Contact Lens & Anterior Eye
Volume 29, Issue 1 , Pages 41-47, March 2006

The effect of short term contact lens wear on the tear film and ocular surface characteristics of tolerant and intolerant wearers

  • M.J. Glasson

      Affiliations

    • Vision CRC, Sydney, Australia
    • School of Optometry and Vision Science, University of New South Wales, Sydney, Australia
    • ,
  • F. Stapleton

      Affiliations

    • Vision CRC, Sydney, Australia
    • Institute for Eye Research, Sydney, Australia
    • School of Optometry and Vision Science, University of New South Wales, Sydney, Australia
    • ,
  • L. Keay

      Affiliations

    • Vision CRC, Sydney, Australia
    • School of Optometry and Vision Science, University of New South Wales, Sydney, Australia
    • ,
  • M.D.P. Willcox

      Affiliations

    • Vision CRC, Sydney, Australia
    • Institute for Eye Research, Sydney, Australia
    • School of Optometry and Vision Science, University of New South Wales, Sydney, Australia
    • Corresponding Author InformationCorresponding author at: Institute for Eye Research, Rupert Myers Building, University of New South Wales, Sydney, NSW 2052, Australia. Tel.: +61 293857412; fax: +61 293867401.

published online 23 February 2006.

Article Outline

Abstract 

The purpose of this study was to determine the effect of contact lens wear on the tear film and ocular surface of people tolerant or intolerant to contact lens wear. Twenty subjects participated; 11 tolerants and nine intolerants. Their baseline tear film (no lens wear) was analysed with a range of clinical measurements and protein analyses (lactoferrin, sIgA and lysozyme). The tests were then repeated at the end of 6h of contact lens wear during the day and while lenses were worn. Both tolerants and intolerants showed statistically significant increases in bulbar and overall conjunctival redness after 6h of lens wear. For tolerants only, there was a statistically significant increase in the tear film meniscus area (0.08mm2±0.04 compared to 0.14mm2±0.06 (p=0.023)) and a statistically significant decrease in the non-invasive tear film break-up time (NI-TBUT; 21.3s±5.7 compared to 3.7s±4.3 (p=0.003)) after 6h of lens wear. There were no changes in other tear film or ocular surface parameters. The protein concentration and lipid layer appearance did not change during lens wear for either population. Prior to lens wear, tolerant subjects had a statistically longer NI-TBUT, higher phenol red thread test and higher tear flow rate. After 6h of lens wear and while wearing lenses, all but NI-TBUT remained statistically different. Lens wear affected only a small number of clinical variables and 6h wear did not effect the concentration of those proteins measured in tears in this study.

Keywords: Comfort, Tear film, Group IV lenses

 

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1. Introduction 

Contact lens induced dry eye is a major cause of contact lens intolerance and discontinuation, and decreased tear volume is implicated in the development of intolerance [1]. Ideally a lens should be submerged in tears, having a tear layer both between the posterior surface of the lens and the epithelial surface (post-lens tear film) and on the lens front surface (pre-lens tear film). The biocompatibility of a lens material in tear fluid is largely dependent upon the wetting nature of the material and this in turn is related to the co-monomers, manufacturing process, rate of deposition of tear film components and tear film of the wearer. The latest technologies for lens materials can alter the first three of these, but the tear film of the wearer is an uncontrollable factor, which seems to be a major determinant of successful lens wear [2].

The tear film is an interactive system that includes mucins, proteins and lipids. These components form a layered or phase-like film, with estimates of the thickness ranging from 40μm [3] to 3μm [4], [5]. The volume of the tear film has been determined with fluorescence techniques to be approximately 6–7μl [6]. The production rate has been measured by various researchers and found to be in the range of <1–1.2μl/min for non-stimulated (basal) tears and greater than 5μl/min for stimulated (reflex) tears [6], [7]. Tear flow rate and tear film stability, which are related primarily to the aqueous phase and tear break-up time (TBUT), are lower in symptomatic contact lens wearers [8], [9]. Decreased tear volume is implicated in intolerance to contact lenses [1]. Evaporation is linked to the protective lipid layer, which impedes evaporation from the tear film. Evaporation has been shown to increase during lens wear [10].

