Patients with presbyopia who wish to wear contact lenses require a lens with more complex goals than a standard spherical lens—namely, to expand the depth of focus or allow the presbyope to see at more than one viewing distance—all with the highest image quality possible. Optics are a crucial aspect of a multifocal contact lens, as they inform how the lens is designed and impact patients differently depending on their anatomical conditions and visual scenarios.
Several recent studies highlight the ongoing development in multifocal contact lens research that may interest today’s practitioners.
|Making sense of multifocal optics can help you better fit presbyopes with the right lens. Photo: Alex Nixon, OD, MS, and Erin Rueff, OD, PhD. Click image to enlarge.|
The changes that occur to the pupil as patients age can have lasting implications on their ability to focus at near, something multifocal lenses have to take into account. Pupil size generally decreases over the age of 50, and the retention of near vision pupil miosis in older eyes can further reduce pupil size for near-viewing conditions where defocus is more likely.1 The smaller pupils in older eyes effectively correct for the increased aberrations found in these eyes.1 However, the decrease in photopic and mesopic pupil diameters in 65-year-old eyes fails to sufﬁciently expand the depth of ﬁeld for observers, who typically require some optical correction to read at near.1
Unfortunately, the same multifocal contact lens optics that help a patient address their decreasing pupil size can also cause other complications. In a recent study published in Contact Lens & Anterior Eye, researchers show that multifocal contact lenses can increase light distortion effects under low light conditions. In addition, the study found the size and shape of the pupil correlates with the size and shape of the distortion.2
The investigators looked at 14 eyes of seven contact lens patients. The light distortion index (LDI) was generally higher with multifocal lenses, varying from 3.7% with single vision lenses to 6.1% with the multifocal center-distance designs and the 5.0mm pupil. Patients with larger pupils weighed in with even higher LDI values when wearing multifocal lenses, shifting from 4.5% (in 3.0mm pupils) to 6.1% (in 5.0mm pupils). The elliptical-shaped pupil produced the largest discrepancy in the distortion size between the vertical and horizontal directions. The team didn’t note any difference between the center-distance and center-near designs.
Optical aberrations play a signiﬁcant role in light disturbances, which are magniﬁed with increasing pupil size. Thus, multifocal lenses with high amounts of spherical aberration (or distance defocus)—i.e., progressives with higher add powers—are more likely to generate signiﬁcant light disturbances under low light conditions.
The authors concluded that, although no statistically signiﬁcant differences were detected, multifocal contact lens wearers will ﬁnd themselves in everyday situations that might compromise their visual performance due to greater light distortion effects, especially with larger pupils. A significant percentage of light distortion effects would be produced by the presence of out-of-focus images given by a multifocal lens, rather than other optical phenomena.2
Up the Power, Up the Success
Fitting multifocal contact lenses can be a time-consuming process, often requiring the trial of several different lens parameters before landing on a good fit for each patient. Knowing this is likely a barrier to the implementation of multifocal contact lenses in practice, researchers tried a modified fitting guide that added +0.25D binocularly to the spherical equivalent distance prescription. Preliminary results suggest the small change could have a big impact.1,2
The researchers fit 183 presbyopic patients using either the traditional or modified fitting guides. The lenses all shared the same common optical design with either lotrafilcon B, nelfilcon A or delefilcon A materials. All participants were current soft contact lens wearers needing presbyopia correction.2
The team found practitioners needed to trial 1.2±0.5 lenses when using the modified fitting guide, compared with 1.4±0.5 lenses with the traditional fitting guide, which met the study’s predetermined criteria for superiority. On the first fitting visit, 82.8% of presbyopic patients required only one multifocal lens to find the right fit using the modified guide, while 65.1% were fit with one lens using the traditional guide. By the second visit, 98% of patients were fit using one or two trial pairs of lenses when the practitioners used the modified fitting guide.2
In addition, more clinicians preferred the modified guide, with 63.6% of participating clinicians giving the modified version the highest ranking for ease of use—only 33.3% did so for the traditional fitting guide.
