Surgical Solutions to Presbyopia
Surgical Solutions to Presbyopia
New ideas that may transform near vision correction.
By Glauco Reggiani-Mello, MD, and Ronald R. Krueger, MD, MSE
Mark Twain famously observed that the two inevitabilities in life are death and taxes. Ophthalmologists would be quick to add a third item to the list: presbyopia. Though less grim than the others, it is in fact responsible for a significant decrease in quality of life.1 The number of people affected by presbyopia is growing fast due to the aging of the population. Presbyopic patients, with or without ametropia and cataracts, are increasingly seeking freedom from glasses.
The main strategy for presbyopia correction — reading glasses for emmetropes or bifocals for ametropes — can be acceptable for many people; however, the modern lifestyle demands options that are better both functionally and aesthetically. Surgical correction of presbyopia has been one of the most intense research fields in ophthalmology. This article reviews many groundbreaking approaches that have been explored during the last decade, some of which are soon to become a reality for practicing ophthalmologists.
The concept of correcting one eye for distance (usually the dominant) and the other one for an intermediate/near target remains popular. It can be performed with laser vision correction or contact lenses in either phakic or pseudophakic patients.
Despite being one of the most performed techniques to correct presbyopia, success is variable and patient satisfaction can be subpar. The drawbacks are having vision in one eye blurred (which requires an adaptation time), reduction in stereopsis and contrast sensitivity.2 It relies on neural adaptation and can be challenging in highly demanding patients, especially those in professions that demand excellent vision for distance or near. Usually patients achieve spectacle freedom for most daily tasks, but there is always some activity that would be better executed with both eyes focusing the target, like night driving in rainy weather or reading in low-light conditions.
Success rates are better for surgical monovision correction than the contact lens approach. LASIK and PRK have great success in correcting low and moderate ametropia, and their patient satisfaction rate of over 95% is highest among the elective procedures.3-5 Thus, the high precision of excimer laser technology makes laser-induced monovision one of the most used techniques to correct presbyopia for a patient with ametropia and no lens opacity.6 Using the excimer laser to correct the dominant eye for distance and the non-dominant eye for near/intermediate vision achieves a success rate of up to 92% of satisfaction7 with an acceptable slight reduction in stereopsis and contrast sensitivity.2
Another advantage is that it can be simply reversed with spectacles or an enhancement of the laser procedure. It can be used in hyperopes or myopes with or without astigmatism, and can be reversed more easily in myopes (as the patient is under corrected and can simply have the ablation completed).
Nevertheless, monovision's shortcomings and trade-offs highlight a need for bilateral near-vision correction. Options are discussed below.
New Laser Ablation Profiles
Innovative laser ablation profiles that create a multifocal cornea to correct presbyopia with fewer side effects than monovision are being studied. Although higher-order aberrations are responsible for decreasing the quality of vision, they also can increase the depth of focus. These optical proprieties have been studied extensively since the development of wavefront technology, which have proved the beneficial effect of spherical aberration8 (a fourth-order aberration that can be negative or positive) on expanding the depth of focus. The amount of aberration that is beneficial seems to vary individually and is not yet determined.
The Adaptive Optics Visual Simulator (AOVS) is a diagnostic device that can simulate the induction of low- and high-order aberrations. It has proven that different higherorder aberrations can have dissimilar effects on visual performance9 and can help determine the adequate type and magnitude of aberration that can expand the depth of focus with minimal effect on quality of vision.
The most promising aberration to induce when intending to expand the depth of focus is negative spherical aberration. In this situation, there is a steeper myopic center surrounded by a flat hyperopic periphery (Figure 1). In theory, this offers the best near vision acuity because it uses the pupil's properties of being smaller when reading and larger when distance focused. During near-vision miosis, it maintains the central myopic rays and suppresses the periphery's distance rays. The opposite effect is found in positive spherical aberration. The literature also suggests a faster neural adaptation response time to this aberration.10
Figure 1. Wavefront image of negative spherical aberration. Steeper center (blue) surrounded by a flatter periphery (red).
