Effect of Environmental Conditions on Tear Dynamics in Soft Contact Lens Wearers

Abstract

purpose. Dry eye symptoms are often associated with soft contact lens (SCL) wear and may be affected by environmental conditions. This study was conducted to determine the effects of temperature and humidity on the tear film in SCL wearers.

methods. All 11 enrolled subjects were males (mean age, 23.5 ± 5.2 [SD] years), and all wore SCL daily. They were exposed in different sessions to four different conditions in an environmental chamber with the air temperature (AT) and relative humidity (RH) set at 5°C/10% (AT/RH), 15°C/20%, 25°C/40%, or 35°C/50%. Two different types of hydrogel SCL (SCL-a and SCL-b; water content 72.0% and 37.5%, respectively) were used. The meniscus tear volume was determined on a video meniscometer by measuring the tear meniscus radius (TMR) with and without SCL. The tear interference patterns on the contact lens (TIPCL) were classified into five grades (the higher the grade, the thinner the film). Using a video interferometer, the non-invasive tear film breakup time (NIBUT) was recorded with and without SCLs; ocular dryness was also scored with and without SCLs.

results. Under the environmental conditions examined, there were no significant differences in the TMR without or with SCL, regardless of their type. As AT and RH decreased, there was a significant increase in the TIPCL grade (CL-a: P = 0.042; CL-b: P = 0.002), a significant decrease in NIBUT (CL-a: P = 0.004; CL-b: P = 0.001), and a significant increase in the dryness score (without SCL P = 0.023; with CL-a P = 0.009; with CL-b P = 0.003). The dryness scores were higher with CL-a than CL-b (P = 0.011 at 15°C/20%). Under identical experimental conditions, we observed no significant change in NIBUT in the absence of an SCL.

conclusions. AT and RH apparently had no effect on the tear volume in the presence of SCLs. As AT and RH decreased, the tear film on the SCL became thinner, NIBUT became shorter, and dryness increased. Dryness was more pronounced in eyes with SCL of the higher water content.

The normal tear film maintains the integrity of the ocular surface epithelium by delicate interactions among lipid, aqueous, and mucus layers. During contact lens (CL) wear, each layer of the tear film may undergo marked changes; the tear film on the soft CL (SCL) is reportedly very thin, unstable, and evaporates easily. ^{1 2} The SCL material may also affect the tear film, in that SCLs with higher water content and thin lens thickness dehydrate easily. ³ More than 50% of SCL wearers reportedly suffer dry eye symptoms ^{4 5} reflective of tear film abnormality; “dryness” is the most common complaint. ⁶  

Previous studies have shown that dry eye symptoms in SCL wearers are affected by environmental conditions such as temperature and humidity ⁷ and that the stability of the prelens tear in SCL wearers is influenced by the relative humidity in a room. In addition, dryness in CL wearers changes with climatic ⁸ and room conditions. ⁹ However, little is yet known regarding the effects of temperature and humidity on the dynamics of the tear film. 

Therefore, we used recently developed noninvasive techniques to assess the effect of various environmental conditions and differences in the SCL water content on tear dynamics. 

Methods

Subjects

All 11 enrolled male subjects (mean age, 23.5 ± 5.2 [SD] years), wore CLs every day for 12 to 18 hours. They exhibited no signs of tear-deficient dry eye by the Schirmer I test without anesthesia (22.6 ± 9.3 mm; mean ± SD; range, 10–35 mm) and had no eye diseases other than myopia. We restricted our study population to 11 subjects, because our purpose was to explore the relationship between prelens tear dynamics and changes controlled environmental conditions. 

Before the study, all protocols were explained fully to each subject and written informed consent was obtained. This study was approved by the institutional review board of our university (No. MCHS-143) and adhered to the tenets of the Declaration of Helsinki. 

Types of SCL

Two commercially available SCL types of similar lens design and made of hydrogel nonionic material with a water content of 72.0% (CL-a) or 37.5% (CL-b) were used (Table 1) . Before the study, we confirmed that both types of the brand-new SCL fit each subject properly and that the lenses were similar in movement, central placement, and comfort. 

