Contact lens wear can induce symptoms of discomfort and dryness and sometimes signs of ocular surface impairment that can be remarkably similar to dry eye conditions in non-lens-wearers, except for the severe cases. The report that emanated from the National Eye Institute and Industry Workshop on Clinical Trials in Dry Eye published in 19951 contained a definition of dry eye as well as a classification scheme for different types of dry eye and recommendations of diagnostic testing procedures for clinical researchers. The definition was “Dry eye is a disorder of the tear film due to tear deficiency or excessive tear evaporation which causes damage to the interpalpebral ocular surface and is associated with symptoms of ocular discomfort.” The report suggested that contact lens dry eye (CL-DE) is a subclassification of the Dry Eye Syndrome, and at that time, not much was known about the epidemiology and etiology of CL-DE. Much has been learnt in the 10 years since that report was published, but ocular discomfort and dryness from contact lens wear still remains an enigma.
Dry eye, which affects up to 34% of the general population,2–4 is a condition resulting primarily from a disturbance of the tear film caused by either deficiency (i.e., reduced tear production or excessive evaporation) or poor quality.5 The poor quality of the tear film, or better expressed as instability, can arise from mucin or lipid deficiency. Tear film instability can also be caused by meibomian gland dysfunction, ocular surface abnormality or blink malfunction resulting in inability to evenly distribute the tear film.6 All of these tear film abnormalities may produce secondary ocular surface damage, rather than the reverse. Dry eye is associated with inflammation. Foulks7 has stated that “the greatest advance in our understanding of the pathophysiology of aqueous-deficient dry eye disease comes from the appreciation of the role of inflammation in dry eye disease.” Treatment of dry eye using therapies that modulate or ameliorate inflammation demonstrate the link between dry eye and inflammation.8–13
Although dry eye disease can be diagnosed by alteration of the volume and quality of the tear film, ocular surface damage, and conjunctival hyperemia, most agree that the diagnosis is most often made on symptoms. Research suggests that clinicians underestimate the severity of their patients’ dry eye symptoms because there may not be a strong correlation between signs and symptoms.2,14,15 These symptoms, which can include a general sense of discomfort, dryness, visual changes, soreness and irritation, burning and stinging, and itching, may ebb and flow in a diurnal pattern, with symptoms intensifying to different degrees toward the evening.4
Contact Lens Wear in an Adverse Environment
Wearing contact lenses in a dry environment such as hot desert-like conditions or artificially heated environments causing significantly reduced humidity during winter months will most likely exacerbate symptoms of dryness or cause symptoms in patients who do not normally have any. We assume that the heated dry environment causes quicker and greater lens dehydration leading to symptoms, although there does not appear to be any experimental evidence to support these claims. On an individual basis, it would seem that lens rehydration with re-wetting drops is the best remedy.
Dry eye can also be considered as an adverse environment for contact lens wear. Severe and, in some cases, moderate dry eye is considered to be a contraindication to contact lens wear, as the eye is invariably aqueous deficient. However, if the ocular surface is seriously compromised, lenses can be prescribed for therapeutic purposes. The various conditions and types of lenses that should be used are beyond the scope of this article.
Contact Lens-Induced Dry Eye
Dry eye symptoms are much more prevalent in patients who wear contact lenses (affecting about 50%) than in the non-lens-wearing population.2,14,16–18 As many as 20% of lens wearers have symptoms that are severe enough for them to reduce their wearing time.18,19 Nichols et al. 18 reported that contact lens wearers are 12 times more likely to report symptoms of dry eye than clinical emmetropes and 5 times more likely to report symptoms than spectacle wearers. The implication from these statistics is that about 18 million contact lens wearers in North America experience dryness symptoms, but on a positive note, about 90% of these patients employ strategies such as reducing their wearing time or use rewetting drops that allow them to continue lens wear.
