End-of-Day Comfort issues
Assessing End-of-Day Comfort
An overview of factors associated with contact lens-induced discomfort and dryness.
|Dr. Fonn is Distinguished Professor Emeritus and the founding director of the Centre for Contact Lens Research at the School of Optometry and Vision Science, University of Waterloo, Waterloo, Ontario, Canada. He is the immediate past Editor-In-Chief of Eye & Contact Lens. Dr. Fonn has served as a consultant or advisor to Alcon Laboratories, Inc. and CooperVision, Inc.|
By Desmond Fonn, Dip Optom, MOptom, FAAO
By estimation, there were about 40 million contact lens wearers in North America during the middle of the first decade of this century. I would say that a reasonable estimate of growth since then might be as high as 10 percent for a number of reasons—wider contact lens accessibility, production improvements, and a greater variety of materials and parameters. There is also an increase in the prevalence of myopia (Vitale et al, 2009), and these are the people who comprise the largest percentage of contact lens wearers.
However, this represents only about 13 percent of the population, in contrast to the more than 50 percent that require vision correction. What has kept the yearly growth rate as low as 1 percent to 2 percent? Premature cessation of contact lens wear is the major reason. Almost as many patients are abandoning lens wear as new wearers enter the market.
The predominant reason for discontinuation of lens wear is discomfort to the point of intolerance (Weed et al, 1993; Pritchard et al, 1999; Pritchard et al, 2002, Richdale et al, 2007; Young et al, 2002). Patients also express the symptoms of dryness, and at least one study has shown a significant correlation between comfort and dryness—as comfort decreases during the day, dryness increases (Fonn et al, 1999), suggesting that the terms can be used interchangeably.
It is questionable whether patients can really distinguish between discomfort and dryness when wearing contact lenses unless an acute or mechanical reaction is provoked by the lens. Of greater significance is how the symptom or variable (comfort or dryness) changes over time, specifically from early to late in the day.
Symptomatic and Asymptomatic Patients
Dryness and discomfort symptoms are experienced by approximately 50 percent of lens wearers, far less than in non-wearers (Guillon and Maissa, 2005; Chalmers and Begley, 2006; Fonn and Dumbleton, 2003). These symptoms typically occur late in the day and are more intense at that time (Chalmers and Begley, 2006).
Clearly what distinguishes these symptomatic and asymptomatic patients are the changes in ratings of the symptoms over the day or wearing time irrespective of the type of lens being worn (Fonn et al, 1999; Fonn and Dumbleton, 2003; Dumbleton et al, 2008). Decreased comfort or increased intensity of dryness over approximately eight hours ranges from 10 percent to 30 percent using visual analog scales to measure the descriptors.
There are mixed reports about the correlation between these symptoms and clinical signs. Glasson et al, (2003) found that tear volume and tear breakup time (TBUT) decreased in intolerant contact lens wearers but not in those who were asymptomatic. This seems to be a pattern (Fonn et al, 1999; Nichols and Sinnott, 2006; Young et al, 2012), highlighting the importance of the interaction between the anterior soft lens surface and the pre-corneal tear film. While contact lenses induce symptoms in some patients (using study methods that appear to be able to differentiate patients into symptomatic and asymptomatic categories), we don’t know how to predictably determine which patients are likely to become symptomatic. It appears that there are many more patients who exhibit symptoms of discomfort/dryness compared to those who have questionable entry criteria for wearing contact lenses.
How Comfort Is Assessed
Unquestionably comfortable contact lens wear is a prerequisite for success and continued use. Of course, there are varying levels of comfort for any patient, and between patients. Acute discomfort is usually incidental, occasional, and rare for any single patient and is almost always accompanied by signs of trauma or infection, for example, but this is not so for the chronic, less intense discomfort.
Comfort varies during the day. Typically, lenses are very comfortable immediately after application—understandably so as a large volume of the disinfection solution or saline rinse accompanies the lens. Some patients report that their eyes feel “better” after lens application than before, but by the end of the day or after some hours of lens wear, that changes to “my eyes feel dry” or “my eyes feel less comfortable.”
Patients may have difficulty in expressing the intensity of the symptom and may resort to removing the lenses and/or abandoning wear. Some practitioners use rudimentary scales for recording levels of discomfort or dryness such as mild, moderate, or severe, or a 0 to 3 scale for tracking purposes, rather than for intervention. Intervention is necessary more often than not, and those strategies will be described later.
Researchers usually employ more sophisticated psychophysical techniques for measuring or assessing subjective expressions such as comfort or vision. Commonly employed tools are Likert Scales, Visual Analog Scales and Numerical Rating Scales. These are well established instruments that are commonly used in contact lens and dry eye research.
