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Eurolens Research, Division of Pharmacy and Optometry, Faculty of Biology, Medicine and Health, The University of Manchester, Oxford Rd, Manchester, M13 9PL, UK
Eurolens Research, Division of Pharmacy and Optometry, Faculty of Biology, Medicine and Health, The University of Manchester, Oxford Rd, Manchester, M13 9PL, UK
Eurolens Research, Division of Pharmacy and Optometry, Faculty of Biology, Medicine and Health, The University of Manchester, Oxford Rd, Manchester, M13 9PL, UK
Eurolens Research, Division of Pharmacy and Optometry, Faculty of Biology, Medicine and Health, The University of Manchester, Oxford Rd, Manchester, M13 9PL, UK
Eurolens Research, Division of Pharmacy and Optometry, Faculty of Biology, Medicine and Health, The University of Manchester, Oxford Rd, Manchester, M13 9PL, UK
This work set out to investigate if there was an association between subjective comfort and both subjective and measured vision during the use of contemporary daily disposable soft toric contact lenses.
Methods
Thirty-eight habitual soft contact lens wearers wore each of three daily disposable toric lenses for one week in a prospective, crossover, randomised, single-masked study. The following clinical measures were recorded at dispensing and follow-up visits: biomicroscopy scores, lens fitting (including rotation and rotational stability), high and low contrast visual acuity, subjective vision quality and subjective ocular surface comfort. Subjective scores were collected using 0–10 numerical grading scales. Comfort scores were analysed using a linear regression model with age, sex, visit, phase of crossover (‘phase’), lens type, lens rotation, lens rotational stability, visual acuity, cylinder power and subjective vision quality as factors of interest and then refined using backward stepwise regression.
Results
Thirty six participants (31.1 ± 13.5 years) completed the study. Comfort scores were found to be associated with subjective vision quality (F = 127.0 ; p < 0.0001), phase (F = 7.2; p = 0.001) and lens type (F = 4.9; p = 0.009). Greater comfort scores were observed with greater subjective vision quality scores. Visual acuity was not statistically significant in the model.
Conclusion
This work suggests that symptoms of ocular discomfort may be more intense if there is also perceived visual compromise in daily disposable soft toric lenses. There was a stronger positive correlation between comfort and subjective vision quality compared with comfort and measured visual acuity.
For more than two decades discomfort has been cited as the primary reason for established contact lens users limiting their wear time and ultimately abandoning their lenses [
]. Despite major innovations in the field over this period, perhaps most notably the introduction of silicone hydrogel materials, contact lens discomfort remains a major unresolved issue for the contact lens industry. These drop-out rates have been reported to range from 12 % to 51 % [
] depending on the methodology and patient populations used to classify the discontinued wearers and from published international prescribing data, it has been estimated that as many wearers discontinue from their lenses as commence in any given year [
] in a multi-site study who recruited 236 lapsed wearers and found that 51 % had originally stopped lens wear as a result of some form of discomfort, but the second most common reason was for vision-related problems which were reported by 13 % of respondents. This same trend has been observed in other national and international investigations [
Discontinuation in toric lens wear may be more common than for spherical lenses. This could be due to increased likelihood of visual problems related to cylinder power, rotation and rotational stability [
]. In these toric users, vision was the most common reason for discontinuation. This was also the case in prospective work investigating new wearers from the same group [
] where drop out for spherical and toric lenses was 19 % and 25 %, respectively (48 % of the toric lapsed wearers cited vision as the major contributing factor). Young et al. [
] also documented a drop out rate of 31 % in toric wearers compared to reusable (91 %) and daily disposable (89 %) spherical lenses when the lapsed wearers were re-fitted with lenses and followed for one month.
