Monovision lenses have been the preferred choice of vision correction for many presbyopes for the decades. However, market research shows that specialty lenses, such as multifocals, are steadily trending upward (increasing 136% since 2005) while the spherical lens market has remained relatively flat.1,2 Multifocal lens visits grew 39% from 2009 to 2010, which is likely due to both new contact lens wearers and patients who switch from other types of lenses.
Presbyopes new to contact lens wear saw the biggest increase in multifocal fits—up 64% in the past year—while multifocal wearers switching from current single vision and toric lenses increased by 35%. Visits by current multifocal wearers increased by 35%.1 This surge in multifocal fitting can largely be attributed to advancement in lens designs and materials, improving success rates, doctor comfort in fitting multifocal products and increased demand.3
There are multiple high oxygen transmission silicone hydrogel and standard HEMA multifocal lenses on the market today to meet the vision and health needs of most spherical presbyopes.
According to the 2010 U.S. Census Bureau, of the 308,745,538 people living in the United States, approximately one-third are over the age of 40 and will need some form of presbyopic correction.4 Presbyopes are projected to be the single largest group of potential contact lens wearers by 2018 (28% of all potential lens wearers or approximately 13.5 million people).5
Because active baby boomers with disposable income started the quest for multifocal contact lenses decades ago, they began lens wear with hard contact lenses and are much more likely to accept GP multifocal lenses.
Unlike the boomers, however, the Gen X generation (born between 1965 and 1980) grew up on soft, often disposable, contact lenses and are unwilling to compromise the comfort and convenience of soft contact lenses. This generation tends to be technology savvy, using a wide variety of mobile high-tech devices with varying working distances.
Going Soft
But, regardless of generation, presbyopes want clear vision at all distances without glasses. Fortunately a wide variety of multifocal lenses are now available and as doctors, we can meet these demands with soft multifocal lenses.
Although soft monovision correction is a viable option for many presbyopes, monovision begins to fail as add power increases and depth perception is disturbed. A 2006 study found that 76% of patients preferred multifocal contact lenses to monovision.6 Another study utilized a BlackBerry hand-held device, facilitating real-time assessment of assigned tasks and found on subjective ratings, monovision was the “lowest performer” while multifocal contact lenses were “highest performer” in early presbyopes.7
Multifocal lenses can offer excellent visual acuity without compromising stereo acuity to the same degree as monovision. So, soft multifocal lenses may provide patients with the functional vision they truly desire.8
The biggest advantage of soft multifocal lenses is that they work over a variety of focal ranges, unlike monovision. Today’s presbyopes are frequent computer users, and therefore the intermediate range plays an important role. Monovision only allows focus of two distinct focal lengths; however, three fields are necessary to function well in today’s world (distance, intermediate and near).
Special Considerations for Multifocal Success
The optical and visual performance of soft multifocal contact lenses depends mainly on five factors: lens design, central optic zone diameter, pupil size, intrinsic spherical aberration and lens centration.9 Visual performance has been evaluated and compared to monovision in terms of contrast sensitivity, high and low contrast visual acuity, and stereopsis.8,10
Near acuity is highly dependent on pupil size. Pupil size varies with light level, patient age and visual task. The larger the pupil size, the greater the decrease in retinal image quality.
For patients with strong intermediate and near tasks, center near lenses are preferred to center distance lenses because the visual performance is better at near, due to convergence-related pupil constriction. Decentration affects the portion of the pupil covered by the central optic zone of the lens.11 Retinal image quality decreases with lens decentration.
Soft multifocal wearers have similar or better high and low contrast visual acuities compared to monovision wearers.8,12 The biggest decrease in acuity in soft multifocal lenses occurs with near tasks in dim illumination. This is why we should remind multifocal wearers that reading glasses may be necessary in low light situations. Numerous studies have shown degradation in intermediate and high contrast sensitivity; however, at low spatial frequencies, multifocal performance is similar to spectacles.13 Monovision wearers also experience more disability from glare and halos.14
Both multifocal and monovision lens wearers must be prepared to undergo a period of adaptation to either modality of correction. In a study comparing the effects of monovision contact lens powers on the function of emmetropic patients, the optimal power for distance and near correction for the monovision contact lens wearer was +1.50D. In emmetropic patients, near vision was improved with increased lens power, but distance vision was degraded objectively and subjectively.15 Therefore, the visual goals of advanced presbyopes are better met with either modified monovision or multifocals.
Multifocal Contact Lens Designs
Soft multifocal contact lenses are based on the principal of simultaneous vision, which means the patient views both distance and near images at the same time. With two different images falling on the retina concurrently, the brain must know to prioritize by selecting either the distance or near image. Although simultaneous vision multifocal contact lenses provide superior stereopsis, the retinal image rivalry significantly reduces contrast sensitivity.