The tear film is necessary to maintain comfort during contact lens wear by lubricating and hydrating the contact lens. However, contact lens wear interferes with normal tear film structure and function. The contact lens separates the tear film into two layers, where the prelens tear film probably contains the superficial lipid layer and aqueous layer and the postlens tear film consists of aqueous and mucin.19,20 As tear film evaporation increases with contact lens wear,21–23 it probably leads to thinning of the pre- and postlens layers because of the separation. Evaporation of the anterior layer can lead to pervaporation of the posterior tear film. It has been shown that pervaporation was responsible for significant corneal staining.24,25 However, corneal staining does not necessarily lead to symptoms of discomfort or dryness, as the lens insulates the cornea during wear.
Other clinical signs associated with CL-DE include inflammation,26 loss of functional visual acuity,27,28 and increased conjunctival staining,29 possibly caused by increased friction resulting from the disruption in the tear film.30 Dryness and discomfort also lead to reduced wearing time and ultimately ceasing lens wear.31,32
The dry eye symptoms associated with contact lens wear are similar to those found in non-lens-wearers, including ocular fatigue, discomfort, redness, itching, dryness, irritation, and scratchiness.15,33 Contact lens wearers are also more likely than non-lens-wearers to experience an increase in the intensity of their dry eye symptoms toward the end of the day, according to the research performed by Nichols and Sinnott,19 though they may experience different dryness-related sensations. Guillon et al.34 noted that contact lens wearers are more likely to report “dryness” symptoms, whereas nonwearers tend to report symptoms of “soreness” and “burning.” Interestingly, Nichols and Sinnott19 found that spectacle wearers also report more dryness symptoms than patients not requiring vision correction. The speculated mechanisms causing contact lens induced discomfort and dryness include inflammation,26,35,36 evaporation and potentially decreased tear production with concurrent increased osmolarity37 linked to corneal hypoesthesia38 and instability of the prelens tear film with reduced TBUT. However, the true cause still remains unknown, and it is very likely that it is multifactorial.
Symptomatic Versus Asymptomatic Contact Lens Wearers.
The difference between asymptomatic (or tolerant) and symptomatic (or intolerant) contact lens wearers is evident in both subjective symptoms and clinical findings. As would be expected, symptomatic contact lens wearers are associated with a decrease in subjective dryness and comfort ratings over time, whereas the ratings of asymptomatic wearers remain relatively constant.33,39 Fonn et al.39 showed that comfort ratings (using typical 0 to 100 visual analog scales) decreased by 30% from morning to afternoon and the same was true for dryness.
Glasson et al.40 found that symptomatic contact lens wearers have significantly more lipocalin, increased lipases and degraded lipids in their tears than tolerant wearers. The clinical relevance of these tear film changes is that they were associated with increased McMonnies dry eye history and symptom scores. In a subsequent study by the same group33 they found that tear volume (meniscus height and phenol red thread test) and tear stability (TBUT) was significantly reduced in intolerant wearers compared with tolerant wearers. The intolerant wearers also reported a greater number of symptoms. They concluded that intolerance was more accurately predicted by symptoms and clinical measures of the tear film but not by protein characteristics of the tear film. Fonn et al.39 also reported a statistically significant decrease in prelens TBUT in symptomatic wearers during a 5-h period, regardless of soft lens type, compared to no significant change of asymptomatic subjects. Similarly, Nichols and Sinnott19 reported that symptomatic wearers demonstrate rapid prelens tear-thinning time: about 2.8 s faster for dry-eyed subjects. They also reported lipid layer thinning in symptomatic subjects, a characteristic that correlated with postlens tear thinning time and tear thinning resulted in increased osmolarity. They suggested that although lenses with high water content are associated with CL-DE symptoms, dehydration of those lenses does not seem to be the mechanism causing the symptoms.
Contact lens materials with poor wettability41,42 and higher water content19,43 have been linked to dry eye symptoms, based on the premise that lenses with a low water content have less capacity to dehydrate and therefore provide higher levels of comfort. However, a number of studies have demonstrated that dryness and discomfort ratings increase (become worse) independently of the amount of dehydration or water content of the hydrogel lenses used.39,44,45 Others46–49 have shown that some high water content lenses that contain phosphorylcholine do not dehydrate as much during the course of the day and cause less discomfort and dryness.