If comfort and vision are important in relation to retention, so too is any relationship between them as one may affect the other. Studies suggest that vision declines during the course of a wearing day in much the same way as comfort [
]. The question of whether or not there is a direct association between vision and comfort in lens wear (or without lenses) remains unanswered but there is some supporting evidence. Although not specific to the ocular surface, it has been demonstrated that images with reduced high spatial frequency information (i.e. ‘blurred’ images) increase asthenopic symptoms [
]. Similarly, contact lens wearers reported improved visual comfort (less ‘eye strain’) when using a computer when their astigmatism was corrected compared with when left uncorrected [
] investigated the effect of visual quality on the perceived ocular surface discomfort in soft, silicone elastomer and rigid lenses under clear, blurred and occluded conditions and reported greater discomfort with blur compared with full correction or occlusion. Rao and Simpson [
] investigated how participants responded to suprathreshold stimulation of the cornea when vision was clear and defocused. They demonstrated that when vision was blurred, discomfort ratings accelerated more quickly than for the clear condition. In an experiment designed to investigate the contribution of wearing a contact lens to induce blur and the effects on ocular comfort, Rao and Simpson again found an association between vision and ocular comfort [
To our knowledge, there is no published work which investigates the association between vision and comfort in soft toric contact lenses. The increased lens diameters and thickness profiles of these lenses compared with their spherical counterparts have the potential to negatively impact upon comfort [
] and the added complexities relating to rotation and rotational stability of the lenses may also affect perceived and measured vision in addition to any surface-related factors which may make the relationship between these two variables different to that in spherical lens wear. As such, this work set out to investigate the relationship between contact lens comfort and both subjective and measured vision in three different daily disposable toric lenses which were each worn for one week.
2. Methods
2.1 Study lenses
The three daily disposable toric lenses investigated in this work are outlined in Table 1 [clariti 1 day toric (C1DT)(somofilcon A, CooperVision Inc.), 1-Day Acuvue Moist for Astigmatism (1DAMfA)(etafilcon A, Johnson & Johnson Vision) and DAILIES AquaComfort Plus Toric (DACPT)(nelfilcon A, Alcon)]. The lenses were chosen as representative examples of contemporary, commonly-prescribed hydrogel and silicone hydrogel toric lenses. The daily disposable modality was chosen in order to mitigate any interactions from accompanying lens care solution systems since previous work has shown that lens/solution combination can affect subjective responses [
]. We also used a narrow range of BVP in order to reduce the variability in lens thickness profiles. Participants were instructed not to use any additional comfort drops or ocular surface preparations for the duration of the study.
All lenses were worn bilaterally for one week (in random sequence) for a minimum of eight hours per day on at least five days per week. Participants were also instructed to wear the lenses for at least two hours before each of the follow-up visits.
2.2 Study design
This was a single-site, prospective, randomised, subject-masked, cross-over study where participants received three sequential interventions (three lens types) in separate treatment phases (one week in each lens type). Ethical approval was granted from the University Research Ethics Committee of The University of Manchester prior to participant recruitment. The study conformed to the tenets of the Declaration of Helsinki and all participants provided written, informed consent prior to enrolment. Inclusion and exclusion criteria are outlined in Table 2.
Table 2Inclusion and exclusion criteria.
Inclusion criteria
Exclusion criteria
18 years and older
Ocular or systemic disorder which would normally contradict contact lens wear
Currently wore soft contact lenses or had done so in the previous 6 months (spherical or toric)
Requirement to use any topical medication (e.g. drops, ointment) during the study
Sphere between −1.00 and −6.00DS in each eye (based on ocular refraction)
Previous history of cataract or refractive surgery or corneal distortion
Cylinder between −0.75 to −2.00DC in each eye (based on ocular refraction)
Pregnant or breast-feeding
Could attain at least 0.20 logMAR high contrast VA in each eye with each of the study lenses
History of anaphylaxis or severe allergic reaction
Could be satisfactorily fit with each of the study lenses
Participation in any contact lens study within 2 weeks of starting the current study
Willing to comply with the wear schedule (at least 8 h per day/5 days per week)
In order to determine the sample size, power analysis was undertaken. A power of 80 % or greater was provided by 33 datasets for subjective comfort on a 0–10 grading scale, assuming a difference of 2.0 points and a standard deviation of 1.0 point. This assumed a two-tailed paired analysis and an alpha of 0.05.