Environmental Condition

During the assessments, the subjects were in a 34.8 m³ aluminum environmental chamber (4.07 m wide, 2.33 m high, 3.67 m deep; Tabai Espec Co. Ltd., Osaka, Japan; Fig. 1a ) that allowed for the independent creation of relative humidity (from 20% to 95%) at temperatures between −10°C and 60°C. For this study, four different conditions were selected: air temperature/relative humidity (AT/RH) 5°C/10%, 15°C/20%, 25°C/40%, and 35°C/50% (Fig. 2) . 

Video Meniscometry

To evaluate the tear volume before and after SCL insertion, a video meniscometer (Fig. 1b , instrument on the left) ^{10 11 12 13} was used. The behavior of the central lower tear meniscus was recorded noninvasively with a digital video recorder. The TMR reflects the total tear volume over the ocular surface (Yokoi N, et al. IOVS 2000;41:ARVO Abstract 65). Our calculation of the TMR differed from that described previously. ¹¹ We captured the tear meniscus image from the recorder with a personal computer (PCG-XR100F/K; Sony Co. Ltd., Tokyo, Japan; with Scion Image software, ver. 4.02; Scion Corporation Co. Ltd, Frederick, MD) to measure the distance between the centers of two black lines (image size, i). The TMR was then calculated using the concave-mirror formula ^{12 13} where TMR = 6.0 × i (mm). 

Video Interferometry

A video interferometer (DR-1; Kowa Co., Ltd., Tokyo, Japan; Fig. 1b , instrument on the right) ¹⁴ was used to observe the interference patterns of the tear film on the SCL. This system uses a white light source, making it possible to observe specular reflected images from the tear surface. With the subjects blinking naturally, recordings are made on a digital video recorder through a half mirror. The device we used is equipped with a charge-coupled device (CCD) video camera. This system allows observation of a 7.0-mm (low magnification) or 2.0-mm (high magnification) circular area in the central cornea or SCL. In the course of the observation period with the subjects blinking the same eye, highly reproducible interference images, in harmony with blinking, appeared repeatedly. 

After preliminary observations on numerous subjects, each image obtained from the surface of the tear film in the center of the SCL was highly magnified, and its tear interference pattern (TIPCL; Fig. 3 ) was assigned a numbered grade: 1, grayish interference, presumably corresponding to a thin lipid layer, with smooth expansion on the aqueous layer on the CL after blinking; 2, grayish interference with multicolored interference below the grayish color, presumably corresponding to a thin lipid and a thin aqueous layer on the CL; and 3, a single colorful interference pattern implying a thin aqueous layer without a lipid layer on the CL. It must be noted that this colorful interference pattern derived from the aqueous layer differed from the thick lipid layer described previously (Yokoi N, et al. IOVS 2000;41:ARVO Abstract 65) in that it moved upward repeatedly in harmony with blinking. The fourth pattern was similar to the third except that the surface of the contact lens was partially exposed immediately after blinking. In the fifth pattern, the surface of the CL was completely exposed immediately after blinking. 

Non-invasive Tear Film Breakup Time

Non-invasive tear film breakup time (NIBUT) ¹⁵ was measured on the cornea before SCL wear and again, after SCL wear, at the center of the SCL, by a video interferometer with low magnification. Subjects were first asked to blink naturally and keep their eyes open for 10 seconds. NIBUT was recorded once, as the time it took for the first major alteration in the interference pattern to appear (Fig. 4) . In subjects who showed no breakup for 10 seconds within the observation period, NIBUT was recorded as 10 seconds. 

Dryness

Awareness of symptomatic ocular dryness was scored before and after SCL insertion: 0, none (not aware of dryness); 1, mild (aware of mild dryness); 2, moderate (aware of moderate dryness); and 3, severe (aware of severe dryness). 

Study Protocol

Examinations before SCL Insertion.

The subjects were instructed not to wear their regular CL on the day preceding and on the day of the experiments. Until this wash-out period, all had been wearing daily disposable CLs as their regular lenses. 

At the inception of the experiment, the subjects and examiners remained in the environmental chamber (25°C/40%) for 15 minutes. The tear volume of the right eye was recorded using a video meniscometer, subsequently, NIBUT on the cornea was evaluated, and a dryness score was assigned. 

Examinations after SCL Insertion.