Contact Lens Dropouts
The number of patients who have permanently abandoned contact lens wear is impossible to estimate with any degree of accuracy. Company estimates extracted from market research data for the United States during the past 10 years were between 10 and 24 million patients. Annualized estimates are probably more accurate but the range of 10% to 35% of wearers is still fairly large, and we do not really know how many of these patients start wearing lenses again and often more than once. Of those who discontinue contact lens wear temporarily, which has been estimated at 30% to 50%, at least half of them do so for 2 years or longer.31 There are about 35 million contact lens wearers in the United States, and although the growth of the contact lens business is reasonable, about 10% in 2005 and about 6% globally, this figure refers to sales. One estimated figure of new contact lens wearers in the United States in 2006 is 12% with an 8% dropout (industry source). This fairly well agrees with the suggested 2 to 3 million dropouts from lens wear each year up to 2004 but there must have been slightly more new wearers that entered the market to account for the small level of growth during this time period.
Similar figures have been reported for the United Kingdom. In 2004 Young, quoting from a Vision Track report, stated that the United Kingdom has one of the highest rates of dropouts in Europe.50 Morgan in 200151 reported that there were about 2.1 million contact-lens dropouts, and then Sulley et al.52 reported in 2002 that there were 1.6 million in the United Kingdom.
The reasons for abandonment of contact lens wear are of much more importance. The more commonly cited reasons are inconvenience/handling of lenses, ocular hyperemia, poor and fluctuating vision particularly when patients become presbyopic and cost. However, contact lens-induced discomfort has consistently been reported as the most common cause and is most often or synonymously described by patients as “dryness.” We have shown a high correlation between comfort and dryness ratings for both symptomatic and asymptomatic subjects.39
Improving Comfort With Contact Lenses
Water Content and Dehydration.
One factor worth considering when it comes to comfort is lens hydration. There has been a marked decline in the use of low water content hydrogel lenses during the last 10 years. As a generality, low-water lenses do not dehydrate as much as high-water lenses;44,53,54 nor do they deposit as much as high water lenses (as a class). Fabrication methods, costs, and disposability would have been as influential on their demise as clinical features. Of course, both hydration and deposition are affected by ionicity. In vivo hydration levels can also be influenced by lens thickness, osmolarity, size of the palpebral aperture (ocular exposure), environmental conditions55 though some studies have found that environmental conditions have minimal effect.56,57
Dehydration can affect the fit of a hydrogel lens by altering the lens parameters in addition to lowering the oxygen transmissibility.54 Although some studies have found a correlation between dehydration of hydrogel lenses and decrease in lens comfort,43,46 others were unable to find a relationship between lens dehydration and decreasing comfort or dryness.39,44 Materials such as omafilcon A (Proclear) and Benz G, which exhibit dehydration resistance reportedly provide enhanced comfort.46–49,58 However, many anecdotal reports suggest that the improvement or greater comfort is dependent more on patient than on lens. In vivo dehydration is most likely a surface phenomenon, which is probably why prelens TBUT decreases during wear. So the debate rages on and it illustrates that we do not completely understand the relation between dehydration and comfort or dryness.
Effects of Silicone Hydrogel Lenses on Ocular Physiology.
Silicone hydrogel lenses have been available for about 7 years. Their development was initially focused on meeting—even surpassing—the oxygen transmissibility requirements for overnight wear. These lenses have allowed researchers and clinicians to study so much more about oxygen deficiency by comparing the effects of these lenses with low-Dk hydrogel lenses. The ocular health benefits of these materials for continuous wear have been thoroughly documented59–64 and more recent studies have confirmed similar advantages for daily wear.65–67 Low-Dk lenses have an effect on all layers of the cornea, but the epithelium has been the subject of attention to determine its role in the link between overnight wear and corneal infection.68–74
However, many other effects on the epithelium by low-Dk lenses compared to high-Dk silicone hydrogels have been discovered. The use of silicone hydrogels for continuous wear has virtually eliminated the presence of epithelial microcysts.59,75,76 High-Dk silicone hydrogel lens materials induce less epithelial thinning than low-Dk hydrogel lenses69–71 and have significantly less effect on the differentiation and proliferation rates of epithelial cells than low-Dk hydrogel lenses.77
Although the chronic effects of hypoxic stress on the corneal epithelium are unlikely to result in any subjective response by patients, there is every reason to believe that increased limbal and bulbar hyperemia60,61,64,78–81 and corneal swelling from Low-Dk lenses will. The secondary effects of increased hyperemia, which has been described as subclinical inflammation, should cause subjective ocular irritation.