Participants attended five study visits in total. At the initial visit (V1), various baseline investigations were carried out including refraction, measured visual acuity (VA) (logMAR) and slit lamp biomicroscopy of the ocular surface before each of the three lens types were fitted which allowed a supply to be ordered for each subject. The tissues of the ocular surface were graded to the nearest 0.1 unit using Efron grading scales [
]. At the second visit (V2), the first lens type to be dispensed was applied and various clinical investigations were carried out at this ‘dispensing’. Lens fit (horizontal and vertical centration, corneal coverage and movement) was graded using -2 to +2 grading scales for each parameter. Anything graded from -1 to +1 was deemed an ‘acceptable’ fit (0 corresponded to ‘optimum’ and +1 or 1 corresponded to ‘slightly excessive or inadequate’) and anything graded as +2 or -2 was recorded as an ‘unacceptable’ fit (+2 or -2 corresponded to ‘extremely excessive or extremely inadequate’). Lens rotation after 10 min of lens settling was recorded as the mislocation in degrees from the zero rotation position (i.e. from the six o’clock vertical marking position on each lens) in primary gaze. Nasal rotation was recorded as a positive number whilst temporal rotation was recorded as a negative number. ‘Absolute’ lens rotation was the overall magnitude of the rotation (i.e. without the sign attached to the value). Lens rotational stability was recorded after 10 min of lens settling as the maximum excursion of the lens in four directions of gaze (up, down, left and right). High illumination logMAR VA (high and low contrast) were recorded. Participants were trained on how to record their subjective responses using numerical grading scales. Quality of vision was recorded with reference to 0–10 numerical grading scales (0=completely dissatisfied, 10=completely satisfied) and ocular surface comfort was recorded with similar scales (0=painful, 10=lenses cannot be felt). Participants were given a supply of contact lenses which were over-labelled to maintain masking and asked to return one week later.
At the third visit (V3), the following ‘follow-up’ clinical investigations were carried out on the first lens type which had been in situ for at least 2 h: quality of vision and ocular surface comfort over the previous week by recall with reference to 0–10 grading scales, high illumination logMAR VA (high and low contrast), lens fit, lens rotation and lens rotational stability. Following removal of the lenses and after a complete biomicroscopic examination, the second lens type was applied and the same dispensing procedures as at V2 were carried out. This process was continued until all three lens types had been dispensed and examined at a follow-up visit (V3-V5). At V5, the participant was discharged.
2.3 Statistical analysis
Statistical analysis was conducted using JMP 14, Version 14.3 (SAS Institute Inc. Cary, NC, USA) using only right eye data where appropriate.
High and low contrast VA, absolute lens rotation and lens rotational stability were each analysed separately using a linear mixed model with participant (random effect), age, sex, visit type (dispensing/follow-up), treatment phase (1, 2, and 3) and lens type as factors of interest. The interaction term ‘lens type X visit type’ was initially included in the model but was removed if it was not significant at p > 0.2. Because the data for the residuals for absolute lens rotation and rotational stability were observed not to be normally distributed, these analyses were repeated after square root transformation of these two variables.
Subjective comfort scores were initially analysed using a linear mixed model with the following factors: participant (random effect), age, sex, visit, phase, lens type, lens rotation, lens rotational stability, high contrast VA, low contrast VA, cylinder power and quality of vision. Backward stepwise regression was then used to build the final model. Non-significant factors at p > 0.2 were individually excluded from the model, starting with the most non-significant factor until none of the variables met the criterion. Post-hoc multiple comparison testing was performed using the Tukey-HSD test. Correlation between variables was assessed using Pearson or Spearman coefficients, as appropriate. Statistical significance was set at p < 0.05.
3. Results
Table 3 details the participant demographics. Of the 38 participants who were dispensed lenses, 36 completed the study, one participant was lost to follow-up and one participant was discontinued due to unacceptable orientation of the toric lens (a DACPT lens).
Lens wearing patterns are shown in Table 4. No differences were seen between the lenses in terms of days per week (F = 0.5, p = 0.64), hours per day (F = 0.5, p = 0.62), comfortable hours per day (F = 0.3, p = 0.73) or wearing time at the follow-up visit (F = 0.3, p = 0.71). All lenses showed acceptable fit at dispensing and follow-up (except for a single 1DAMfA lens displaying ‘extremely inadequate’ movement at follow-up). Slit lamp biomicroscopy grades at the follow-up visit were all acceptable and within normal clinical limits for successful lens wear.
Fig. 1 shows high and low contrast VA at the dispensing and follow-up visits. Lens type, phase, visit type and sex had no signifcant effect on high or low contrast VA (all p > 0.05). Low contrast VA was significantly influenced by age (F = 4.7, p = 0.04), such that increasing age was associated with poorer VA. This finding was not observed for high contrast VA (F = 1.1, p = 0.30).