A brand-new SCL (CL-a or CL-b) was inserted into the right eye, and the subject remained in the environmental chamber for 15 minutes in one of the environmental conditions studied. Then the tear volume in the right eye was evaluated as above; and TIPCL, NIBUT on the SCL, and dryness were recorded. Subsequently, the SCL was replaced with a brand-new SCL of the other type (CL-b or CL-a) and the experimental steps were repeated. 

On different days, the experimental steps were repeated in each of the other environmental conditions (5°C/10%, 15°C/20%, or 35°C/50%) until each subject had undergone study under all four conditions. 

Statistics

TMR, TIPCL, NIBUT, and dryness were expressed as the mean ± SD. One-way analysis of variance (ANOVA) was used to compare the TMR in the different environments. Multiple comparisons were made with the Tukey-Kramer test. Student’s paired t-test was used to compare the recorded TMR when the subjects wore the two different types of SCLs. We used the Kruskal-Wallis test to compare nonparametric parameters (TIPCL, NIBUT, and dryness) at the different environmental conditions imposed and the Fisher protected least-significant difference test (PLSD) for multiple comparisons. The Mann-Whitney test was used for comparison of the nonparametric parameters obtained with the two types of SCL. P ≤ 0.05 was considered significant. 

Results

Video Meniscometry

The TMR before and after insertion of the two types of SCL at the four environmental conditions are listed in Table 2 . There was no significant difference in TMR, indicating that the tear volume was not affected by AT and RH in the environmental conditions imposed and that neither the absence or presence of SCLs, nor their water contents, had an effect (Fig. 5) . However, we cannot exclude the possibility that conditions not tested in this series may affect TMR. 

Video Interferometry

The TIPCL obtained with CL-a and CL-b at the environmental conditions studied is shown in Table 3 . Significant differences were found for CL-a at 15°C/20% and 35°C/50% (P = 0.029), and for CL-b at 5°C/10% and 35°C/50% (P = 0.016), at 15°C/20% and 25°C/40% (P = 0.002), and at 15°C/20% and 35°C/50% (P = 0.001). These results demonstrate that irrespective of the CL water content, TIPCL was affected by AT and RH (Fig. 6) and confirmed earlier observations that the tear film on the SCL becomes significantly thinner as both AT and RH decrease. On the contrary, at identical environmental conditions, the water content of the SCL had no significant effect on TIPCL. 

Non-invasive Tear Film Breakup Time

Before the insertion of the two types of SCL, there was no significant difference in the second NIBUTs at all experimental conditions studied (Table 4) . Significant differences in NIBUT were recorded at 15 minutes after the insertion of the two different types of lenses (CL-a: P = 0.004; CL-b: P = 0.001). For CL-a, the difference was significant at 5°C/10% and 25°C/40% (P = 0.041), at 5°C/10% and 35°C/50% (P < 0.001), at 15°C/20% and 25°C/40% (P = 0.041), at 15°C/20% and 35°C/50% (P < 0.001), and at 25°C/40% and 35°C/50% (P = 0.041). For CL-b the difference was significant at 5°C/10% and 25°C/40% (P = 0.003), at 5°C/10% and 35°C/50% (P < 0.001), at 15°C/20% and 25°C/40% (P = 0.002), and at 15°C/20% and 35°C/50% (P < 0.001). These findings demonstrate that without SCLs, NIBUTs were independent of environmental conditions, However, with either type of SCL, AT and RH exerted an effect (Fig. 7) . In addition, the stability of the tear film on the SCL was affected by decreases in AT and RH, although the water content of the lens did not play a significant role. 

Dryness

As shown in Table 5 , environmental conditions had a significant impact on the dryness score of eyes without and with SCL of both types. As AT and RH decreased, eye dryness increased (Fig. 8) . Moreover, the water content of the SCL played a role. Under identical environmental conditions, eye dryness was more pronounced in the presence of the lens with the higher water content (CL-a). This difference was statistically significant at 15°C/20% (P = 0.011). 