Pre-corneal non-invasive tear break-up time (NI-TBUT) can range in time from very poor <10s to very good >30s. Symptomatic contact lens wearing patients have a corneal NI-TBUT in the region of 3–10s, which is similar to that seen in mild KCS patients [11]. In most studies, tears begin to break-up within 2–3s on the front surface of a rigid gas permeable contact lens and 5–6s on soft contact lenses [12], [13]. Young and Efron [14] demonstrated that tear break-up occurs within 3–10s on the front surface of hydrogel lenses, and that longer break-up times were generally associated with higher water content lenses. This latter finding is consistent with the thicker aqueous layer present on higher water content lenses observed in the same study [14].

Recently, we have demonstrated that, in the absence of contact lens wear, there are differences in the tear film of people tolerant or intolerant to contact lenses. Tolerant subjects have fewer symptoms of discomfort, a more stable tear film (as measured by higher maximum forced interval between blinks, phenol red thread test of tear film volume, tear meniscus height and volume, non-invasive tear film break-up time and type of drying pattern), lower levels and activity of secretory phospholipase A2, lower concentration of lipocalin and lower levels of peroxidised lipids in the tear film [2], [15]. There were no differences in the conjunctival or limbal redness, lipid layer appearance, tear flow rate, tear film osmolality, and total protein, lactoferrin, lysozyme or secretory IgA concentrations in the tear film between tolerants and intolerants in the absence of lens wear [2]. This current study was designed as a prospective case control study to investigate the effect of contact lens wear on the tear film and ocular surface characteristics of tolerant and intolerant subjects.

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2. Methods 

2.1. Subject selection 

Subjects with a history of contact lens wear participated in this study. Subjects were not habitual contact lens wearers and we chose this group in order to avoid any possibility of changes in tear film due to habitual contact lens wear. Twenty subjects were recruited for tests over 3 days. All subjects signed informed consent, the protocol was approved by the Institutional Ethics Committee of the University of New South Wales, and the guidelines of the Association for Research in Vision and Ophthalmology and the Declaration of Helsinki were adhered to. Subjects were classified as tolerant or intolerant to contact lens wear as described previously [2]. Intolerant subjects were classified as those who would prefer not wear lenses for longer than 6h during the day. Subjects had not worn lenses for atleast 1 week prior to enrolment. Subjects came to the clinic in the morning and had their eyes evaluated clinically and tears collected for subsequent analysis. The stability of a subjects’ ocular characteristics over the day (at 10a.m., 1p.m. and 4p.m. without CLs), and their tears in terms of total protein content were analysed to remove the possibility that changes seen after lens wear were due solely to the fact that these measures were conducted in the afternoon, whereas measurements taken without lens wear were recorded in the morning prior to fitting lenses. Group IV hydrogel lenses (Focus Monthly, CIBA Vision, Duluth, GA, USA) were then fitted and subjects wore lenses for 6h, after which time they returned to the clinic for ocular evaluation and tear collection. During the contact lens phase of the project, all tests were conducted with the lens in situ.

2.2. Maximum blink interval (MBI) 

This interval was the length of time a subject could hold their gaze (stare) on an object (approximately 3m distance) before ocular irritation occurred and without reflex tearing [16]. This usually meant passing the first urge to blink, which occurred in 3–5s, and concentrating on the next ocular feeling of dryness or irritation and then blinking. A stopwatch was used to record the MBI, and three consecutive readings for each eye were averaged (the intrasubject variability was approximately 17% of the mean).