Fig. 1High contrast (A) and low contrast (B) VA (a cross represents the mean, a horizontal line the median, the box represents the 25th and 75th percentiles and the whiskers represent maximum and minimum values). C1DT; clariti 1 day toric, 1DAMfA; 1-Day Acuvue Moist for Astigmatism, DACPT; DAILIES AquaComfort Plus Toric.
Lens absolute rotation and rotational stability are shown in Fig. 2. Lens absolute rotation was not influenced by lens type (F = 1.4, p = 0.26), visit type (F = 1.4, p = 0.23), phase (F = 0.25, p = 0.78), sex (F = 0.1, p = 0.75) or age (F = 0.0, p = 0.88). Lens rotational stability was significantly influenced by lens type (F = 7.7, p = 0.0006), but not by visit type (F = 0.2, p = 0.68), phase (F = 0.4, p = 0.69), sex (F = 0.1, p = 0.81), or age (F = 0.8, p = 0.38). Post-hoc analysis demonstrated better rotational stability of DACPT compared with 1DAMfA (least square mean (LSM) 2.4 vs. 4.0) ; p = 0.0006) and C1DT lens (LSM 2.4 vs. 3.6) ; p = 0.01). The results of the analyses using the square root transformation were similar to those without.
Fig. 2Lens rotation (A) and rotational stability (B) (a cross represents the mean, a horizontal line the median, the box represents the 25th and 75th percentiles and the whiskers represent maximum and minimum values). C1DT; clariti 1 day toric, 1DAMfA; 1-Day Acuvue Moist for Astigmatism, DACPT; DAILIES AquaComfort Plus Toric.
Subjective comfort scores were found to be associated with subjective vision quality (F = 127.0 ; p < 0.0001), phase (F = 7.2 ; p = 0.001) and lens type (F = 4.9; p = 0.009). Greater subjective comfort scores were correlated with greater subjective vision quality scores (r = 0.66, P < 0.0001) (Fig. 3). Post-hoc analysis showed that subjective comfort scores were significantly greater at phase 3 compared with phase 1 (LSM 8.7 vs. 8.0; p = 0.0006). The C1DT lens showed significantly better comfort than the DACPT lens (LSM 8.2 vs 7.7; p = 0.007). Measured visual acuity was not statistically significant in the model (Fig. 4).
Fig. 3Relationship between comfort and vision quality. Relationship between comfort and vision quality. To avoid overlapping data, random jitter was applied to each data point in the x and y directions (n = 220 observations).
Fig. 4Relationship between comfort and measured VA (A) (n = 220 observations) and between subjective vision quality and measured VA (B) (n = 221 observations). To avoid overlapping data, random jitter was applied to each data point in the x and y directions.
All three daily disposable toric lenses investigated in this work showed good clinical performance for measured VA, lens fit, biomicroscopy evaluation and subjective response.
Measured VA at high and low contrast was similar for all three lens types and no other factors investigated had an effect on VA except for age, but only for low contrast. Here, increasing age was associated with worse VA. Reductions in VA with age have been reported before [
] but it is interesting that such an observation was detected in this work with a small number of participants using conventional acuity measurement approaches (as distinct from a full contrast sensitivity work-up). Measured VA was similar at dispensing and follow-up suggesting that a short settling period of about 10 min is enough time to assess the final visual performance with these toric lenses.
Toric lenses have additional rotational, movement-related design complexities compared to spherical lenses which can affect vision [
]. In this study there was a difference in rotational stability between the lens types, presumably as a result of differences in the methods used for stabilisation (Table 1). Toric lenses are impacted by all the usual factors which can affect vision in soft lenses, and foremost amongst these is degradation of the lens surface (particularly in between blinks) which is likely to be due to pre-lens tear film break-up and poor wetting which ultimately affect the quality of the retinal image [
In general there were very few differences observed which would be considered clinically significant. Each lens was worn for a similar amount of time in each treatment phase and participants reported a similar number of comfortable hours per day (just over 9 h).
The C1DT silicone hydrogel showed marginally better comfort than the DACPT hydrogel lens but it is difficult to pinpoint precisely why this might have been since there are many potential confounding factors which can influence comfort when different lens types are compared, such as differences in lens design (e.g. edge shape and overall contour), surface characteristics (e.g. coefficient of friction and wettability) and differences in modulus between the lenses.
