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
1. Lemp MA. Report of the National Eye Institute/Industry workshop on clinical trials in dry eyes. CLAO J 1995;21:221–32.
2. Doughty MJ, Fonn D, Richter D, Simpson T, Caffery B, Gordon K. A patient questionnaire approach to estimating the prevalence of dry eye
symptoms in patients presenting to optometric practices across Canada. Optom Vis Sci 1997;74:624–31.
3. Schaumberg DA, Sullivan DA, Buring JE, Dana MR. Prevalence of dry eye
syndrome among US women. Am J Ophthalmol 2003;136:318–26.
4. Begley CG, Chalmers RL, Mitchell GL, Nichols KK, Caffery B, Simpson T, DuToit R, Portello J, Davis L. Characterization of ocular surface
symptoms from optometric practices in North America. Cornea 2001;20:610–18.
5. Lin PY, Tsai SY, Cheng CY, Liu JH, Chou P, Hsu WM. Prevalence of dry eye
among an elderly Chinese population in Taiwan: the Shihpai Eye Study. Ophthalmology 2003;110:1096–101.
6. Holly FJ, Lemp MA. Tear physiology and dry eyes. Surv Ophthalmol 1977;22:69–87.
7. Foulks GN. What is dry eye
and what does it mean to the contact lens
wearer? Eye Contact Lens
8. Stern ME, Beuerman RW, Fox RI, Gao J, Mircheff AK, Pflugfelder SC. The pathology of dry eye
: the interaction between the ocular surface
and lacrimal glands. Cornea 1998;17:584–9.
9. Pflugfelder SC, Solomon A, Stern ME. The diagnosis and management of dry eye
: a twenty-five-year review. Cornea 2000;19:644–9.
10. Marsh P, Pflugfelder SC. Topical nonpreserved methylprednisolone therapy for keratoconjunctivitis sicca in Sjogren syndrome. Ophthalmology 1999;106:811–16.
11. Sall K, Stevenson OD, Mundorf TK, Reis BL; CsA Phase 3 Study Group. Two multicenter, randomized studies of the efficacy and safety of cyclosporine ophthalmic emulsion in moderate to severe dry eye
disease. Ophthalmology 2000;107:631–9.
12. Kunert KS, Tisdale AS, Stern ME, Smith JA, Gipson IK. Analysis of topical cyclosporine treatment of patients with dry eye
syndrome: effect on conjunctival lymphocytes. Arch Ophthalmol 2000;118:1489–96.
13. Stevenson D, Tauber J, Reis BL; The Cyclosporin A Phase 2 Study Group. Efficacy and safety of cyclosporin A ophthalmic emulsion in the treatment of moderate-to-severe dry eye
disease: a dose-ranging, randomized trial. Ophthalmology 2000;107:967–74.
14. Chalmers RL, Begley CG, Edrington T, Caffery B, Nelson D, Snyder C, Simpson T. The agreement between self-assessment and clinician assessment of dry eye
severity. Cornea 2005;24:804–10.
15. Schein OD, Tielsch JM, Munoz B, Bandeen-Roche K, West S. Relation between signs and symptoms of dry eye
in the elderly. A population-based perspective. Ophthalmology 1997;104:1395–401.
16. Nichols JJ, Mitchell GL, Nichols KK, Chalmers R, Begley C. The performance of the contact lens dry eye
questionnaire as a screening survey for contact lens
-related dry eye
. Cornea 2002;21:469–75.
17. Caffery BE, Richter D, Simpson T, Fonn D, Doughty M, Gordon K. The Canadian dry eye
epidemiology study. In: Sullivan DA, Dartt DA, Meneray MA, eds. Lacrimal Gland, Tear Film
, and Dry Eye
Syndromes, Part 2: Basic Science and Clinical Relevance. New York: Plenum Press; 1998:805–6.
18. Nichols JJ, Ziegler C, Mitchell GL, Nichols KK. Self-reported dry eye
disease across refractive modalities. Invest Ophthalmol Vis Sci 2005;46:1911–4.
19. Nichols JJ, Sinnott LT. Tear film
, contact lens
, and patient-related factors associated with contact lens
-related dry eye
. Invest Ophthalmol Vis Sci 2006;47:1319–28.
