Retinal Straylight Measurements for Assessing Wear-Related Changes in Monthly Soft Contact Lenses

Abstract

Visual quality in the human eye depends on the integrity of ocular structures. Soft contact lenses interact directly with these structures and can also serve as an external source of straylight. The purpose of this study was to quantify changes in optical quality among habitual wearers of monthly disposable silicone hydrogel soft contact lenses (SCLs) by comparing retinal straylight with new versus month-old lenses. Retinal straylight was measured using a C-Quant straylight meter in 33 young adults (22.0 ± 1.4 years) wearing either comfilcon A (n = 17) or lotrafilcon B (n = 16) lenses. Measurements were first performed with month-old SCLs that had been worn for ≤4 h that day; after lens replacement and a 15 min adaptation period, measurements were repeated with new SCLs. The mean decimal logarithm of the straylight parameter, log(s), was significantly higher with month-old SCLs (0.97 ± 0.17) than with new SCLs (0.86 ± 0.15; paired t-test, p < 0.001), yielding an average increase of Δlog(s) = 0.11 ± 0.08. No significant difference was found between materials. Thirty-six percent of participants reported end-of-cycle visual discomfort. These findings indicate that monthly SCLs at the end of the replacement period can measurably increase retinal straylight.

1. Introduction

Discomfort is a common complaint among contact lens wearers, typically arising either by the end of the day or over the course of the lens lifespan [1,2,3]. It remains one of the primary reasons why users discontinue contact lens wear [4]. A variety of factors contribute to this discomfort, including reduced lens wettability, the accumulation of surface deposits, tear film disruptions and related issues [5,6]. Changes in optical quality are usually not directly related to discomfort, as physical sensations such as dryness or irritation often occur independently of visual clarity [6]. Several studies have shown that daily disposable lenses tend to provide better comfort compared to reusable lenses. Grupcheva [7] analyzed factors associated with the switching from monthly to daily disposable contact lenses, with each lens type worn for a four-week period with the recommended usage guidelines. The results showed that daily disposable lenses provided better visual comfort than monthly lenses after the four-week trial period. In that study, visual discomfort was defined as instability of vision. Dumbleton et al. [8] demonstrated that new monthly disposable silicone hydrogel contact lenses provided greater comfort at the end of the first day of wear compared to the comfort levels at the end of the replacement period, indicating the potential discomfort that accumulates with continued wear.

One of the main factors influencing overall satisfaction with contact lenses is vision quality [9]. Given the greater discomfort associated with reusable lenses, it could be hypothesized that the vision quality with reusable contact lenses may be inferior to that experienced with daily disposable lenses. Several studies have investigated changes in vision quality among contact lens wearers over the wearing period [7,10,11,12]. These studies found no clinically significant changes in vision quality between daily and monthly disposable soft contact lenses throughout the wear period.

Vision quality can be assessed using a variety of tests, with visual acuity serving as the primary measure of visual function in most clinical settings [13]. Visual acuity with monthly disposable soft contact lenses remains relatively stable throughout the daily wearing period and over the course of a typical month-long replacement cycle. Grupcheva et al. [7] reported no significant change in visual acuity over four weeks of wear with monthly silicone hydrogel lenses. Similarly, Pucker et al. [10] found that visual acuity remained unchanged throughout daily use and across multiple follow-up visits during a one-month period. These findings suggest that, in healthy eyes and with proper lens care, visual acuity does not change during the month of lens wear.

While visual acuity is an important parameter, it may not fully capture all aspects of vision quality. Contrast sensitivity is more sensitive to changes in optical quality and is a more precise measure of vision quality [14]. Belda-Salmeron et al. [11] observed fluctuations in contrast sensitivity among contact lens wearers throughout the day, though these changes were statistically significant only for some spatial frequencies.

Another critical but less frequently studied measure of vision quality is retinal straylight, often associated with disability glare. Retinal straylight refers to the scattering of light within the eye, which can degrade image quality even when visual acuity remains unaffected. Sources of straylight include the cornea, crystalline lens, vitreous and retina, but scatter can also be influenced by external optics such as glasses, contact lenses or intraocular lenses [15]. Straylight-related complaints often occur independently of reductions in visual acuity or contrast sensitivity, highlighting the need for its assessment in clinical practice [16]. Retinal straylight has also been demonstrated to be a sensitive parameter for evaluating visual quality after cataract and refractive surgery. After cataract surgery, patients frequently report fewer problems with night glare despite only modest improvements in contrast sensitivity. Quantitative testing confirms that the straylight parameter typically drops significantly following lens extraction, even when pre- and postoperative visual acuity are comparable [17]. Retinal straylight measurement has been shown to be a sensitive parameter for evaluating visual quality not only after cataract surgery, but also following refractive surgery, as it can reveal changes that are not captured by visual acuity or contrast sensitivity [18].