2.3. Phenol red tear test (PRTT) 

Patients were asked to keep their eyes open (blinking gently if necessary) for 15s while a phenol red impregnated cotton thread (ZONE-QUICK; Menicon Co. Ltd., Japan) was placed in their lower conjunctival sac. This test is based on the Hamano cotton thread test and estimates tear volume in the lower meniscus sac [17]. Three consecutive readings for each eye were averaged and results reported as millimetres (mm) of tear wetting (the intrasubject variability was approximately 22% of the mean).

2.4. Non-invasive tear break-up time (NI-TBUT) 

The tear break-up time was determined non-invasively using a custom-made tearscope on a modified slitlamp. The technique was based on that of Guillon and Guillon [13] as described in Carney et al. [20]. NI-TBUT was the time measured, in seconds, between the full opening of the eyelids following a complete blink and the first break in the tear film. Three consecutive readings for each eye were averaged (the intrasubject variability was approximately 8% of the mean).

2.5. Meniscus height and area 

The inferior tear meniscus height (tear prism) was recorded using a Sony 3 CCD video camera attached to a Zeiss 30 SL/M slit-lamp biomicroscope [2]. The tear prism near the middle of the lid margin was observed using an optic section with the microscope at 45° to the light path to form a specular reflection of the prism. The magnification was fixed for each measurement where 1mm=60μm. The images, one for each eye, were measured manually for height and prism area (triangular shape) and averaged. Measurements were adjusted for magnification.

2.6. Conjunctival redness 

The CCLRU decimalised grading scale [21] (range 1–4) was used to assess the redness of both eyes in the nasal, temporal, superior and inferior quadrants for both the limbal and bulbar regions [21] and then averaged to give an overall redness score for the conjunctiva of the eye. Intrasubject variability did not exceed 16% of the mean.

2.7. Lipid layer appearance 

Slit lamp examination of a subject's ocular lipid layer in both eyes was measured on a graded scale (0–5) [13], where 0 is no lipid, 1 is an open meshwork, 2 is a tight meshwork, 3 is a flow pattern and 4 is an amorphous pattern and 5 has coloured fringes.

2.8. Tear collection 

Basal (open-eye) tears were collected with glass microcapillary tubes [7], [18], [19]. The time taken for tears to reach a specified marked point on the microcapillary tube was recorded as flow rate (μl/min). Tear collection continued until a minimum of 15μl was collected on 1 day from a combination of both eyes. In some cases this took up to 40min without reflex stimulation and no irritation (and often subjects rested for 5min between each 5μl collected). After collection, tears were centrifuged at 1000×g for 5min to remove debris, then placed into smaller aliquots and stored at −80°C until all subjects had completed clinical examination.

2.9. Total protein content of tears 

Total protein was determined using a semi-quantitative assay (bicinchoninic acid; Bio-Rad, Richmond, CA, USA). Standards of bovine serum albumin ranged from 0–1mg/ml in 0.1M Tris base, pH 11 buffer. Tear samples (10μl) were diluted 1/50 or 1/100 in Tris pH 11 buffer. The data was expressed as the mean of two samples from the eye.

2.10. Lactoferrin and sIgA 

A commercially available enzyme linked immunosorbent assay (ELISA; Oxis International Inc., Portland, OR, USA) was used for lactoferrin according to the manufacturer's instructions and a published sandwich ELISA method was used for sIgA [22]. Tears were diluted to 1/10,000 and 1/20,000 in sample diluting phosphate buffer supplied for lactoferrin and tears were diluted 1/1000 in phosphate buffered saline (pH 7.4; PBS) containing 0.1% (v/v) Tween 20 for sIgA. The data were expressed as the mean of two samples from the eye.

2.11. Lysozyme 

A turbidimetric assay [23] was used and included 20μl human milk lysozyme standards (Sigma–Aldrich, St. Louis, MO, USA; 0.016–1.0mg/ml) and 20μl tear samples (diluted 1 in 2 or 1 in 4 in PBS). Samples/standards were mixed with 200μl Micrococcus lysodeikticus (Sigma–Aldrich, St. Louis, MO, USA; 1mg/ml in PBS). Changes in optical density after 15min at 35°C were measured at 450nm and converted to μg/μl of active lysozyme concentration using a standard curve. The data were expressed as the mean of two samples from the eye.

