Graphical abstract
Highlights
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Simultaneous measurement of contact lens dewetting behavior and visual performance is possible.
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After 13.1 s ± 17,4 s, 20.83 s ± 21,81 s, 34.67 s ± 29,11 s participants lost one, two or three log units of visual acuity.
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Visual loss of one log unit occurred 5 s after first dewetting.
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Visual loss due to dewetting of pre lens tear film was unaffected by amount of daily wearing time (5 min vs 8 h).
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Visual performance was reduced without any dewetted are within the pupil zone in more than 50 % of all cases.
Abstract
Purpose
To introduce a novel in vivo method (Visual Acuity Dry Up; VADU) for testing the visual performance during the dewetting of the pre-lens tear film on a contact lens (CL).
Methods
Thirty-nine subjects were fitted with daily disposable CL (Nelfilcon A). Visual performance and dewetting characteristics of the pre-lens tear film were simultaneously assessed using a modified multifunctional topographer (Keratograph 5 M, OCULUS Optikgeräte GmbH, Wetzlar, GERMANY) and the Non-Invasive Keratograph Dry-Up Time (NIKDUT) method. Measures were taken after five minutes and eight hours of CL wear and included the Visual Acuity Dry-Up Time (VADUT; time between last blink and visual breakdown) and the Visual Acuity Dry-Up Area (VADUA; dewetted CL area at visual breakdown) at one (VA +0.1logMAR ), two (VA +0.2logMAR ) and three (VA +0.3logMAR ) log units below maximal visual acuity (VA).
Results
Participants lost one, two or three log units of VA after 13.1 ± 17.4 s, 20.83 ± 21.81, 34.67 ± 29.11 (VADUT), corresponding to a dewetted CL area of 4.82 ± 6.64 mm², 9.5 ± 8.26 and 13.0 ± 8.68 (VADUA), respectively. Differences in VADUT und VADUA for all visual requirements were significant (all p < 0.05). VADUT and VADUA did not vary with CL wear duration (all p > 0.05). A median VA loss of one log unit occurred five seconds after the first dewetting.
Conclusions
The novel VADU method can be used to analyze the role of the tear film stability on the visual performance during CL wear. Hereby, visual loss is quantified based on the threshold definition of the psychometric function.
1
Introduction
Soft contact lenses (CL) are a common form of ametropia correction, with market penetration rates varying from 5% in Australia and Europe to 16 % in the US. [ ] However, dropout rates of up to 30 % are driven by wearers frequently experiencing discomfort and poor vision. [ ]
The tear film overlying a CL plays an important part in comfort and vision. A consistently wettable CL surface ensures a stable, lubricious tear film and enables good vision and comfort by reducing friction during blinking. [ , ] Optimal wettability of the CL front surface depends on several parameters, including material [ ], individual blister solution [ ] and lens care solution [ , ]. An inhomogeneous CL front surface resulting from dewetting may reduce contrast sensitivity [ ] and visual quality [ ].
Currently, reports on the simultaneous measurement of the pre-lens tear film stability and the visual performance in CL wearers are limited. Using videokeratoscopy, Szczesna-Iskander et al. reported a decrease in visual performance at 3.5 s after the first dewetting of the CL surface, and a modest but significant correlation between visual loss and the initial dewetting time within the pupil zone, as well as the overall corneal zone. [ ]
In a pre-study, a method to simultaneously measure the dewetting characteristics of the CL surface and the visual performance of the lens wearer was developed by modifying a multifunctional topographer. ( Gunkel , S. et al.: Correlation Between Tear Film Stability and visual performance – Development and Experimental Testing of a New Measuring Procedure, 2015, JenVis Research, data on file) The data suggests a correlation between the first dewetting of the pre-lens tear film and the first decrease in contrast sensitivity, thus indicating a reduction in visual performance.
This study evaluated a novel in-vivo method of assessing visual performance reduction as a function of CL surface dewetting time and area.
2
Methods
This open-label, randomized study was approved by the ethics committee of the Friedrich-Schiller University, Jena and participants signed an informed consent form. The study followed the tenets of the Declaration of Helsinki.
2.1
Study population
Thirty-nine (28 female and 11 male) habitual soft CL wearers (mean 26 ± 4 years) were enrolled in this study. All subjects had binocular vision and a visual acuity (VA) of 20/25 or better. Subjects with an astigmatism greater than 1.0 D were excluded from the study.
The spherical refractive error ranged from -6.0 to +0.5 D, with a mean of -2.36 ± 1.68 D, and the astigmatism amounted to 0.25 ± 0.29 D and ranged from 0.00–1.00 D. The resulting spherical CL correction ranged from -0.5 to -6.5 D with a mean of -2.42 ± 1.62 D. The test eye was chosen based on the lowest astigmatism; in case of a tie, the right eye was chosen. Subjects with a Non-Invasive Keratograph Tear Film Breakup Time (NIKBUT f ) for the first detected break up ≤ 5 s were excluded. [ ]
2.2
Definition of visual acuity drying up time and visual acuity drying up area
For this study, two new values were defined: 1) the Visual Acuity Drying-Up Time (VADUT) and 2) the Visual Acuity Drying Up Area (VADUA). Both describe the visual performance as a function of the pre-lens tear film dewetting. VADUT measures the time (in seconds) between a blink and the subject’s visual resolution limit. Visual breakdown was defined at the first occurrence of three errors in a row of five fixed-sized optotypes presented in a randomized and ongoing manner (psychometric function) (see Fig. 1 ). The visual loss was most likely caused by an inhomogeneous CL front surface following dewetting (see Fig. 2 ).