Have Silicone Hydrogel Contact Lenses Improved Comfort and Reduced Dryness Symptoms?
The answer to this question is yes, but with some reservation. First, let us set aside the issue about initial discomfort caused soon after lens insertion or during the first week or 2 weeks after switching from hydrogels to silicone hydrogels. There have been many anecdotal reports about patients who experience these, which I term “nuisance factors”. Their “discomfort” is primarily caused by mechanical irritation linked to modulus, edge thickness, lens design, and fit of the lens (usually loose or flat fitting lenses). As there are a variety of silicone hydrogel lenses to choose from, all of these problems should easily be overcome.
The more important issue is whether silicone hydrogels decrease discomfort and reduce dryness symptoms at the end of the day. Few reports suggest that there is little difference between these and conventional low-Dk hydrogels,45,82 but as expected, studies investigating the relative performance of these lens groups report conflicting results. However, many more clinical trials have found that patients wearing silicone hydrogels report higher levels of comfort.61,65,66,81,83–88 It is expected in clinical trials that patients will react differently to intervention, and the studies in which no difference was found might be accounted for by lack of power (a sample size that was too small) or simply selecting enough patients whose symptoms did not improve with silicone hydrogels. A classic example came from Schafer et al.,83 who found that both the frequency and severity of dryness symptoms decreased significantly with silicone hydrogels compared with their habitual hydrogel lenses. Although this study proves that silicone hydrogels can make a difference, the percentage of asymptomatic patients increased from about 45% to 65%.
There are a number of possible reasons for the reduction in discomfort and dryness symptoms with silicone hydrogels besides increased oxygen supply. Silicone hydrogels have wettability similar to that of conventional hydrogels, and continuous wear of these lenses enables the adherence of biocompatible tear film components to the lens surface.87 A number of laboratory studies have found significantly less protein deposition on silicone hydrogel lenses,89,90 a characteristic that may increase wettability and decrease friction.
Silicone hydrogel lenses appear to exhibit reduced dehydration in an in vitro model compared with conventional lenses,91 and Efron and Morgan54 found the same in in vivo studies. However, unlike conventional hydrogel lenses, reduction of water from silicone hydrogel lenses will increase the oxygen permeability. Most dehydration occurs within the first few hours of lens wear, whereas symptoms of dryness tend to surface after 5 to 6 h of lens wear.39,44
Adding Polyvinyl Alcohol and Polyvinyl Pyrrolidone to Materials.
Now that hypoxia has effectively been eliminated for the vast majority of patients with silicone hydrogel lenses, the next most important element left to improve is wettability of the lens surface (this is the “Holy Grail”). How to mimic the ocular surface so that the prelens TBUT of about 20 s is achieved and maintained throughout the wearing time will be a challenge. However, in-eye “wetting” is more complex than just TBUT. Lubrication is another criterion of performance, but this involves the measurement of the coefficient of friction under eyelid load, which is a nonclinical surrogate of the stability of the dynamic in-eye wetting behavior.92
The attempts that have been made include the incorporation of polyvinyl alcohol (PVA) or polyvinyl pyrrolidone (PVP) into contact lens materials to enhance the wettability of the lens, and to date, these appear to be encouraging. Both of these monomers have been used in artificial tears and rewetting drops for contact lenses.