2.12. Statistical analysis 

The Statistical Package for Social Sciences (SPSS; Windows version 10.0.05 Chicago, Il, USA) was used. Using the results of a pilot study measuring tear volume, which we chose as the primary outcome measure of the study based on previous data [2], we determined the number of subjects required for statistical significance. This was estimated using the GPower programme Version 2.0 [24], [25]. This indicated that two subject groups (tolerant versus intolerant) of atleast eight people each (power 80%; confidence 95%—i.e. alpha-value of 0.05; intrasubject differences σ=4.8; significant difference aimed to be detected between groups δ=7) were needed.

Averages of repeated measurements were made for each subject; for example, duplicates in the biochemical assays or results for each eye were averaged. All variables were checked for outliers using boxplots to identify subjects; this allowed certain outlying individual results (possibly contaminated samples) to be removed from the final group for each test. Not more than one result was ever removed, thereby maintaining the required number of people in each group (≥8). All differences between the tolerant and intolerant subject groups or between pre-lens wear and during lens wear were compared using the independent group t-test for parametric data and the Mann–Whitney test for the non-parametric data (the lipid layer appearance). Diurnal readings were analysed with a repeated measures analysis of variance (ANOVA). Variables were considered statistically different if p<0.05.

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3. Results 

Table 1 gives the population demographics. There were no significant differences between the two populations in terms age or gender. The measurement of tear break-up time did not vary for three consecutive readings for each eye at each time point (Table 2). Similarly maximum forced blink interval (MBI) was repeatable over consecutive readings, between eyes and during the day. The phenol red thread test and total protein concentration measured once at each time point during the day and for each eye were also repeatable (Table 2). Previously, it has been shown that redness responses do not alter significantly during the day (8h of open-eye) in the absence of lens wear [26]. Therefore, differences in the clinical ocular responses and tear film biochemical characteristics are likely to be due to lens wear and not to differences that might occur due to taking measurements at different times of the day.

Table 1. Population demographics
Contact lens preferenceNumber in each groupAge (years)Gender (M/Fa)
Tolerant1125–392/9
Intolerant923–401/8

aThe number of females using oral contraceptives at the time of each study was three tolerant and two intolerant contact lens wearers.

Table 2. Analysis of variation between readings for some variables used in this study prior to lens wear
Between eyesDiurnal 10a.m., 1p.m., 4p.m.a
NI-TBUTp=0.900p=0.323, F=1.177
MBIp=0.805p=0.984, F=0.016
Total proteinp=0.244p=0.588, F=0.541

aDiurnal readings were measured at these different time points prior to beginning the full study and analysed with a repeated measures ANOVA. The differences between eyes tested was analysed with the paired t-test.

Table 3, Table 4 show the results for tolerant and intolerant subjects, respectively. For tolerant subjects, 6h of lens wear resulted in statistically significant increases in limbal and conjunctival redness, an increase in the tear meniscus area but a decrease in the NI-TBUT. There were no other statistically significant changes, in the parameters measured, to the eye during lens wear. Also, there were no changes to the protein concentrations (total or individual proteins) during 6h of lens wear. For intolerant subjects there were fewer changes to the eye during lens wear. The only significant changes were to limbal and conjunctival redness, both of which increased during lens wear. No other significant changes were detected for intolerant subjects. Fig. 1 shows the variables that were significantly different between the tolerant and intolerant groups prior to lens wear and during lens wear. While NI-TBUT, the phenol red thread test and tear flow rate were all reduced in intolerant wearers compared to tolerant wearers prior to lens wear, only phenol red thread test and tear flow rate were persistently reduced in intolerant wearers following 6h of wear. The NI-TBUT of tolerants was reduced during lens wear to a value similar to that of intolerant subjects in the absence or presence of a lens.