20. Nichols KK, Nichols JJ. Dry eye
update 2003. Contact Lens
21. Hamano H, Hori M, Mitsunaga S. Measurement of evaporation rate of water from the precorneal tear film
and contact lenses. Contacto 1981;25:7–14.
22. Cedarstaff TH, Tomlinson A. A comparative study of tear evaporation rates and water content of soft contact lenses. Am J Optom Physiol Opt 1983;60:167–74.
23. Mathers W. Evaporation from the ocular surface
. Exp Eye Res 2004;78:389–94.
24. Holden BA, Sweeney DF, Seger RG. Epithelial erosions caused by thin high water content lenses. Clin Exp Optom 1986;69:103–7.
25. Orsborn GN, Zantos SG. Corneal desiccation staining with thin high water content contact lenses. CLAO J 1988;14:81–5.
26. Pisella PJ, Malet F, Lejeune S, Brignole F, Debbasch C, Bara J, Rat P, Colin J, Baudouin C. Ocular surface
changes induced by contact lens
wear. Cornea 2001;20:820–5.
27. Timberlake GT, Doane MG, Bertera JH. Short-term, low-contrast visual acuity reduction associated with in vivo contact lens
drying. Optom Vis Sci 1992;69:755–60.
28. Goto E, Yagi Y, Matsumoto Y, Tsubota K. Impaired functional visual acuity of dry eye
patients. Am J Ophthalmol 2002;133:181–6.
29. Guillon M, Maissa C. Bulbar conjunctival staining in contact lens
wearers and non lens wearers and its association with symptomatology. Cont Lens Anterior Eye 2005;28:67–73.
30. Guillon M, Styles E, Guillon JP, Maissa C. Preocular tear film
characteristics of nonwearers and soft contact lens
wearers. Optom Vis Sci 1997;74:273–9.
31. Weed K, Fonn D, Potvin R. Discontinuation of contact lens
wear. Optom Vis Sci 1993;70 (Suppl):140.
32. Pritchard N, Fonn D, Brazeau D. Discontinuation of contact lens
wear: a survey. Int Cont Lens Clin 1999;26:157–62.
33. Glasson MJ, Stapleton F, Keay L, Sweeney D, Willcox MD. Differences in clinical parameters and tear film
of tolerant and intolerant contact lens
wearers. Invest Ophthalmol Vis Sci 2003;44:5116–24.
34. Guillon M, Maissa C. Dry eye
symptomatology of soft contact lens
wearers and nonwearers. Optom Vis Sci 2005;82:829–34.
35. Willcox MD, Lan J. Secretory immunoglobulin A in tears: functions and changes during contact lens
wear. Clin Exp Optom 1999;82:1–3.
36. Schultz CL, Kunert KS. Interleukin-6 levels in tears of contact lens
wearers. J Interferon Cytokine Res 2000;20:309–10.
37. Gilbard JP, Gray KL, Rossi SR. A proposed mechanism for increased tear-film osmolarity in contact lens
wearers. Am J Ophthalmol 1986;102:505–7.
38. Mathers WD. Why the eye becomes dry: a cornea and lacrimal gland feedback model. CLAO J 2000;26:159–65.
39. Fonn D, Situ P, Simpson T. Hydrogel lens dehydration and subjective comfort and dryness ratings in symptomatic and asymptomatic contact lens
wearers. Optom Vis Sci 1999;76:700–4.
40. Glasson M, Stapleton F, Willcox M. Lipid, lipase and lipocalin differences between tolerant and intolerant contact lens
wearers. Curr Eye Res 2002;25:227–35.
41. Thai LC, Tomlinson A, Simmons PA. In vitro and in vivo effects of a lubricant in a contact lens
solution. Ophthalmic Physiol Opt 2002;22:319–29.
42. Guillon M, Guillon JP. Hydrogel lens wettability during overnight wear. Ophthalmic Physiol Opt 1989;9:355–9.
43. Efron N, Brennan NA. A survey of wearers of low water content hydrogel contact lenses. Clin Exp Optom 1988;71(3):86–90.
44. Pritchard N, Fonn D. Dehydration, lens movement and dryness ratings of hydrogel contact lenses. Ophthalmic Physiol Opt 1995;15:281–6.