A Likert scale offers response options dependent on the question or statement. For example, “My lenses are comfortable” is intended to solicit a response from the patient that could determine frequency (all the time, most of the time, some of the time, etc.) or severity (mild, moderate, severe, etc.).
Visual Analog Scales (VASs) are designed to have subjects place a demarcation on a line (on a printed page or computer screen), which may be anchored at one end with a zero and descriptor (very uncomfortable) and 100 at the other end (very comfortable) to denote a subject’s comfort level. The demarcation is measured by someone other than the responder, and that result is the comfort score. There are many variations of the VAS: orientation of the line, anchors and descriptors along the line, etc.
Another variation of the VAS is termed Numerical Rating Scale (NRS). Subjects are asked to select a number from the NRS, descending from 100; the intervals (numbers) can have descriptors. Not surprisingly, it has been substantiated that grading scales with finer grading (0 to 100) are more sensitive compared to (0 to 3). Any of these scales can be modified and put to good use by practitioners, particularly for purposes of collecting comparative clinical information over time.
Clinical Observations Associated With Discomfort and Dryness
Pre-lens BUT or other measures of tear stability on the front surface of the lens decrease over time (hours per day) and more so in symptomatic patients (Young et al, 2012). This is understandable, as tear evaporation rate increases with lens wear (Cedarstaff and Tomlinson, 1983; Tomlinson, 2012) and tear volume decreases (Chen et al, 2011), although these two variables are not clinical measures.
Naturally, tear film osmolality increases with soft contact lens wear because of decreased tear volume (Kojima, 2011), but the significance of the change is still in question. More recent observations of the conjunctiva such as lid wiper epitheliopathy (LWE) (Korb et al, 2002) and lid parallel conjunctival folds have been linked to symptomatic contact lens wearers (Pult et al, 2009), although other groups have not found those associations.
Wearing Time and Its Association With End-of-Day Comfort
Most successful wearers can wear their lenses “all day.” Wearing time can vary quite significantly among patients, but the average wearing time range is 10 to 14 hours. You would expect that wearing time is dependent on the level of comfort toward the end of the day, and therefore symptomatic patients should have reduced wearing time compared to asymptomatic patients. Dumbleton et al (2008) found that total wearing time was always longer compared to comfortable wearing time (about 2 hours), but in this study the subjects were not stratified as symptomatic and asymptomatic. In a very recent internet survey using a social media it was found that the average wearing time among approximately 4,200 wearers and lapsed wearers was 12.0 hours per day, and those who discontinued wear reported a shorter wearing time during the day compared to continuing wearers (Dumbleton, 2013). However, end-of-day comfort scores may not correlate with wearing time, so Dr. Nancy Keir and colleagues developed a metric termed “Cumulative Comfort” that combines comfort scores with wearing time (Keir et al, 2012).
Etiology of End-of-Day Discomfort
There are many causes of lens-induced discomfort that can be termed acute, occasional, sporadic, and are typically unilateral. This condition is usually accompanied by signs linked to mechanical, traumatic, or toxic reactions that can invariably be easily remedied. However, chronic end-of-day discomfort, or increasing dryness, is an enigma simply because we (researchers and clinicians) do not know what causes patients to become symptomatic. Most will agree that lens dehydration on the eye impacts the front surface of the lens. As one measure of the effect, pre-lens BUT is reduced or expressed clinically—wettability is reduced, adversely affecting the interaction of the tarsal conjunctiva with the front surface of the lens. But there are other factors to consider. For example, wearing a contact lens for many hours is likely to induce a sub-clinical inflamed eye simply by using limbal hyperemia as the index. As lens wear continues throughout the day, substantially less is known about the effect of the lens on the post-lens tear film and the posterior lens surface’s interaction with the cornea/limbus and conjunctiva.
While the etiology of end-of-day discomfort and dryness is being unravelled, efforts to improve and prolong the wettability of soft lenses (especially maintaining hydration of the front surface) through any means makes sense.
A variety of strategies have been employed. The incorporation of a humectant in the material attracts and retains moisture to keep the lens hydrated throughout the wearing day. Another method is the inclusion of polyvinyl alcohol (PVA) into the lens—the non-bound PVA purportedly elutes into the tear film over the course of the day. Surface acting rewetting agents such as surfactants and demulcents have also been tried, and similar wetting agents have also been incorporated into disinfecting solutions. A water gradient (with dramatically different core and surface water content) material has been developed for a daily disposable silicone hydrogel lens.
The most pressing need is to solve the problem of end-of-day discomfort and dryness that plagues at least half of all contact lens wearers. While the contact lens industry continues to develop new materials with emphasis on the biocompatibility of the lens surface, further effort is required to explain the fundamental neurobiology and physiology of this subjective phenomenon. In the meantime, clinicians should continue to assess this important clinical measure to evaluate the relative merits of advances in contact lens technology. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #212.