Table 3. The effect of wearing a Group IV HEMA-type contact lens on ocular and tear film characteristics of tolerant (N=11) wearers
Variable tested0h Baseline mean±S.D. or median/interquartile range6h Contact lens wear mean±S.D. or median/interquartile rangep value
Lipid layer appearance3/1–42/1–3ns
Limbal redness (av)2.07±0.342.42±0.290.018
Bulbar redness (av)1.69±0.231.83±0.24ns
Conjunctival redness (av)1.88±0.182.13±0.180.005
Meniscus height (mm)0.50±0.190.63±0.18ns
Meniscus area (mm2)0.08±0.040.14±0.060.023
NI-TBUT (s)21.3±5.713.7±4.30.003
Tear flow rate (μl/min)1.51±0.582.82±1.45ns
Maximum blink interval (s)27.8±16.637.8±17.4ns
Phenol red tear test (mm)16.9±6.117.9±6.1ns
Total protein (mg/ml)4.18±1.923.22±1.49ns
Lysozyme (mg/ml)1.56±0.291.37±0.33ns
Secretory IgA (mg/ml)0.86±0.140.77±0.25ns
Lactoferrin concentration (mg/ml)2.68±0.632.58±0.95ns

NI-TBUT: non-invasive tear break-up time; Bulbar and Limbal redness: average of the four quadrants graded; Conjunctival redness (av): average of limbal and bulbar conjunctival redness to show overall redness of the ocular surface; ns: not significantly different.

Table 4. The effect of wearing a Group IV HEMA-type contact lens on ocular and tear film characteristics of intolerant (N=9) wearers
Variable tested0h Baseline mean±S.D. or median/interquartile range6h Contact lens wear mean±S.D. or median/interquartile rangep value
Lipid layer appearance3/1–32/1–3ns
Limbal redness (av)2.09±0.262.64±0.230.002
Bulbar redness (av)1.92±0.342.03±0.28ns
Conjunctival redness (av)2.01±0.272.64±0.230.030
Meniscus height (mm)0.48±0.160.57±0.11ns
Meniscus area (mm2)0.07±0.050.11±0.03ns
NI-TBUT (s)13.7±2.812.7±4.6ns
Tear flow rate (μl/min)0.76±0.440.82±0.36ns
Maximum blink interval (s)29.8±19.348.3±37.1ns
Phenol red tear test (mm)9.2±3.47.7±3.7ns
Total protein (mg/ml)4.74±1.345.51±1.36ns
Lysozyme (mg/ml)1.32±0.181.26±0.26ns
Secretory IgA (mg/ml)0.81±0.240.97±0.18ns
Lactoferrin concentration (mg/ml)2.51±0.332.99±0.75ns

NI-TBUT: non-invasive tear break-up time; Bulbar and Limbal redness: average of the four quadrants graded; Conjunctival redness (av): average of limbal and bulbar conjunctival redness to show overall redness of the ocular surface; ns: not significantly different.

  • View full-size image.
  • Fig. 1. 

    Differences between tolerant and intolerant contact lens wearers. NI-TBUT: non-invasive break-up time of tears. Tear flow rate given at 10 times the rate so values should be divided by 10 to give μl/min. After 6h of lens wear, NI-TBUT was not statistically significantly different between the groups.

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4. Discussion 

The effect of contact lens wear on the tear variables of both tolerant and intolerant contact lens wearers was to increase limbal and conjunctival redness. Limbal hyperaemia has previously been shown to increase during 4 and 16h of soft contact lens wear, and this was thought to be due to local hypoxia at the lens edges [26]. The stability of the tear film in tolerant contact lens wearers was affected by contact lens wear more than the intolerant contact lens wearers. The NI-TBUT decreased more dramatically in the tolerant contact lens wear group. The NI-TBUT of intolerant subjects was significantly lower initially before lens wear and remained low over the 6h of lens wear. NI-TBUT of severe dry eye subjects is commonly under 10s if the tear film is present at all [27], [28], [29], [30]. Intolerant contact lens wearers, although having a faster NI-TBUT than tolerant contact lens wearers, did not have the very fast tear break-up times observed by KCS patients [11], [31]. Measures of tear thinning rates show that contact lenses give higher tear thinning rates than tears on the corneal surface [32]. The pre-lens tear film may be considerably thinner than the pre-corneal tear film [4] leading to higher tinning rates. However, even if these films are of roughly similar thickness as has been proposed [33], the pre-lens tear film is still much less stable [33].