45. Fonn D, Dumbleton K. Dryness and discomfort with silicone hydrogel contact lenses. Eye Contact Lens
46. Young G, Bowers R, Hall B, Port M. Six month clinical evaluation of a biomimetic hydrogel contact lens
. CLAO J 1997;23:226–36.
47. Young G, Bowers R, Hall B, Port M. Clinical comparison of Omafilcon A with four control materials. CLAO J 1997;23:249–58.
48. Lemp MA, Caffery B, Lebow K, Lembach R, Park J, Foulks G, Hall B, Bowers R, McGarvey S, Young G. Omafilcon A (Proclear) soft contact lenses in a dry eye
population. CLAO J 1999;25:40–7.
49. Hall B, Jones S, Young G, Coleman S. The on-eye dehydration of proclear compatibles lenses. CLAO J 1999;25:233–7.
50. Young G. Why one million contact lens
wearers dropped out. Cont Lens Anterior Eye 2004;27:83–5.
51. Morgan P. Is the UK contact lens
market healthy? Optician 2001;221:22–6.
52. Sulley A, Rogers J, Griffin P. Lapsed wearers—a bigger problem that we thought. Poster presented at the British Contact Lens
Association Conference, Birmingham; 2002.
53. Efron N, Brennan NA. A survey of wearers of low water content hydrogel contact lenses. Clin Exp Optom 1988;71:86–90.
54. Efron N, Morgan PB. Hydrogel contact lens
dehydration and oxygen transmissibility. CLAO J 1999;25:148–51.
55. Andrasco G. Hydrogel dehydration in various environments. Int Contact Lens
56. Morgan PB, Efron N, Morgan SL, Little SA. Hydrogel contact lens
dehydration in controlled environmental conditions. Eye Contact Lens
57. Brennan NA, Efron N, Bruce AS, Duldig DI, Russo NJ. Dehydration of hydrogel lenses: environmental influences during normal wear. Am J Optom Physiol Opt 1988;65:277–81.
58. Quesnel NM, Giasson CJ. On-eye dehydration of proclear, resolution 55G and Acuvue contact lenses. Cont Lens Anterior Eye 2001;24:88–93.
59. Covey M, Sweeney DF, Terry R, Sankaridurg PR, Holden BA. Hypoxic effects on the anterior eye of high-Dk soft contact lens
wearers are negligible. Optom Vis Sci 2001;78:95–9.
60. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Vascular response to extended wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci 2001;78:147–51.
61. Brennan NA, Coles ML, Comstock TL, Levy B. A 1-year prospective clinical trial of Balafilcon A (PureVision) silicone-hydrogel contact lenses used on a 30-day continuous wear schedule. Ophthalmology 2002;109:1172–7.
62. Fonn D, MacDonald KE, Richter D, Pritchard N. The ocular response to extended wear of a high Dk silicone hydrogel contact lens
. Clin Exp Optom 2002;85:176–82.
63. Malet F, Pagot R, Peyre C, Subirana X, Lejeune S, George-Vicariot MN, Bleshoy H, Long B. Clinical results comparing high-oxygen and low-oxygen permeable soft contact lenses in France. Eye Contact Lens
64. Maldonado-Codina C, Morgan PB, Schnider CM, Efron N. Short-term physiologic response in neophyte subjects fitted with hydrogel and silicone hydrogel contact lenses. Optom Vis Sci 2004;81:911–21.
65. Long B, McNally J. The clinical performance of a silicone hydrogel lens for daily wear in an Asian population. Eye Contact Lens
66. Dumbleton K, Keir N, Moezzi A, Feng Y, Jones L, Fonn D. Objective and subjective responses in patients refitted to daily-wear silicone hydrogel contact lenses. Optom Vis Sci 2006;83:758–68.
67. Roth H, Amon K, Bruckmann P, Degle S, Pilz P, Schmitt-Lieb K, et al. A silicone hydrogel for daily, flexible and extended wear. Optician 2005;230:20–5.
68. Cavanagh HD, Ladage P, Yamamoto K, Li SL, Petroll WM, Jester JV. Effects of daily and overnight wear of hyper-oxygen transmissible rigid and silicone hydrogel lenses on bacterial binding to the corneal epithelium: 13-month clinical trials. Eye Contact Lens
69. Cavanagh HD, Ladage PM, Li SL, Yamamoto K, Molai M, Ren DH, Petroll WM, Jester JV. Effects of daily and overnight wear of a novel hyper oxygen-transmissible soft contact lens
on bacterial binding and corneal epithelium: a 13-month clinical trial. Ophthalmology 2002;109:1957–69.