Discussion

We found that the tear film on SCLs became thinner and unstable and that in the presence of an SCLs, symptoms of dryness increased with a decrease in AT and RH. In addition, dryness was greater in the presence of the SCL with the higher water content. The tear volume with either type of SCL in place was not affected by decreases in AT and RH. To assess the tear volume at the meniscus during SCL wear, we adopted a video meniscometric method ^{10 11 12 13} that measured the radius of the tear meniscus at the lower lid margin. This measurement is thought to reflect the tear volume at the meniscus and thus over the ocular surface. Of note, although the radius of the tear meniscus was not affected by the temperature or humidity of the environment, the tear film on the SCL was greatly affected. These results suggest that the total tear volume over the ocular surface is less responsive to environmental temperature and humidity than are the tears on the SCL. Although we are currently unable to explain this finding, we posit that reflex tear secretion through the reflex loop ¹⁶ may compensate for the gradual changes in total tear volume induced by changes in temperature and humidity. These environment-related evaporative changes in the small, very thin tear film on the SCL may occur too fast for adequate compensation by reflex tear secretion. 

Although the tear film on the SCL is thinner than on the precornea, ¹ the effects of environmental conditions are currently unknown. We found that the tear film on the SCL became thinner with decreases in AT and RH. When both AT and RH decreased, the tear film on the SCL showed a loss of lipid interference and a very unstable aqueous layer. Others ^{17 18} have suggested that disappearance of the lipid layer may facilitate evaporation from the SCL and result in dehydration. ^{19 20} In addition, factors such as low environmental humidity have been shown to play a role in SCL dehydration. ^{21 22} Our finding that, at low AT and RH, NIBUT on the SCL was significantly shortened points to facilitated evaporation, because the tear film on the SCL lost lipid-layer coverage at low AT and RH. Our results are compatible with those of Nilsson and Andersson ⁷ who have shown that the tear breakup time (T-BUT) on the SCL surface is significantly shortened when the relative humidity is less than 31%. However, they did not report the effects of AT and RH on dryness. We found that dryness in SCL wearers was exacerbated at low AT and RH, conditions that mimicked those prevalent during the local winter season. ²³ Although dry eye symptoms are reflective of environmental conditions, the mechanism(s) underlying dryness in SCL wearers is not clearly understood. We posit that dryness may be attributable to tear film instability, a hypothesis that is supported by the observation that T-BUT on the SCL surface was shortened under conditions of low humidity. ⁷ A further contributing factor may be mechanical friction between the upper palpebral conjunctiva and the altered SCL surface after tear film breakup. Korb et al. ²⁴ have shown a correlation in SCL wearers between dryness and fluorescein and rose bengal staining of the upper lid margin. This stained lesion is called lid-wiper epitheliopathy. We found that dryness was significantly worse in the SCL with higher water content, although the lens water content did not have a significant effect on NIBUT. Although at present we cannot explain this discrepancy, we posit that it is attributable to differences in deposits on the surface of the different types of SCL. A short T-BUT has been linked to reports of subjective discomfort. ^{7 25 26} For a better understanding of the dryness in SCL wearers, other factors such as the effect of evaporation in response to changes in AT and RH on tear osmolarity, the mechanical interactions of the SCL with the ocular surface, and the effect of vasodilatation subsequent to body temperature increases, must be investigated. 

In the present study, we used an environmental chamber that allows imposition and maintenance of a controlled adverse environment and that has been used by others to explore the pathophysiology of dry eye. ²⁷ Because the tear dynamics in subjects wearing contact lenses had not been investigated, we examined the effect of AT and RH and identified the occurrence of dramatic changes in the tear film covering the SCL. The thin tear film on the SCL is similar to that in patients with severe tear-deficient dry eye, and we suggest that our model may be appropriate to analyze the tear dynamics in these patients. 

In conclusion, we demonstrated dynamic changes in the tear film on the SCL under various environmental conditions. The tear film on the SCL became thinner and unstable, and dryness increased as AT and RH decreased. The tear film condition produced at low AT and RH may be a model for the winter season, when complaints of dryness by SCL wearers tend to increase. Based on our observations, we recommend that low-water-content SCLs be prescribed for individuals who live and work in environments in which AT and RH are low. 

 Presented in part at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, May 2003.