VADUA describes the dewetted area in mm² at the visual breakdown and is based on the evaluation routine of the Non-Invasive Keratograph Dry-Up Time (NIKDUT) method described by Marx and Sickenberger. [ ] Hereby, illuminated ring patterns are projected and reflected on a contact lens surface. A uniform reflection indicates a wettable, and thus stable and homogeneous contact lens surface. Localized distortions and gaps occurring over the course of the measurement indicate signs of surface drying.
A virtual grid overlay consisting of eight concentric rings with twelve meridians was applied to a video that was recorded during the measurement ( Fig. 3 ). This grid divides the area of the front surface of the CL in 192 segments that are marked subjectively as dewetted in each frame of a video file, i.e. 32 times per second. The corresponding pre-lens tear film area in mm² was calculated for each segment. VADUT and VADUA can be evaluated for a loss of any visual requirement, e.g. maximal visual acuity or one, two, three etc. log units below.
2.3
Study setup
The Visual Acuity Dry Up (VADU) routine includes the evaluation of VADUT and VADUA and requires a modification of a multifunctional topographer (Keratograph® 5 M [K5M], OCULUS Optikgeräte GmbH, Wetzlar, GERMANY). A micro-display was built into the observation path of the topographer by using a semi-transparent mirror and a magnification system ( Fig. 4 ), allowing the presentation of optotypes (maximum: logMAR -0.3).
The VADU routine uses white light (as opposed to infrared light, used for NIKBUT) to ensure a higher video quality and a more precise subjective interpretation of the dewetting process. The VADU routine for a specific visual requirement starts automatically after the subject completed two blinks (c.f. the standard NIKBUT test integrated in the K5M). While refraining from blinking, the subject used a numeric keypad to select one of eight possible orientations of presented Landolt-Cs. Any answer immediately triggered another random optotype orientation of the same size (fixed visual requirement). No answer within two seconds counted as a wrong answer. Subject answers were recorded and analyzed in real time. The routine ended after the visual breakdown (three wrong answers out of five given answers) or the maximum time limit of 96 s was exceeded.
Subjects received CLs in both eyes. The CL at the non-tested eye effectively reduced the subject’s compulsion to blink. No further methods for blink suppression were applied. Videos featuring blinking or incomplete blinking were not analyzed. CL movement was only observed instantly after the initial blink. No dewetting or visual breakdown occurred before lens stabilization, hence CL movement did not influence VADUT or VADUA measurements.
2.4
Study procedures
All participants presented for a baseline examination (visit 1), consisting of a case history, visual acuity test, binocular vision check, subjective refraction and anterior segment slit-lamp biomicroscopy examination. Corneal topography, radii, eccentricities and diameter (white to white), as well as NIKBUT f were measured with the modified K5M.
Participants were fitted with daily disposable Nelfilcon A CLs in both eyes in the morning (visit 2) and returned after 8 ± 0.5 h of lens wear (visit 3). The VADU routine was conducted twice, after five minutes of wear (visit 2) and after eight hours of wear (visit 3). The visual breakdown at three different visual requirements presented in a randomized order was successively assessed for each subject. The visual requirement was linked to the individual maximal visual acuity (VA max ). The VA max was reduced by one, two and three log units, e.g. for a VA of -0.1 logMAR, the visual requirements were set to 0.0 logMAR (=VA max +0.1log units), +0.1 logMAR (=VA max +0.2log units) and +0.2 logMAR (=VA max +0.3log units).
2.5
Statistical analysis
Data was analyzed using SPSS Statistics 21.0 (IBM, Armonk, New York, USA). A dependent t -test for paired samples (paired t -test) was used to compare the means and their 95 % confidence intervals (CI) of the VADUT and VADUA values based on wear time and visual requirement. Normality was ascertained based on the central limit theorem [ ]. An alpha level of 0.05 was considered statistically significant. No comparable data for the primary endpoints (VADUT and VADUA as a function of visual requirement) were available, therefore no sample size calculation was possible. A post-hoc power analysis was conducted using the software package GPower [ ]. The effect size was individually calculated from group parameters (mean and SD value 1, mean and SD value 2, correlation between groups). Post-hoc analyses revealed that the statistical power for this study was at least 0.92. Specific power values can be obtained within the results.
3
Results
3.1
VADUT and VADUA in relation to CL wear duration
Analysis revealed no statistically significant difference between the dry-up characteristics and wear duration (five minutes and eight hours), for any visual requirement ( Tables 1 and 2 ). Hence, no group differentiation for further analyses is necessary.
| Mean | Differences and 95 % CI t-value, p-value | |
|---|---|---|
| VADUT VA+0.1logUnit_5min | 12.62 s | .87, -8.37—6.63; t (38) = -.24; P = .82 |
| VADUT VA+0.1logUnit_8h | 13.49 s | |
| VADUT VA+0.2logUnit_5min | 21.49 s | 1.31, -7.46—10.08; t (38) = .30; P = .77 |
| VADUT VA+0.2logUnit_8h | 20.18 s | |
| VADUT VA+0.3logUnit_5min | 37.79 s | 6.25, -6.49—19.00; t (38) = .99; P = .33 |
| VADUT VA+0.3logUnit_8h | 31.54 s |
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