70. Ren DH, Yamamoto K, Ladage PM, Molai M, Li L, Petroll WM, Jester JV, Cavanagh HD. Adaptive effects of 30-night wear of hyper-O2
transmissible contact lenses on bacterial binding and corneal epithelium: a 1-year clinical trial. Ophthalmology 2002;109:27–39.
71. Ladage PM, Yamamoto K, Ren DH, Li L, Jester JV, Petroll WM, Cavanagh HD. Effects of rigid and soft contact lens
daily wear on corneal epithelium, tear lactate dehydrogenase, and bacterial binding to exfoliated epithelial cells. Ophthalmology 2001;108:1279–88.
72. Zaidi TS, Fleiszig SM, Preston MJ, Goldberg JB, Pier GB. Lipopolysaccharide outer core is a ligand for corneal cell binding and ingestion of Pseudomonas aeruginosa
. Invest Ophthalmol Vis Sci 1996;37:976–86.
73. Fleiszig SM, Lee EJ, Wu C, Andika RC, Vallas V, Portoles M, Frank DW. Cytotoxic strains of Pseudomonas aeruginosa
can damage the intact corneal surface in vitro. CLAO J 1998;24:41–7.
74. Imayasu M, Petroll WM, Jester JV, Patel SK, Ohashi J, Cavanagh HD. The relation between contact lens
oxygen transmissibility and binding of Pseudomonas aeruginosa
to the cornea after overnight wear. Ophthalmology 1994;101:371–88.
75. Keay L, Sweeney DF, Jalbert I, Skotnitsky C, Holden BA. Microcyst response to high Dk/t silicone hydrogel contact lenses. Optom Vis Sci 2000;77:582–5.
76. Sweeney DF. Clinical signs of hypoxia with high-Dk soft lens extended wear: is the cornea convinced? Eye Contact Lens
77. Ladage PM, Ren DH, Petroll WM, Jester JV, Bergmanson JP, Cavanagh HD. Effects of eyelid closure and disposable and silicone hydrogel extended contact lens
wear on rabbit corneal epithelial proliferation. Invest Ophthalmol Vis Sci 2003;44:1843–9.
78. Papas EB, Vajdic CM, Austen R, Holden BA. High-oxygen-transmissibility soft contact lenses do not induce limbal hyperaemia. Curr Eye Res 1997;16:942–8.
79. Papas E. On the relationship between soft contact lens
oxygen transmissibility and induced limbal hyperaemia. Exp Eye Res 1998;67:125–31.
80. du Toit R, Simpson TL, Fonn D, Chalmers RL. Recovery from hyperemia after overnight wear of low and high transmissibility hydrogel lenses. Curr Eye Res 2001;22:68–73.
81. Coles C, Brennan N, Jaworski A, Woods J. Ocular signs and symptom in patients completing 3 years with silicone-hydrogel contact lenses in 30-day continuous wear. Optom Vis Sci 2001;78 (Suppl):201.
82. Coles-Brennan C, Brennan NA, Connor HR, McIlroy RG. Do silicone-hydrogels really solve end-of-day comfort problems? Invest Ophthalmol Vis Sci 2006;47:E-abstract 106.
83. Schafer J, Barr JT, Mack C. A characterization of dryness symptoms with silicone hydrogel contact lenses. Optom Vis Sci 2003;80 (Suppl):187.
84. Chalmers RL, McNally JJ, McKenney CD, Robirds SR. The role of dryness symptoms in discontinuation of wear and unscheduled lens removals in extended wear of silicone hydrogel lenses. Invest Ophthalmol Vis Sci 2002;43:E-abstract 3088.
85. Liu J, Thota S, Ladage PM, Quintero S, Parker K, Bynum R, Hanson K, Chagpar N, Perrigin D, Leach N, Bergmanson JP. The relationship of initial wettability and comfort in silicone hydrogel contact lens
wearers. Optom Vis Sci 2004;81 (Suppl):82.
86. Chalmers RL, Dillehay S, Long B, Barr JT, Bergenske P, Donshik P, Secor G, Yoakum J. Impact of previous extended and daily wear schedules on signs and symptoms with high Dk lotrafilcon A lenses. Optom Vis Sci 2005;82:549–54.