Focus DAILIES (CIBA Vision) incorporates polymerized PVA which is bound within the matrix of the lens. During the Light Stream process, the nonfunctionalized (extra nonbound) PVA elutes or leaks into the tear film during a 24-h period.93,94 The action of blinking helps to release the additional PVA into the tear film, thus maintaining tear stability and patient comfort in an action CIBA Vision has termed “AquaRelease.”93,94
Acuvue Oasys (Johnson and Johnson) also incorporates a high-molecular-weight wetting agent from the PVP family, and is marketed as Hydraclear Plus. Inclusion of this monomer enables the lens to achieve wettability without subsequent surface treatment.95 Hydraclear technology was first used by Johnson and Johnson in their Acuvue Advance silicone hydrogel lens. The high-molecular-weight molecule (PVP) acts as a hydrophilic humectant, that is, it attracts and retains moisture, keeping the lens hydrated throughout the wearing day.96
It has been shown22,97 that subjects’ precontact lens tear film evaporates much more quickly than their precorneal tear film, regardless of lens material or water content. This dehydration or evaporation is an obvious cause of contact lens intolerance expressed as dryness, as the tear film is a necessary component to ensure lubricity between the lens and the tarsal conjunctiva. Rewetting (or comfort) drops, which aim to mimic or supplement natural tears, are able to temporarily ease discomfort caused by dryness. Although rewetting drops have met with some success, it has been challenging to develop a solution able to provide sustained comfort and relief of dry eye symptoms during the course of a wearing day. Despite their viscosity-enhancing ingredients, instilled drops tend to have a short ocular residence time, draining through the patient’s nasolacrimal duct quickly after instillation, with the remainder being quickly absorbed by the cornea, conjunctiva, and nasal mucosa—with at least 90% loss for each application.98 As a result, drops usually need to be re-instilled frequently throughout the day to provide effective comfort.
The lens surface can be modified to enhance its wettability with the use of a wetting agent, such as surfactants, demulcents, or hyaluronic acid. Surfactants include PVA and Tetronic 1304. Both in vitro99 and ex vivo100 studies have demonstrated that with the instillation of Tetronic 1304, the lens surface adsorbs the surfactant and increases the lens’s wettability. Demulcents include carboxymethylcellulose, povidone, and hydroxymethylcellulose. Hyluronic acid has been described as the next-generation comfort ingredient.101
All the major lens care manufacturers have modified their multipurpose soft lens disinfecting solutions in an attempt to improve the comfort and surface wetting properties of lenses.
Complete Moisture Plus contain hydroxypropyl methylcellulose and propylene glycol as moisturizing lubricants to maximize comfort and provide longer wearing time.
ReNu with MoistureLoc was specifically designed to optimize comfort with the balafilcon A material, incorporating a novel wetting agent called “MoistureLoc.”102–104 MoistureLoc comprised a unique combination of two wetting agents, poloxamer 407 and polyquaternium-10. The combination of these agents bind to the surface of the balafilcon material, resulting in enhanced comfort at the end of the day. Varikooty et al.105 have shown that ReNu with MoistureLoc produced greater end of day comfort and less end of the day dryness than Opti-Free Express.
Alcon Replenish MPS was designed to enhance contact lens comfort by retaining moisture on the surface of the lens. It contains Tetronic 1304, a surfactant that helps lenses retain moisture, and C9-ED3A (nonanolyethylenediaminetriacetic acid), a novel surface-active wetting agent. These components apparently work together, with natural tears, to retain moisture at the lens surface during the course of the day.106
CIBA Vision’s lens care solution, Aquify MPS, features HydroLock formulation that includes Dexpant-5 (an ingredient found in dry eye products) and Sorbitol (a natural ingredient that attracts moisture), which together help keep lenses from drying out. As a result, lenses would stay moist and comfortable around the clock.
The contact lens industry has made significant strides in developing new materials and surface modifications to improve comfort for all patients, and it appears that these changes are translating into real clinical benefits. More extensive market research is required to determine if drop outs are decreasing significantly and whether the vast majority of patients can wear their lenses discomfort free for long periods. The future looks particularly encouraging for contact lens wearers of silicone hydrogels with “clever” surface technology that can mimic the ocular surface.
I thank Alisa Sivak for her help in preparing this manuscript.
I do not have any financial or other interests/arrangements with the products/companies mentioned in the article.
School of Optometry
University of Waterloo
200 University Avenue West
Waterloo, ON N2L 3G1, Canada
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