The impact of soft contact lenses on retinal straylight has been investigated in several previous studies. The cornea is a major source of straylight in young, healthy individuals [19], and even short periods of contact lens wear may induce corneal edema, potentially increasing straylight [20]. Meulen et al. [21] found no statistically significant difference in retinal straylight between soft contact lens wearers and non-wearers, though they reported a significant increase in straylight with rigid contact lenses. Cerviño et al. [22] investigated retinal straylight in subjects wearing both tinted and clear soft contact lenses. Their results suggested that clear soft contact lenses do not significantly increase retinal straylight when compared to the absence of lens wear. Similarly, Gaurisankar et al. [23] reported that retinal straylight values were not statistically different when myopic eyes were corrected with spectacles compared to new soft contact lenses. Although previous studies have examined the effect of contact lens wear on retinal straylight, none have characterized how this parameter changes over the full replacement cycle of monthly disposable lenses. The current study aims to investigate potential changes in retinal straylight with monthly disposable soft contact lenses at both the beginning and the end of the replacement period.

2. Materials and Methods

A total of 33 young adults (mean age of 22 ± 1.4 [SD] years) who regularly wore monthly disposable silicone hydrogel contact lenses (SCLs) and had no history of ocular diseases participated in this study. The contact lenses were worn in daily wear mode and removed during sleep. All participants had a visual acuity of at least 1.0 (decimal units) and showed no signs of ocular pathology. The spherical equivalent refraction of participants ranged from −9.00 D to −1.00 D, with an average refraction of −2.75 D. Of the participants, 17 wore contact lenses made from comfilcon A material (Biofinity, CooperVision, Inc., San Ramon, CA, USA), while 16 wore lenses made from lotrafilcon B (Air Optix, Alcon, Geneva, Switzerland). There were no statistically significant differences in age and refraction between the two groups. Written informed consent was obtained from each participant prior to the study, and ethical approval was granted by the Research Ethics Committee of the Faculty of Biology and the Faculty of Geography and Earth Sciences at the University of Latvia.

Retinal straylight was measured with the C-Quant straylight meter (Oculus Optikgeräte GmbH, Wetzlar, Germany), which is based on the compensation comparison method [24]. This technique involves presenting a flickering (~8 Hz) stimulus comprising a concentric annulus and a central test field divided into two halves (Figure 1). Because of intraocular light scattering, part of the flicker from the annulus reaches the test field, producing a weak flicker perception in both halves. Counterphase compensation light is added to one half of the central field, while the other half contains none. The subject task is to decide which side flickers more strongly. By presenting a series of such comparisons with varying amounts of compensation light, the instrument records the subject’s responses in a two-alternative forced-choice paradigm. A psychometric function is fitted to these responses using a maximum-likelihood algorithm, yielding the straylight parameter s expressed as log(s). The point of subjective equality—where both halves appear to flicker equally—represents the level of counterphase modulation that exactly compensates for the straylight flicker caused by intraocular scatter [24].

Due to time constraints, the retinal straylight measurements were initially conducted with monthly worn SCL that had been worn for no more than 4 h. The subject then removed the old lenses and inserted new SCL. After a 15 min adaptation period, straylight measurements were repeated with the new lenses. At least three measurements were performed for each condition. Measurements with an estimated standard deviation SD greater than 0.08 or a quality parameter below 0.5 were excluded and repeated. All measurements were conducted in a dimly lit room (~9 lux) (Konica Minolta Illuminance Meter T-10). The dominant eye was used for the measurements, while the non-dominant eye was covered with an eyepatch.

Participants were required to complete a survey and answer questions regarding the type of contact lens material they use (comfilcon A or lotrafilcon B), the number of days the old monthly contact lenses were worn and whether they experience discomfort as the end of the contact lens wear period approaches. In this study, visual discomfort was defined as the physical or sensory unpleasantness experienced during contact lens wear.