Creating a multifocal cornea by inducing spherical aberration in a LASIK procedure is called presbyLASIK. Strategies can entail creating a steeper center for near vision and a flat periphery for distance (central presbyLASIK, Figure 2)11 or a flat center for distance and a steep periphery for near (peripheral presbyLASIK, Figure 3).12,13 The first induces negative spherical aberration; the second, positive spherical aberration. The literature shows good early results with both techniques, but since there was loss of best spectacle-corrected visual acuity in some patients and decrease in quality of vision, further clarification is necessary. The AOVS can help in future selection of patients and magnitude of aberration in a custom fashion.
Figure 2. Topography pattern of central PresbyLASIK.
Figure 3. Wavefront representation of a peripheral presbyLASIK. Flatter center (red) surrounded by a steeper periphery (blue).
One newly developed technique to correct presbyopia is the IntraCor procedure performed with the Femtec femtosecond laser from Technolas Perfect Vision of Heidelberg, Germany. It is based on an "intrastromally controlled" biomechanical manipulation.14,15
In this procedure, femtosecond laser pulses are used to perform corneal "intrastromal-only" incisions in a cylindrical shape pattern. The incisions heal fast, since there is no damage to the epithelium. The incisions biomechanically induce a hyperprolate, negatively aspheric corneal shape, and an aberrated refractive profile with both negative spherical aberration and positive secondary spherical aberration.14 First results show an increased depth of focus and refractive corneal stability over the first year with no patients showing loss of best-corrected visual acuity.
Our knowledge about corneal biomechanics has been increasing over the last decade with the development of new diagnostic tools and cross-linking procedures. However, the fear of biomechanical disasters16,17 resulting in irregular astigmatism due to corneal instability (which occurred in procedures like hexagonal keratotomy, automated lamellar keratoplasty and radial keratotomy) are still fresh in our minds and long-term follow-up is needed to prove safety and stability.
When correcting presbyopia in patients with lens opacity, cataract surgery followed by multifocal or accommodative IOL is the standard treatment. However, if the patient does not have a lens opacity, refractive lens exchange with a premium IOL for presbyopia could be performed but the surgeon should be aware that the risk of severe complications such as endophthalmitis and retinal detachment is much greater than in laser corneal corrections.18,19
Apodized diffractive multifocal IOLs, in which there is a split of light into two foci (distance and intermediate/near) have shown better outcomes compared to refractive multifocal IOLs (different zones of focus on the IOL that provide various focal points) because they provide better optical quality,20 and are today the most used IOLs to correct for presbyopia.
The Acrysof Restor from Alcon is the main representative in this category and provides excellent distance and near visual acuity. Intermediate vision is one of the main limitations of this IOL (since it is limited to two foci); however, there was considerable improvement with intermediate vision in the +3.00D model compared to the +4.00D. Patient selection and education are the most important factors to achieve success when considering a multifocal IOL. Halos and glare can be functionally limiting, depending on the patient's activity. Another limiting factor is corneal astigmatism that ideally should be less than 1.00D (if greater, it can be corrected with LRI or LASIK). Highly aberrated corneas (e.g., decentered ablations, high spherical aberration) are not suitable for multifocal IOLs.
The concept of an accommodative intraocular lens is in theory the ideal for presbyopia correction, since it mimics the normal young human lens. There is no splitting of the focused light, which is responsible for the major problems with multifocal IOLs (halos, glare and the high luminosity needed). However, the magnitude of accommodation needed for reading comfortably is around 6.00 to 8.00 D (the amount of accommodation typical at age 30, Figure 4), since only 50-60% of accommodative amplitude should be used to avoid fatigue (3.00 D is needed to focus 33 centimeters). Available technology is only capable of much lower amplitude (around 1.00 D to less than 3.00 D). In addition, long-term results are limited by factors such as capsular fibrosis and opacification. Consequently, the expectation is good intermediate vision with less than ideal near vision. Pseudo-accommodative factors such as mini-monovision and pupil-dependent depth of focus play an important role in the success of the current technology.
Figure 4. Accommodation of eight diopters in a young eye. Left: focused near. Right: focused distance.