The need for multiple optical powers in one lens is often avoidable by employing pinhole optics, which will alleviate presbyopia symptoms through enhancing the depth of focus.1 Many factors affect the selection of pinhole pupil size, but clinicians must strike a balance when improving defocused near vision without compromising focused distance vision.1 Three parameters are key: pupil axial location, pupil size and whether to employ a ﬁxed pupil or one that varies with light level.1
As pupil size decreases, the role of diffraction blur in degrading image quality is ampliﬁed, but the impact of higher-order aberrations (HOAs) is reduced, which produces a peak image quality in a focused eye for pupil diameters between 2.0mm and 3.0mm.1 At low photopic and mesopic light levels, reducing retinal illuminance lowers contrast sensitivity due to photon noise effects, and the combined effects of diffraction and reduction in retinal illuminance can signiﬁcantly impair distance vision when pupil diameters are decreased below 2.5mm.1
A study based in Seoul, South Korea, evaluated the efficacy and safety of a novel presbyopia-correcting pinhole soft contact lens. The Eyelike Pinhole II (Koryo Eyetech) includes an additional light-transmitting ring in the lens’ mid-periphery. The researchers found that the lens improved distance-corrected near visual acuity (VA). The binocular-corrected distance VA was not affected by the pinhole contact lens.3
This prospective clinical study enrolled 29 patients with presbyopia who wore the Eyelike Pinhole II for more than three hours per day for one week. The mean distance-corrected near VA of the treated eye and the mean binocular distance-corrected near VA improved after pinhole contact lens wear from −5.00D to −1.00D. Although the mean corrected distance VA of the treated eye deteriorated, there was no significant change in the mean binocular corrected distance VA.
The researchers believe their findings suggest that the pinhole lens can allow the wearer to perform daily tasks such as reading books or newspapers, texting on mobile phones and working on the computer in a more convenient manner. VA values before lens wear did not significantly exceed those after lens wear in any case, which indicated that the lens didn’t seem to worsen the vision quality.
The majority of participants were satisfied with the overall outcome, and 66% recommended the lens to others. Although the scores for visual symptoms and discomfort were lower than the work performance scores, the overall satisfaction level was not affected. Participants were willing to tolerate slight discomfort if they could perform their tasks effectively.3
|Achieving the desired level of multifocality requires larger, radially varying power changes in the correcting lens of a center-near design compared with a center-distance design. Image: Robert L. Davis, OD. Click image to enlarge.|
Design and Aberrations
Radially symmetric or concentric multifocal lenses are the most common designs. The concentric multifocal design is rotationally insensitive and includes two or more powers contained in geometrically separate zones located at different distances from the lens center.1 Designs that incorporate two powers in alternating annular zones often also include signiﬁcant regions of the lens in which there is a gradual power change with increasing radial distance.1
When the outer zone of a two-zone concentric design is defocused, the defocused point-spread-function will be an annulus. In this case, the resulting annular halo will increase in size as the pupil dilates and be most visible when the stimulus contrast is highest, as it is when viewing lights at night—a common source of visual disturbance clinically reported with multifocal optics.1
In addition to reducing the size of these haloes by reducing the add power (with either low add multifocal lenses or extended depth-of-focus lenses), an alternative strategy proposed is reducing the size of the defocused halo by coupling positive defocus with negative spherical aberration (including negative spherical aberration in the add zone) and vice versa. This produces a smaller but higher contrast halo.
Nevertheless, the naturally occurring changes in the refractive state across the pupil will add to (or subtract from) any multifocality provided by a contact lens.1 Therefore, the signiﬁcant positive spherical aberration exhibited by older eyes and the corneas of pseudophakes may augment any center-distance multifocal that also contributes more positive power with increasing radial distance from the lens center, or positive spherical aberration.
