Currently, there are two options for the contact lens correction of presbyopia, monovision and multifocal contact lenses. The advantages and disadvantages have been examined by many studies. However, major advances in contact lens design, along with improvements in measurement techniques of visual performance and subjective quality of vision, called for a reevaluation of the current presbyopic contact lens options.
The concept of monovision was first proposed by Westsmith in the 1960s. He described a system of fitting only one eye for distance and the other eye for near. Like with any optical correction, visual acuity is a critical factor in the success of monovision. Campbell and Green first demonstrated acuity loss with loss of binocularity. The acuity loss in monovision with high-contrast charts and adequate lighting is generally agreed to be less than one line. High-contrast visual acuity loss is relatively independent of the add power. There is approximately a one-letter decrease in binocular acuity per diopter of add power. Thus, most patients are able to maintain adequate vision with monovision correction.
There is some disagreement as to whether stereoacuity is important for distance depth perception. It is thought that beyond approximately 20 feet, monocular cues are used to judge distances. Cues such as perspective, overlap, motion parallax, and size may be more important to depth perception at distance than stereoacuity. The distance stereoacuity of patients with monovision is generally accepted to be 20 to 40 sec arc, but studies show a wide range of stereoacuity loss at near with monovision fitting. Ong reported that depth perception decreased with increasing anisometropia up to +2.00 D. Levy showed a direct linear relationship between unequal acuity and loss of stereoacuity. Later studies demonstrated that stereoacuity loss worsens with increasing monocular add powers.
The first soft bifocal contact lenses became available in the United States in the 1980s. In the past 2 decades, there have been major advances in disposable soft multifocal contact lenses. With multifocal contact lenses, visual acuity varies widely depending on the design of the lens, lighting, and contrast. Studies using early designs of multifocal contact lenses found that high-contrast distance acuity was decreased by between 0.5 to 1.0 lines and low contrast by one to two lines. More recent studies using newer designs have demonstrated that high-contrast acuity is not significantly affected with bifocal or multifocal contact lenses.
Stereoacuity can also be affected by multifocal contact lenses. Reports by Sheedy on early designs of multifocal lenses showed that stereoacuity was reduced by 32 to 36 sec arc compared with binocular correction. Other studies have shown losses up to 100 sec arc. More recent multifocal designs have shown improvement such that there is no significant decrease in stereoacuity with multifocal contact lenses vs. spectacle correction.
In 2000, there were approximately 100 million presbyopes in the United States; an additional four million people become presbyopic each year. The percent of contact lens fits and refits into monovision or multifocal soft contact lenses in 2004 was 16% in the United States and ranged from 1% to 40% worldwide. Bausch & Lomb’s SofLens Multifocal (Rochester, NY) claimed approximately 50% of the market share for soft multifocal contact lenses in 2005, the largest share of any soft multifocal brand (Health Products Research data, third quarter 2005). According to a recent survey, presbyopic contact lens fits still tend toward monovision over multifocal contact lenses. Perhaps this is because of the ease of fitting monovision or because of the wide range of available lens types and powers monovision allows, or it could be because monovision is a superior modality to multifocal contact lenses. To date, no study has been conducted to evaluate both the subjective and objective performance of the SofLens Multifocal and monovision correction in patients suited for both modalities. It was the purpose of this study to determine which presbyopic lens modality performs better and is better accepted by patients.
DISCUSSION
This study demonstrated that patients can achieve better than 20/20 high-contrast distance and near visual acuity with both multifocal and monovision contact lenses. The average high-contrast binocular acuity for both lens types was, at most, two letters worse than the best-corrected spectacle acuity and not significantly different. These findings are consistent with previous studies of monovision and multifocal contact lenses that found less than one line of visual acuity loss on high-contrast charts.
Low-contrast visual acuity was worse with presbyopic contact lens correction. At distance, low-contrast visual acuity with both multifocal and monovision contact lenses was three to four letters worse than with best-corrected manifest refraction. At near, multifocal wearers lost five to six letters of vision, and monovision wearers lost two letters compared with best correction. These findings are also consistent with previous studies showing decreased low-contrast visual acuity for both lens types.