The statistical modeling in the present work showed that there was a treatment phase effect for comfort i.e. better comfort was reported at visit 3 compared to visit 1. This result implies that the observed treatment effects will depend on the order in which they were received, reinforcing the need for randomisation of lens order (as used here) in this form of clinical study. The reasons for the increasing comfort with study phase are not immediately clear but it is possible that this is due to the sensitivity or the reliability of the grading instrument itself [
] or that the study participants reported reduced comfort at the first visit because they had just been taken out of their habitual lenses, with which they were presumably content and as the study progressed they became more accustomed to wearing different lens types. However, it is unlikely that any carryover effects for subjective comfort for a lens would meaningfully influence the reporting of comfort for a subsequent product (a week later). If there is some effect due to this phenomenon, the robustness of the work is maintained because the study was randomised, meaning that any effect on an individual lens type is further minimised. In addition to this, any ongoing effect from moving through a series of lenses is mitigated by the incorporation of ‘phase’ as part of the statistical model.
The dataset presented here allowed the investigation of the relationship between measured VA and subjective vision quality. These relationships can be seen in Fig. 4B and show that for both high and low contrast measures, better in-clinic, measured VA scores are associated with better vision quality subjective scores but these correlations are weak. They demonstrate that patients can have relatively poor measured VA and still be satisfied with their perceived vision (and vice-versa). This is not the first time a weak correlation between these two variables in contact lens wear has been reported. Jong et al. [
] reported on the relationship between VA measured at various distances and subjective vision ratings (1–10 numeric rating scales) in a retrospective analysis of studies using five different multifocal lenses. They demonstrated weak to moderate relationships between objective and subjective measures and concluded that subjective vision ratings were better indicators of satisfaction (and therefore willingness to purchase lenses) than in-office VA measurements. This finding has important practical ramifications and suggests that eyecare practitioners fitting soft toric lenses should specifically ask patients about their opinion of their vision (and act accordingly if the patient is not satisfied, given the potential for discontinuation in the event of poor subjective vision [
The literature on the association of vision with comfort in contact lens wear is sparse and the authors have no knowledge of this having been investigated with toric soft lenses previously. Certainly, a direct link between vision and comfort has not been shown before. One of the first reports which investigated if ocular discomfort was affected when vision is poor reported that reducing visual quality was associated with decreased perceived ocular comfort [
]. In the investigation, vision was fully corrected, optically blurred (by +4.00DS) or completely occluded when participants wore each of three different lens types: silicone hydrogel, silicone elastomer and rigid lenses. Unsurprisingly, there were differences in comfort between the lens types but more interestingly, participants reported greater ocular surface discomfort when blur was present compared with both full correction or occlusion. The amount of blur induced in the experiment is significantly greater than would be experienced during normal contact lens wear and it raises the question of whether or not small amounts of blur are significant in this respect? The work in the current investigation indeed suggests comfort is reduced by much smaller changes in vision and this phenomenon could affect many regular contact lens wearers.
Two papers published in 2015 and 2016 by Basuthkar Sundar Rao and Simpson who investigated the impact of vision on ocular discomfort are pertinent to this discussion. The first paper attempted to correlate the discomfort felt during suprathreshold stimulation of the cornea using pneumatic stimuli when vision was both clear and defocused (using 0–100 numerical rating scales) [
]. They found an exponential relationship between ocular surface discomfort and stimulus value such that when vision was blurred, comfort became worse compared to when vision was clear. Interestingly, they did not find any statistically significant differences in the raw subjective ratings of discomfort or intensity between the two visual conditions suggesting a possible higher order sensory integration between vision and discomfort (or pain). Although not directly related to contact lens wear these findings were important in that they demonstrated an association between ocular surface sensation and quality of vision. A contact lens on the ocular surface is likely to stimulate the corneal (as well as the conjunctival) sensory nerves through similar mechanical interaction mechanisms as the aesthesiometer pneumatic stimuli, but changes to ocular tissue physiology in contact lens wear may also contribute.