The normality of data distributions was evaluated using the Kolmogorov–Smirnov test (socscistatistics.com). Mean values between data groups were compared using a one-tailed t-test for dependent or independent samples. To determine whether the slope of the regression line significantly differs from that of a hypothetical value, a regression t-test was conducted. A significance level of 0.05 was used for all statistical analyses. The calculations were performed using MS Excel for Microsoft 365 (Version 2509, Build 19231.20194).

When retinal straylight measurements were conducted, the program G*Power 3.1.9.7 [25] was used to determine if there was an appropriate level of statistical power in this research. A power level of at least 0.8 is recommended for studies involving humans [26]. Considering the groups’ mean values and standard deviations, calculations indicated that a sample size of 33 participants is sufficient to achieve the desired power level.

3. Results

3.1. Retinal Straylight with New and Worn Contact Lenses

Figure 2 shows the average log(s) values obtained with both new and month-old SCLs. A one-tailed t-test for dependent samples was used to compare the mean straylight values. There was significantly greater retinal straylight with month-old SCLs (M = 0.97, SD = 0.17) than with new SCLs (M = 0.86, SD = 0.15), t(32) = 7.49, p < 0.0001. The average increase in retinal straylight with old contact lenses was Δlog(s) = 0.11 ± 0.08.

Figure 3 shows the relationship between straylight values with new and monthly worn contact lenses. There is a significant correlation between retinal straylight values obtained under different conditions. Pearson’s correlation coefficient was r = 0.88, p < 0.00001. The mean standard deviation of participants’ straylight measurements was 0.03 for new contact lenses and 0.04 for used lenses. The slope of the linear regression line was 1.0014, which was not significantly different from 1.0 (p = 0.99, regression t-test). This indicates that the increase in retinal straylight observed with monthly worn SCLs occurs independently of the initial straylight values obtained with new lenses. This finding suggests that the observed straylight increase results from general optical changes in the lens–eye system rather than from individual variability in baseline ocular scatter.

3.2. Contact Lens Material and Discomfort

No significant differences in retinal straylight were observed between the measurements taken with comfilcon A and lotrafilcon B contact lenses, regardless of whether the lenses were new or worn for at least 30 days (p > 0.05) (Figure 4).

The participants’ questionnaire responses showed that 12 subjects (36%) reported experiencing discomfort toward the end of the contact lens wear period. To evaluate whether the groups with and without reported discomfort differed in retinal straylight levels, the mean log(s) values for each group were compared when using month-old contact lenses (old SCLs). A one-tailed t-test revealed no statistically significant differences in retinal straylight between the group with discomfort (M = 0.96, SD = 0.17) and the group without discomfort (M = 0.97, SD = 0.15), t(31) = 0.26, p > 0.05).

One of the questionnaire items asked participants about their adherence to the recommended replacement schedules for their contact lenses. Approximately 30% of participants (10 out of 33) reported wearing their monthly contact lenses beyond the prescribed period, extending use up to 40 days. To evaluate whether wearing contact lenses for more than 30 days further increases retinal straylight compared to lenses worn within the 30-day replacement period., the increase in straylight with worn versus new lenses was compared between the two groups. No statistically significant difference was found (p > 0.05, t-test for independent samples) between the group adhering to the replacement schedule (Δlog(s) = 0.11 ± 0.01) and the group that did not (Δlog(s) = 0.09 ± 0.03).

4. Discussion

The results showed that retinal straylight was significantly higher with monthly worn disposable soft contact lenses compared to new lenses. The average increase in retinal straylight with worn SCLs was Δlog(s) = 0.11 ± 0.08. De Wit et al. [27] suggest that an increase of at least log(s) = 0.10 is required for wearers to notice a difference in visual quality. Thus, the observed rise in retinal straylight with monthly worn disposable SCLs could have clinical significance.

Previous studies have shown that soft contact lenses do not increase retinal straylight when compared with eyes without contact lenses [21,22]. Cerviño et al. [22] tested only soft contact lenses shortly after insertion and reported no change in retinal straylight, while van der Meulen et al. [21] likewise found no straylight increase among habitual soft lens wearers but did not specify how long their monthly lenses had been in use. Although the present study did not include a direct no-lens control condition, the higher straylight values observed after one month of lens wear suggest that, under certain conditions—such as prolonged use or lens aging—soft contact lenses may increase retinal straylight when compared with eyes without lenses.