What is more important is whether the loss of a few letters of acuity is clinically relevant and important to the patient. The worst acuity for both contact lens modalities was 20/32 at near. A good benchmark for near acuity requirements is 20/40, but it is not likely that patients would be able to read so close to their threshold level for long periods of time. Also, the patients’ visual acuity was not measured at low illumination. Studies have shown a decrease in acuity from one to three lines with both monovision and multifocal contact lenses under decreased illumination. Either low illumination alone or in conjunction with low-contrast print may compromise vision to unacceptable levels with either lens modality. It is likely that reduced illumination would decrease vision with multifocal contact lenses more than with monovision, because vision with multifocal lenses is more dependent on pupil size. Therefore, it is important to consider that some patients may require reading glasses or an alternate correction for extended or difficult near reading tasks.
A limitation of this study is the fact that it did not allow for a crossover in fitting either the monovision or multifocal lenses. The dominant eye was always fitted for distance, whereas the nondominant eye was fitted for near viewing. Pilot work by Schor and Erickson claimed that correcting the wrong eye for distance can impair the success of a monovision fit. However, for most of their experiments, they only used five to eight subjects, most of whom were not presbyopic. Later studies by Schor reexamined this point using patients with presbyopia and demonstrated that ocular dominance is not a critical part of a successful monovision fit. Back and Holden also concluded that ocular dominance does not seem to play a critical role in the successful fitting of monovision. Multiple studies showed that fitting either the dominant or nondominant eye for distance had no effect on visual acuity or performance. However, if a patient’s occupation includes demanding near tasks, they may be better fitted with the near lens on their dominant eye. Both monovision and multifocal lens fitting should be adapted to the individual patient’s visual needs.
On average, stereoacuity was 79-sec arc better with the multifocal compared with monovision contact lenses. Previous studies have demonstrated a range of stereoacuity from 40-sec arc to 400-sec arc depending on the type of lens, add power, and test of stereoacuity used. Not surprisingly, most studies have consistently found better stereoacuity with multifocal contact lenses than with monovision.
This study is unique in that the patient’s stereoacuity was compared with his or her habitual near correction and thus provides more information about visual performance in the real world. There was a large range of stereoacuity and a lower average stereoacuity than expected because patients were often undercorrected for near on entrance to the study. Thus, the average stereoacuity with multifocal lenses surpassed the baseline stereoacuity measurement by approximately 30-sec arc. Overall, however, the average stereoacuity with multifocal contact lenses was not as good as either what would be expected with full binocular near correction or as measured in previous studies. This may be the result of the fact that the stereoacuity test used in this study did not have monocular cues and was therefore a more accurate representation of true stereoacuity than findings from previous studies. The loss of stereoacuity may also be the result of the near blur (20/32 low-contrast acuity) found with the multifocal contact lens. Finally, approximately 34% of our subjects wore the multifocal in a “modified monovision” format (one high add and one low add). The fitting was done according to the manufacturer’s fitting guidelines, and this technique is a relatively common practice used by practitioners with this and other bifocal and multifocal contact lenses. Although this fitting technique may not accurately assess full multifocal to monovision performance, it is a better assessment of how patients are fitted outside a research setting and how they would experience the lenses if they were fitted in an average practice. The modified monovision fitting may be the reason for the decrease in stereoacuity seen in our patients with multifocal contact lenses.
The NEI-RQL instrument was developed to measure how refractive error and vision correction affect a person’s daily living. It is known that common in-office testing such as Snellen visual acuity or stereoacuity with monocular cues fail to evaluate all aspects of visual function. In fact, Berry and coworkers found that even when patients are corrected to 20/30 or better, they still have complaints about their vision and correction. Furthermore, the means of correction plays an important role in patient satisfaction. In Berry’s survey, the frequency of positive to negative comments in those patients wearing glasses was approximately equal. With contact lenses, positive comments outweighed negative comments by two to one.
In this study, no differences were found from the NEI-RQL results for expectations, diurnal fluctuations, dependence on correction, or worry. This was an expected result, because these subscales are more pertinent to patients with ocular disease or undergoing surgery. Also, the questions about suboptimal correction were not valid for this study. These patients were instructed to wear their contact lenses as much as possible during the study. Thus, because these questions asked about the frequency of using one type of correction over another, they were not relevant to the current study.