Their second paper attempted to tease out the effect of wearing a contact lens to induce blur on ocular surface comfort from inducing blur without a lens [
]. They found that comfort ratings were similar when an image was viewed with a +6.00DS contact lens and when a spatially filtered image (blurred to a nominal +6.00DS equivalent defocus) was presented without a lens in place. This is particularly surprising since all participants were non-lens wearers and were wearing lenses presumably for the first time. Ocular surface comfort was worse in the blurred contact lens situation compared with the clear non-lens situation which intuitively is what would be expected, but what is of particular interest is that comfort was worse in the non-lens spatially blurred condition compared to the non-lens clear viewing condition. No lens was worn in either of these conditions – the only difference was the clarity of the image being viewed. Equally surprising was that there was no difference in comfort between the no-lens spatially blurred and the contact lens blurred conditions. The change in vision ratings did not match the change in comfort ratings as vision was rated significantly worse when targets were dioptrically blurred compared to when they were spatially blurred. Their experiment also showed that when visual structure was absent during occlusion or ganzfield (full field) viewing, comfort was unaffected and appeared similar to clear viewing conditions. This latter finding supports those of Papas and colleagues who found that comfort was better in the various contact lenses when participants were occluded compared to when they were blurred [
] but there is currently no evidence that sensations of vision and ocular surface discomfort can interact via integration in the brain since they are known to have different pathways. The visual pathway relies on retinal photoreceptors to transduce light into neural signals which then travel via ganglion cells to the optic nerves and eventually reach the visual cortex in the occipital lobe. Ocular surface pain (and presumably also ‘discomfort’) which is not neuropathic in origin, results from the stimulation of mechanical/polymodal nociceptors or thermoreceptors in the cornea and conjunctiva [
]. These impulses are then carried along the trigeminal nerve endings to the trigeminal brainstem nuclear complex, the trigeminal subnucleus interpolaris/caudalis transition regions, upper cervical cord junction and then to the posterior thalamus [
]. Third-order neurons then relay information to the supra-spinal centres, including the somato-sensory cortex. Pain is a highly complex sensory process and is thought to be made up from multiple functional mechanisms i.e. sensory (intensity), affectiveness (unpleasantness) and cognitive dimensions [
The sensory and affective components of pain: are they differentially modifiable dimensions or inseparable aspects of a unitary experience? A systematic review.
]. Indeed, the effectiveness and cognitive dimension mechanisms may allow some form of integration with vision. Most of the referenced literature places emphasis on how vision can affect comfort but it is also possible that any potential cause/effect relationship may be the other way around i.e. perceived vision could deteriorate with increasing discomfort - however this seems clinically less likely.
The association between comfort and subjective vision demonstrated here is likely to be dependent on various factors related to the design of the investigation such as the lenses used, the age of the population, the experience of the lens wearers and the instrument used to describe the subjective symptoms. The type of numerical grading scale used here for both vision and comfort has been used in many contact lens investigations and has the advantage of being easy to administer, but their precise utility and validity has not been fully explored. It is not known whether or not these results could be translated to different lens brands or to reuseable lenses.
Overall, this work suggests that it is important for contact lens practitioners to be vigilant to patient reports of blurred vision during toric lens wear as this may increase the perception of discomfort; both discomfort and reduced vision are known reasons for contact lens discontinuation. The phenomenon of reduced vision causing subjective discomfort may also be important in other eye conditions such as dry eye disease, post refractive surgery and cataract.
5. Conclusions
This work suggests that symptoms of ocular discomfort may be more intense if there is also perceived visual compromise in daily disposable soft toric lenses. Therefore, it is important for eyecare practitioners to routinely and pro-actively ascertain the level of visual satisfaction experienced by wearers in addition to inquiring about the comfort of the lenses. These subjective vision measures are more likely to give an indication of fitting success and likely continuation than measured visual acuity. Every effort should be made to tackle all sources of potential visual compromise by paying careful attention to toric lens fitting parameters such as rotation, rotational stability and the amount of cylinder corrected.
Funding
This work was funded by CooperVision Incorporated. Gary Orsborn and Jose Vega worked with the other authors to design the clinical study and reviewed the manuscript prior to submission.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
We thank our clinical, logistical and administrative colleagues at Eurolens Research for their input into the acquisition of data for this study.
The sensory and affective components of pain: are they differentially modifiable dimensions or inseparable aspects of a unitary experience? A systematic review.
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