87. Sweeney D, duToit R, Jalbert I, Sankaridurg PR, Holden BA, Skotnitsky C, Stephensen A, Covey M, Rao GN. Clinical performance of silicone hydrogel lenses. In: Sweeney D, ed. Silicone Hydrogels: Continuous Wear Contact Lenses. 2nd ed. Oxford, UK: Butterworth-Heinemann; 2004:164–216.
88. Fonn D, Pritchard N, Dumbleton K. Factors affecting the success of silicone hydrogels. In: Sweeney D, ed. Silicone Hydrogels: Continuous Wear Contact Lenses. Oxford, UK: Butterworth-Heinemann; 2000:214–34.
89. Senchyna M, Jones L, Louie D, May C, Forbes I, Glasier MA. Quantitative and conformational characterization of lysozyme deposited on balafilcon and etafilcon contact lens
materials. Curr Eye Res 2004;28:25–36.
90. McKenney C, Becker N, Thomas S, Castillo-Krevolin C, Grant T. Lens deposits with a high Dk hydrophilic soft lens. Optom Vis Sci 1998;75 (Suppl):276.
91. Jones L, May C, Nazar L, Simpson T. In vitro evaluation of the dehydration characteristics of silicone hydrogel and conventional hydrogel contact lens
materials. Cont Lens Anterior Eye 2002;25:147–56.
92. Tighe B. Silcone hydrogels: structure, properties and behaviour. In: Sweeney D, ed. Silicone Hydrogels: Continuous-Wear Contact Lenses. 2nd ed. Oxford, UK: Butterworth-Heinemann; 2004:1–27.
93. Nick J, Winterton L, Lally J. Enhancing comfort with a lubricating daily disposable. Optician 2005;229:30–2.
94. Peterson RC, Wolffsohn JS, Nick J, Winterton L, Lally J. Clinical performance of daily disposable soft contact lenses using sustained release technology. Cont Lens Anterior Eye 2006;29:127–34.
95. Tighe B. Contact lens
materials. In: Philips AJ, Speedwell L, eds. Contact Lenses. 5th ed. Oxford, UK: Butterworth-Heinemann; 2004:50–92.
96. Osborn K, Veys J. A new silicone hydrogel lens for contact lens
-related dryness, Part 1: Material properties. Optician 2005;229:39–41.
97. Thai LC, Tomlinson A, Doane MG. Effect of contact lens
materials on tear physiology. Optom Vis Sci 2004;81:194–204.
98. Tonge S, Tighe B, Franklin V, Bright A. Contact lens
care, Part 6: Comfort drops, artificial tears and dry-eye therapies. Optician 2001;222:27–32.
99. Ketelson HA, Meadows DL, Stone RP. Dynamic wettability properties of a soft contact lens
hydrogel. Colloids Surf B Biointerfaces 2005;40:1–9.
100. Subbaraman LN, Bayer S, Gepr S, Glasier MA, Lorentz H, Senchyna M, Jones L. Rewetting drops containing surface active agents improve the clinical performance of silicone hydrogel contact lenses. Optom Vis Sci 2006;83:143–51.
101. Szczotka-Flynn LB. Chemical properties of contact lens
rewetters. Contact Lens
102. Wolffsohn J, Borazjani R, Groeminger S, Jani G. ReNu with MoistureLoc: a literature review. Optician 2005;229:16–18.
103. Ammon D. Multi-purpose solutions designed for silicone hydrogel lenses. Rev Optom 2005;142(9):4–5.
104. Bailey GM, Bowling E, Huh S, Raheja MK, Salamone J, Stone R. The science behind contact lens
solutions, part 1. Rev Optom 2005;142(6s):12–16.
105. Varikooty J, Situ P, Jones L, Fonn D. Clinical performance of alexidine-based and polyquad-based multipurpose solutions when used with daily wear balafilcon lenses. Optom Vis Sci 2005;82:E-abstract 055097.
106. Stiegemeier MJ, Friederichs GJ, Hughes JL, Larsen S, Movic W, Potter WB. Clinical evaluation of a new multi-purpose disinfecting solution in symptomatic contact lens
wearers. Cont Lens Anterior Eye 2006;29:143–51.