Contact lens wear also increased the tear meniscus area of tolerant wearers only. This increase in meniscus area may be in response to the slight irritation felt by having a contact lens in the eye and a faster blink frequency when wearing lenses [34], [35], [36] resulting in a greater volume of tears being released onto the ocular surface. However, the phenol red thread test did not show an increase in tear volume. Tomlinson et al. [37] have argued that the phenol red thread test does not measure tear production or volume but may reflect simply the ability of different tears to move up the thread. If there was a greater tear volume in tolerant subjects this did not result in a more stable tear film; the NI-TBUT, independent of tolerance and for each group, was reduced during contact lens wear. This increase in tear meniscus area during lens wear was not associated with an increase in tear meniscus height, indicating that the increase in area was probably due to movement of the lower lid margin away from the globe or a radius change. In contrast to the finding in the current study, Miller et al. [38] demonstrated that there was no difference between the tear meniscus height of non-lens wearers compared to adapted successful lens wearers. The reasons for this are unknown, although it may simply be due to differences in populations or methodologies.

All protein variables were not significantly affected by either lens type over 6h wear time, for either the tolerant or intolerant contact lens wearers. This finding has also been observed by other researchers. Carney et al. [39] found over a 6-month soft lens wear period that lactoferrin (0.3–0.7mg/ml) did not significantly change over time. Other researchers have shown that the average lactoferrin concentration was 1.6mg/ml in reflex tears at 6 and 9 months of contact lens wear which was also not significantly different from basal tear film concentrations [40], [41]. Additionally, no significant differences for long-term soft lens wear was found for lysozyme concentrations [39], [42], [43].

Comparing tolerant contact lens wearers to intolerant contact lens wearers at baseline, differences in NI-TBUT, PRTT and tear flow rate were found, all being lower to begin with in the intolerant contact lens wearing group. These differences have been shown previously [2]. This potentially confirms the clinical relevance of these tests in predicting likely problems with contact lens tolerance. In the present study we did not demonstrate differences in MBI and meniscus height or area that had been seen previously in the absence of contact lens wear [2]. This appeared to be due to the MBI and meniscus measures being higher in the intolerant subjects in the current study. After 6h of lens wear, the NI-TBUT of tolerant wearers reduced to the extent that this was no longer different to intolerants. However, the phenol red thread test and tear flow rate were still significantly different between the two groups after 6h of wear. Again, this indicates, perhaps, the instability of the tear film on the surface of these lenses.

In conclusion, this study has found that it is the tear film of subjects tolerant to contact lenses that change more dramatically than the tears of intolerant subjects during 6h wear of a Group IV hydrogel lens. The tear film of intolerants did not alter significantly probably due to the fact that there was less volume and relatively poor stability even without contact lenses. Irrespective of tolerance, adding a contact lens into the eye rendered the tears unable to form a stable film over the lens surface. Thus, a contact lens with improved tear film holding ability might be a more successful; helping to maintain the tear film structure of atleast tolerant wearers. Intolerant wearers are probably intolerant due to as yet unknown defects in tear film structure even prior to lens wear.

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Acknowledgement 

This study was partly supported by the Australian Federal Government through the Co-operative Research Centres Programme.

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PII: S1367-0484(06)00008-7

doi:10.1016/j.clae.2005.12.006

Contact Lens & Anterior Eye
Volume 29, Issue 1 , Pages 41-47, March 2006

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