.[11]  Despite this, some studies have found no improvement in discomfort or dryness with SiH lenses compared to conventional hydrogel lenses.[12, 13]

SiH lenses have unique material properties, differing greatly from conventional hydrogels with respect to their polymer bulk chemistry and surface characteristics.  Three specific areas that are significantly different between SiH lenses and conventional hydrogels, and could potentially impact comfort, include oxygen permeability, water content, and lens surface wettability.

Oxygen permeability
In addition to clinical signs, symptoms such as corneal fatigue, dryness and reduced comfort have been associated with chronic hypoxia.[14, 15] With that in mind, the question is whether or not symptoms of discomfort and dryness can be alleviated by increasing the oxygen permeability of a lens.  SiH lens materials have enhanced oxygen permeability compared to conventional hydrogels as a result of the inclusion of silicone.  A proposed mechanism for reducing discomfort by increasing oxygen availability to the cornea is by preventing the inflammation that accompanies hypoxia.[6]  While this may be true, it is difficult to demonstrate a direct relationship as there are potentially many other underlying factors involved with CL wear.  Intuitively, however, if chronic hypoxia can be avoided, the discomfort and dryness that has been found to be associated with it should be minimized. 

Water content
The water content of a lens can influence the potential for lens dehydration throughout the course of the wearing period, which can affect the pre- and post-lens tear film.  Lens discomfort and dryness have been attributed to lens dehydration, and less discomfort and dryness has been found with conventional hydrogels that do not dehydrate as much during the course of the day.[16]  Nichols and Sinnott[17] found that CL materials with higher water content have been linked to dry eye symptoms; however, they suggest that dehydration does not seem to be the mechanism behind this.  Furthermore, Fonn et al[18] were unable to show a correlation between lens dehydration and subjective symptoms of discomfort and dryness with conventional hydrogels.[18]  A number of other studies have demonstrated that discomfort and dryness become worse independently of the amount of dehydration or water content of the hydrogel lenses used,[12, 18, 19] suggesting that the relationship between dehydration and discomfort and dryness is uncertain.

The water content of current SiH lenses varies considerably but it has been shown that certain SiH lenses exhibit less in vitro dehydration[20] and in vivo dehydration[21] compared to conventional hydrogels.[20]  Further in vivo dehydration studies with SiH lenses are still necessary in order to determine whether this finding has a clinical impact on subjective symptoms of discomfort or dryness.

Lens surface wettability
Lens wettability is a measure of the quality of the pre-lens tear film over the anterior surface of a CL.  There has been a lot of attention to surface wettability of SiH lenses due to the hydrophobic nature of silicone.  Surface-treated and non-surface-treated SiH lenses are currently on the market, each with unique processes for overcoming this hydrophobicity.  Despite these efforts, SiH lens surfaces have been shown to attract relatively high levels of lipid deposits compared to conventional hydrogels,[22] which can result in reduced in vivo wettability.[23] Guillon et al[24] have reported that in vivo wettability is associated with better comfort; however, few other studies have found a direct link between these two measures.  In vivo wettability seems to be an inconsistent indicator of CL-related dryness and discomfort as patients with poor in-eye wettability can be completely asymptomatic. Investigations in this area are ongoing and will be necessary to fully understand the relationship between lens surface wettability and subjective symptoms of discomfort and dryness.     

Summary
The precise etiology of discomfort and dryness with CLs remains essentially unknown; however, it is clear that these conditions are mulitfactorial and complex.  The impact of lens choice on comfort is complicated, as lens design, lens modulus, surface characteristics, oxygen permeability and water content are all contributing factors.  It is also necessary to consider the interaction between CL materials and care systems (especially for daily wear), as the long and/or short term effect of solution incompatibility on discomfort and dryness is still uncertain.  Lens fit is also critical, as reduced comfort has been found with SiH lenses fitted too flat or unacceptably loose.[25] 

Despite these complexities, SiH lenses, while not eliminating symptoms of discomfort and dryness, can have a positive impact for certain individuals.  To further understand the etiology of discomfort and dryness with CL wear, we need to improve our understanding of the biocompatibility of CL materials and CL solutions and determine their effect on the ocular surface.  In the meantime, the literature provides persuasive evidence to suggest that SiH lenses can be a viable tool for reducing CL-related discomfort and dryness.

References:

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