Importantly, ocular spherical aberration may help or hinder the multifocal optics, depending on the type of design being ﬁt and may likely contribute to the variable patient responses common during multifocal contact lens fits.1 In the case of center-near designs that inherently contain negative spherical aberration, for example, ocular positive spherical aberration will subtract from the add power provided by the multifocal lens.1
Achieving the desired level of multifocality in the corrected eye requires larger radially varying power changes in the correcting lens of a center-near design compared with a center-distance design. Although it could be simple to add the extra power needed for the center-near designs, high levels of spherical aberration in a contact lens will introduce more coma as the lens decenters.1 Peak and overall image quality are ultimately affected more by lens decentration in the center-near design because of the higher levels of lens spherical aberration required to achieve multifocality.1
A study conducted by researchers at the Brien Holden Vision Institute assessed the effect of spherical aberration as a function of power by evaluating the optical power profiles of all the most commonly prescribed multifocal contact lenses across a wide range of prescription powers. The researchers found that power profiles can vary widely between the different lens types; however, they also observed certain similarities between some of the center-near designs.4
“For the more recently released lens types, there seems to be a trend emerging to reduce the add amplitude, include negative spherical aberration, keep the power profiles consistent across the power range and offer lenses in at least three add powers and a daily disposable wearing mode,” the researchers wrote in their paper.4
The study measured power profiles of 38 types of multifocal contact lenses—in powers of +6.00D, +3.00D, +1.00D, −1.00D, −3.00D and −6.00D (three lenses each). The study identified three basic types of power profiles: center-near, center-distance, and concentric-zone ring-type designs. For most of the lens types, the relative plus with respect to prescription power was lower than the corresponding spectacle add. For some lens types, however, the measured power profiles were shifted by up to 1.00D across the power range relative to their labeled power.
In most lenses, the measured add amplitude was substantially lower than what would be required to provide the full reading compared with the corresponding recommended spectacle add power. Most of the lenses were designed with noticeable amounts of spherical aberration. The researchers noted that the sign and magnitude of spherical aberration can be either power-dependent or consistent across the power range.4
“When the first bi- and multifocal soft contact lenses appeared on the market, the lenses were labeled with their distance and add power,” the researchers noted. “In recent years, most of the newly released lenses only use descriptors like low, medium and high add power and make reference to the equivalent spectacle add power in the fitting guide.”4
Although some of the reasons for dissatisfaction with multifocal soft lenses are generally age-related discomfort and handling issues, unwarranted visual compromise does not make them an attractive option for the potential presbyopic lens wearer. Power profiles give insight into the distribution and magnitude of relative plus with respect to the prescription power in multifocal contact lenses. Practitioners can use such profiles to discriminate lens designs and to correlate design features with visual performance.4
A key understanding of the strengths and limitations of multifocal contact lens optics and how they might be applied in clinical practices is critical. Making sense of the optics —and changing up their design as new ideas come out—can help you better fit presbyopes with the right lensIn clinical practice, doctors must provide the best image quality or vision correction possible. With this in mind, multifocal contact lenses can provide patients good vision at all distances and, theoretically, optimal image quality with the right design.
1. Kollbaum PS, Bradley A. Multifocal contact lenses can provide patients good vision at all distances. Clin Exp Optom. 2020;103(1):21-30.
2. Monsalvez-Romin D, Gonzazlez-Meijome J, Esteve-Taboada J, et al. Light distortion of soft multifocal contact lenses with different pupil size and shape. Cont Lens Ant Eye. December 4, 2019. [Epub ahead of print].
3. Jun I, Cho JS, Kang MG, et al. Clinical outcomes of a novel presbyopia-correcting soft contact lens with a small aperture. Cont Lens Ant Eye. December 24, 2019. [Epub ahead of print].
4. Kim E, Bakaraju RC, Ehrmann K. Power profiles of commercial multifocal soft contact lenses. Optom Vis Sci. 2017;94(2):183-96.