The differences in near and distance vision were not significant, yet there was a significant drop in clarity of vision with both contact lens modalities compared with the patient’s habitual correction. The near and far vision questions ask how the patient functions on many common visual tasks such as reading and driving. Patients felt that they could function well with their contact lenses and did not have difficulty performing most visual tasks; however, the clarity of vision subscale includes more general questions about whether the patient experienced any blur or distorted vision. It is understandable that patients would experience more transient blur with contact lenses then with their habitual correction (usually reading or multifocal spectacles), especially if they were new contact lens wearers.
Likewise, it was not surprising to find an increase in symptoms with both contact lens types. The NEI-RQL symptoms questions included discomfort, dryness, tearing, itching, and soreness, all of which can be very common in new contact lens wearers. Because this study excluded patients with previous multifocal or monovision contact lens experience, over two thirds of the subjects were new wearers. It is interesting to note, however, that there were no significant differences in symptoms between the two contact lens modalities.
The differences on activity limitations as a function of correction were not significant. Contact lenses could have been expected to improve the patients’ ability to participate in recreational activities. However, even at baseline, numerous subjects scored at the maximum level, so there was not much room for improvement in this area. This is consistent with previous studies using this survey.
Contrary to older studies, there was no statistically significant increase in glare symptoms with presbyopic contact lenses. Many studies have shown that glare was a major complaint with both multifocal and monovision contact lenses. Our results showed no difference in glare symptoms for monovision, multifocal, or baseline. This could be the result of the fact that the multifocal is a newer design. Additionally, some of the earlier studies used traditional lenses that may have caused corneal edema, whereas this study used disposable lenses. Furthermore, a recent study by McDonnell found no difference in glare symptoms when comparing monovision wearers with single-vision corrected presbyopes.
It is interesting to note that the subjects were significantly happier with their appearance in contact lenses compared with their previous correction. Both multifocal and monovision scored over 30 points higher than baseline on the appearance subscale—a large difference on a 100-point scale.
There was a trend toward higher overall satisfaction with multifocal contact lenses compared with either monovision contact lenses or the previous correction. Unfortunately, with this sample size, it was not possible to achieve statistical significance. The final study outcome, lens preference, speaks to the same question. Patients were asked to choose which presbyopic contact lens modality they preferred—multifocal or monovision. Because eligible patients could not have had prior experience with either lens modality before entering the study, there is no evidence to suggest their responses were biased by previous events.
Comparing both the objective and subjective results, there was only one test in which multifocal contact lenses were significantly superior to monovision: stereoacuity. Patients had better stereoacuity with multifocal contact lenses than with monovision, in some cases even better than what they were accustomed to with their previous correction. The importance of the loss of stereoacuity is often downplayed. Previous authors have claimed that studies have failed to show a subjective impairment related to the decreased depth perception and that patients do not complain of a loss of stereoacuity. However, the loss of depth perception may play a larger role in the success or failure of monovision than once thought. More recent examinations of monovision found that failed monovision wearers had larger losses of distance and near stereoacuity than successful monovision patients. Du Toit also established that the loss of near stereoacuity was greater in an unsuccessful group of monovision patients than in her successful group.
Patients may not realize that it is a loss of stereoacuity that causes difficulties with task performance and visual function, and the average patient may not have an adequate enough understanding of stereoacuity to volunteer such information. However, Papas found that when patients were given the opportunity to wear both monovision and bifocals and were asked if they noticed difficulty judging distances, 43% reported difficulty with monovision but only 5% reported difficulty with bifocals.
After trying the Bausch & Lomb SofLens Multifocal and SofLens 59 monovision contact lenses in close succession to allow for a true comparison, patients preferred multifocal lenses three to one over monovision. The reason patients prefer the multifocal contact lens is most likely because the multifocal provided comparable visual acuity without compromising stereoacuity to the same degree as monovision.
Conclusion
The majority of our patients preferred multifocals to monovision, most likely because the Bausch & Lomb SofLens Multifocal provides excellent visual acuity without compromising stereoacuity to the same degree as monovision.
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