 Supported in part by Grant-in-Aid for Scientific Research 0718003 from the Japanese Ministry of Health, Labor, and Welfare; Grant 14571683 from the Japanese Ministry of Education, Culture, Sports, Science and Technology; and research grants from the Kyoto Foundation for the Promotion of Medical Science, the Intramural Research Fund of Kyoto Prefectural University of Medicine.

 Submitted for publication October 28, 2003; revised March 22, 2004; accepted March 25, 2004.

 Disclosure: K. Maruyama, None; N. Yokoi, None; A. Takamata, None; S. Kinoshita, None

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Table 1.

 

Details of Contact Lenses

Contact Lens	Water Content (%)	Polymer Material*	FDA Group	Nominal Parameter, †	Dk/L, ‡	Trade Name	Manufacturer
CL-a	72.0	DMAA/NVP [NI]	Group IV	8.50/−3.00/14.0/0.10	34.0	Menicon Soft S	Menicon Co.
CL-b	37.5	HEMA/MA [NI]	Group I	8.40/−3.00/13.8/0.09	10.0	Menicon Soft MA	Menicon Co.

*  DMAA, dimethyl acryl amide; NVP, n-vinyl pyrolidone; MA, methacrylic acid; [NI], non-ionic material.

†  Back optic zone radius (mm)/lens power (D)/total diameter (mm)/center thickness (mm).

‡  Oxygen transmissibility; unit: ×10⁻⁹ (cm/sec) (mLO₂/mL × mm Hg).

Figure 1.

 

External appearance of the (a) environmental chamber, with the (b) video meniscometer (left) and the video interferometer (right) located in the chamber.

Figure 2.

 

The four different environmental conditions set in the chamber. The conditions were (AT/RH) 5°C/10%, 15°C/20%, 25°C/40%, and 35°C/50%.

Figure 3.

 

TIPCL scoring: grade 1, grayish interference color with smooth expansion on the aqueous layer after blinking; grade 2, grayish interference color with colorful interference beneath the grayish color, showing the presence of a thin lipid layer and a thin aqueous layer on the CL; grade 3, single colorful interference pattern, showing a thin aqueous layer without a lipid layer on the CL; grade 4, very similar to grade 3, except that the surface of the CL is partially exposed immediately after blinking; grade 5, the CL surface is completely exposed immediately after blinking. Lower images represent the cross-sectional view of the tear film on the CL (gray: contact lens surface; blue: aqueous layer; tan: lipid layer).

Figure 4.

 

Representative images of the noninvasive breakup of the tear film (a, c) immediately after blinking and (b) 9 and (d) 5 seconds after blinking, respectively; (a, b) tear lipid layer interference images at the center of the cornea before lens application; (c, d) tear lipid layer interference images at the center of the SCL after lens application. Arrows: NIBU.

Table 2.

 

TMR Before and 15 Minutes After SCL Insertion

	CL-a		CL-b		Without CL
5°C 10%	0.25 ± 0.08		0.26 ± 0.09		0.27 ± 0.08
15°C 20%	0.28 ± 0.12		0.26 ± 0.12		0.27 ± 0.10
25°C 40%	0.26 ± 0.08	NS	0.26 ± 0.07	NS	0.26 ± 0.07	NS
35°C 50%	0.25 ± 0.08		0.28 ± 0.12		0.25 ± 0.09

 Mean ± SD. NS, not statistically significant at the 5% level.

Figure 5.

 

Changes (mean ± SD) at the different environmental conditions studied in the TMR in the presence or absence of the CL.

Table 3.

 

TIPCL at 15 Minutes After SCL Insertion

 

Figure 6.

 

Changes (mean ± SD) in TIPCL at 15 minutes after CL insertion into the right eye in various environmental conditions.

Table 4.

 

NIBUT Before and 15 Minutes After SCL Insertion

 

Figure 7.

 

Changes (mean ± SD) in NIBUT before and 15 minutes after the insertion of the CL in various environmental conditions.

Table 5.

 

Dryness Score Before and 15 Minutes After SCL Insertion

 

Figure 8.

 

Changes (mean ± SD) in the dryness score before and 15 minutes after CL insertion in various environmental conditions.

The authors thank Tatsuo Terashima (Biostatistics Department, Santen Pharmaceutical Co., Ltd., Tokyo, Japan) for providing statistical assistance.