The three simultaneous vision lens designs are the diffractive, concentric (annular) and aspheric designs.
• Diffractive designs. Diffractive lenses utilize concentric phase plates to diffract light, creating the desired multifocal effect. This lens design induces significant ghosting of images and dramatically decreases acuity in dim illumination. Due to these limitations, there are no current diffractive multifocal contact lenses on the market (the Hydron Echelon lens [CooperVision] was discontinued in March 2006), although multifocal intraocular lenses are still being implanted with good success.16
• Concentric (annular) design. Concentric multifocal lenses have either a distance or near optical zone overlying the pupil. Therefore, pupil size and lens centration are critical in predicting the visual performance of the lens. Concentric multifocal designs include:
– Distance centered. The distance prescription overlies the pupil surrounded by the add power. An increased pupil size in dim illumination allows light from the add power to enter the eye, creating halos and ghost images. On the flip side, bright illumination may reduce near acuity.
– Near centered. The near add is centered with the distance power surrounding. Miosis induced by accommodation enhances image clarity by assuring that the pupil stays within the near optical zone.
– Pupil-intelligent design. A distance-centered zone is surrounded by alternating near and distance zones positioned over the pupil. The goal is to maximize acuity in varied lighting environments.
– Aspheric-blended transition. Eliminating the abrupt power change between the distance and near optical zones, this design consists of an aspheric-blended transition.
A concentric lens design performs best for early presbyopes with low-to-moderate bifocal add requirements. Increasing the add power with this lens design significantly decreases low-contrast acuity as well as the overall quality of vision. If multiple near optic zones sizes are available, consider prescribing the smaller zone for the dominant eye and the larger zone for the non-dominant eye.
• Aspheric design. Aspheric lenses offer a gradual change in power from the center to the periphery. This progressive change may provide more optimal intermediate acuity. Aspheric multifocals have either a posterior or anterior aspheric surface. Aspheric multifocal designs include:
– Posterior surface aspheric. The plus power increases toward the periphery. The posterior surface aspheric design may be more appropriate for patients requiring detailed distance vision in photopic (bright) conditions.
– Anterior surface aspheric. Plus power is greatest in the center of the lens progressing to more minus in the periphery. This design may be better suited for individuals with greater near demands in bright lighting conditions.
Keep in mind that aspheric geometry will induce spherical aberrations. The high plus power located centrally in a front surface aspheric lens creates greater central positive spherical aberrations that progressively decrease toward the periphery.
The opposite is true for back surface aspheric lenses. This is advantageous only if the lens design cancels out the patient’s corneal and lenticular aberrations.9
Modified monovision works particularly well with these designs. The availability of two lenses of similar design having different power gradients provides the ability to overcome problems related to poor contrast sensitivity. Poor contrast sensitivity, as opposed to improper lens power, may be the indication for a particularly sensitive patient’s inability to achieve satisfactory distance vision.
Future Designs
Future multifocal contact lenses may be individually designed based on the patient’s unique aberration profile. Knowing the spherical aberrations of the cornea is essential to predict the likely clinical performance of any contact lens.9 Positive powered contact lenses exhibit positive spherical aberration, while negative powered contact lenses exhibit negatively powered spherical aberrations.
Some manufacturers claim to reduce or “correct” the spherical aberrations created by the lens in air with an aspheric front surface. This philosophy works beautifully if an aspheric front surface is what is needed to negate the spherical aberration of that cornea.17 In some instances, an aspheric front surface will induce additional aberrations.
Therefore, in order for a lens to adequately correct for corneal-induced aberrations, the specific aberration profile of the cornea must be known.18 Because no two corneas are exactly alike, this can only be measured by an aberrometer. Advancements in aberrometry analysis and contact lens designs may make aberration control lenses the preferred lens choice of the future.
As the presbyopic population continues to grow, so do our multifocal contact lens options. Ironically, it may be the doctor who may pose a barrier to the growth of his or her own practice by not embracing the new technology and the ever-present demands from the patient.3 Nevertheless, if the trends of the past decade continue to hold true, the multifocal market will continue to grow and prosper.
Dr. Sindt is the Co-Chief Clinical Editor of Review of Optometry. She is also the Director, Contact Lens Service, and an associate professor of clinical ophthalmology at the University of Iowa, as well as president of Women of Vision. Dr. Mataya Pietig is in private practice in Ankeny, Iowa. Her clinical focus includes advanced contact lens fitting and pediatric optometry.
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