In the present study, the compensation comparison method was used to assess retinal straylight. This method has been shown to provide reliable and repeatable measurements of intraocular light scatter and demonstrates strong agreement with objective optical techniques for quantifying light scatter in the eye [16]. Cerviño et al. [28] further reported that psychophysical straylight measurements obtained with the C-Quant instrument closely correspond with objective estimates of intraocular forward scatter derived from Hartmann–Shack spot-pattern analysis. Comparative evaluations have also indicated that the compensation comparison method performs more consistently than other glare tests, particularly in terms of validity and discriminative ability [29]. Consequently, the results of this study were compared only with those of studies employing the same method for retinal straylight measurements.

Retinal straylight increases complaints of glare [19]. If soft contact lenses increase retinal straylight, a higher frequency of glare-related complaints would be expected compared to emmetropic individuals. This assumption is partly supported by studies assessing quality of life in myopic subjects using different methods of vision correction. Queirós et al. [30] found significantly worse glare scores in soft lens users compared to emmetropes, while González-Pérez et al. [31] observed a similar trend, although the difference was not statistically significant. These findings suggest that retinal straylight may be elevated in some contact lens wearers, and the current results provide partial support for this observation. In the present study, 36% of participants reported a reduction in visual quality after wearing contact lenses for one month. However, Pucker et al. [10] found no significant increase in discomfort among monthly lens wearers at the end of the wear period. The discrepancy between the present findings and those of Pucker et al. [10] may be explained by differences in study design, participant selection, lens materials and the type of questionnaire used to assess discomfort. Another important factor that could influence such findings is wearer compliance with the recommended replacement schedule. In the present study, only about 30% of participants reported consistently replacing their monthly contact lenses within the prescribed period, which may have contributed to variations in subjective comfort and optical quality. Importantly, extending lens wear beyond the recommended replacement period also increases the risk of complications, particularly infectious keratitis, as prolonged use promotes microbial contamination and compromises corneal physiology [32].

Due to time constraints, visual acuity was not assessed with both new and worn disposable SCLs in this study, leaving open the question of whether there is a reduction in visual acuity after a month of soft contact lens wear. As previously noted, straylight-related complaints often occur independently of changes in visual acuity [16], so an increase in retinal straylight does not always affect visual acuity. Other research has similarly shown that visual acuity does not significantly decrease after a four-week period of disposable contact lens wear [7,10].

Łabuz et al. [33] hypothesized that the material of the contact lenses could influence retinal straylight levels. In this study, however, no significant differences were found between the lotrafilcon B and comfilcon A materials, suggesting that the material of soft monofocal contact lenses may have little impact on retinal straylight. This result implies that the type of material used in SCLs is not a major determinant of straylight increase.

A limitation of the present study is that measurements with used SCLs were not performed immediately after insertion but only after the lenses had already been worn for at least one hour and up to four hours. This makes it plausible that the observed increase in retinal straylight with worn SCLs reflects not only changes accumulated over a month of wear but also short-term effects such as corneal edema. Belda-Salmerón et al. [11] investigated how visual performance changes over a day of contact lens wear. Visual acuity and contrast sensitivity were measured every two hours during a 12 h period of continuous wear, and both parameters remained stable during the first four hours and beyond. Other studies showed that discomfort generally appears only after about six hours of continuous lens use with computer work [34].

However, previous research has shown that biometric parameters in soft contact lens wearers typically stabilize within hours to several days after lens removal [35,36]. Nonetheless, a small residual effect may persist, such as transient alterations in corneal curvature, epithelial thickness, or hydration that occur shortly after lens removal. These minor changes could, in theory, influence optical quality.

The relatively wide range of refractive errors among participants may also have affected retinal straylight and visual quality. Earlier studies have demonstrated that greater axial length and higher degrees of myopia are associated with increased straylight values [37]. Therefore, the potential influence of refractive status on the present results cannot be excluded.

5. Conclusions

The results of this study indicate that in habitual wearers of monthly disposable silicone hydrogel contact lenses, retinal straylight can significantly increase by the end of the wearing period. Monthly worn soft contact lenses, worn for at least 30 days, appear to reduce visual quality, as retinal straylight was significantly higher with used lenses compared to new ones. These findings further demonstrate that retinal straylight assessment is a sensitive method for detecting optical changes associated with prolonged contact lens wear.