There are two designs being explored by industry: single- and dual-optics. Bausch+Lomb's Crystalens is a singleoptic accommodative IOL and currently the only accommodating IOL approved in United States. The concept is that the IOL should be in a posterior position in the capsular bag for a distance target (Figure 5). Accom modative effort would displace the IOL anteriorly, causing an increase in its relative power (Figure 6). There is an improved intermediate visual acuity, with optical symptoms (halos and glare) comparable to a monofocal IOL.21 However, as expected due to a limited real accommodation, the results for uncorrected near visual acuity are not as good as multifocal IOLs.22 In addition, capsular fibrosis can cause reduced accommodation and IOL tilt (Z syndrome).23
Figure 5. The Crystalens accommodating intraocular lens in a distance position (posterior).
Figure 6. Crystalens in an intermediate/near position. Note the anterior movement of the IOL.
A new, dual-optic accommodating IOL (Synchrony from Abbott Medical Optics) is under investigation in the United States. It consists of a high plus-powered front optic coupled to a minus posterior optic.24 The first studies show real accommodative amplitude ranging from 1 to 2.5 D,25 better near vision compared to monofocal IOLs and a lower rate of posterior capsular opacity attributed to the non-collapse of the capsular bag with the dual optic design.26 Further studies are needed to better understand this promising technology.
This is an area of intense research, with many promising IOL designs and lens refilling procedures currently under investigation.
Femtosecond Laser-Assisted Accommodation Restoration
The concept of laser modification of the crystalline lens was first reported in 1998.27 Laser micro-perforations within the hard lens nucleus were proposed to enhance the sliding of lens fibers and thereby increase lens flexure. In 2001, a study in cadaver eyes proved that it was possible to reverse the agerelated loss in lens elasticity by selective intra-lenticular photodisruption.28
Safety studies in an animal model showed that treatment with the femtosecond laser in the crystalline lens generated pinpoint opacities in the nucleus, but no progressively developing cataracts were observed.29 A computer model of the crystalline lens (Figure 7) was then created to assess the impact of various photo-disruption algorithms (Figure 8) to enhance the sliding of lens fibers.
Figure 7. Computer model of the lens fibers and sutures used to simulate the best pattern of cuts that would increase sliding of lens fibers.
Figure 8. Location of incisions in the lens to allow increased sliding of the lens fibers.
Studies in monkeys showed no cataract formation in a long-term follow-up and now a human clinical trial is being performed internationally to establish safety and efficacy. Early results showed variability in accommodation gain and newer algorithms are now being tested. Figure 9 shows a slit lamp photograph of a treated patient. Restoring accommodation via femtosecond laser needs further investigation but is possible, and once approved should be considered in emmetropic patients with presbyopia before refractive lens exchange.
Figure 9. Slit lamp appearance of a lens treated with a femto-second laser for accommodation restoration.
Other methods such as scleral expansion and monovision with conductive keratoplasty have been under research, with limited success. Scleral expansion has shown limited increase of accommodation30 in a invasive procedure (risk of scleral thinning) and conductive keratoplasty induces more irregular astigmatism, high-order aberrations and offers less stable results compared to LASIK surgery.31 Refractive surgeons in the US and Europe are investigating new corneal inlays such as the Flexivue, Acufocus and PresbyLens.
Presbyopia is the most prevalent disease in ophthalmology, and due to the aging of the population, the percentage of the people affected is increasing. The impact of presbyopia in the modern lifestyle is requiring other alternatives than conventional reading glasses. Exploring the surgical correction of presbyopia is an exciting field of investigation; intense research is already leading to better outcomes, breaking old paradigms and improving the quality of life. OM
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|Glauco Reggiani Mello, MD, is a fellow in cornea, cataract and refractive surgery at the Cole Eye Institute of the Cleveland Clinic Foundation.Ronald R. Krueger, MD, is the medical director of the department of refractive surgery at the Cleveland Clinic Cole Eye Institute and professor of ophthalmology of the Cleveland Clinic Lerner College of Medicine of Case Western Reserve University.
Ophthamology Management, Issue: October 2010