Ординатура / Офтальмология / Английские материалы / Wavefront Customized Visual Correction The Quest for Super Vision II_Krueger, Applegate, MacRae_2003

the myopic transition zone, but the hyperopic refractive effect exceeds the myopic correction effect and the result is a steepening of this part of the cornea. In the hyperopic transition zone, the ablation profile changes from a concave to a convex profile. However, considering the 6 mm zone ablation and the 1.50 mm decentration, this hyperopic transition zone is located 4.50 mm from the optical center and therefore has little effect on the final refractive result. In the other direction, on the side of the myopic ablation but outside the central 3 mm zone, the myopic correction juxtaposes the hyperopic transition zone, which starts at this level. Unlike the myopic transition zone, which has an ablation profile similar to the myopic central ablation zone but with a decreasing curve, the hyperopic transition zone has an ablation curve that is inverted compared to the central hyperopic zone. The effect of the hyperopic transition zone is to flatten the corneal curvature in this area; therefore, this effect is additive to the concurrent myopic ablation that also flattens the cornea. In light of these considerations on the hyperopic transition zone profile, we think that there is a greater limitation in decentering the hyperopic ablation than the myopic ablation in order to keep this transition zone outside of the eye’s optical center. Also, the largest hyperopic ablation diameter possible should probably be used.

In our experience, the maximum decentration distance used for the retreatment of severe decentration cases was 2 mm.

We think that there is a greater limitation in decentering the hyperopic ablation than the myopic ablation in order to keep this transition zone outside of the eye’s optical center. Also, the largest hyperopic ablation diameter possible should probably be used.

In conclusion, cases of moderate and marked decentration, after an initial excimer laser refractive surgery, can be corrected or markedly improved with the current laser technology. The decentration retreatment techniques, using a decentered myopic ablation or a combination of myopic and hyperopic ablation, were successful in recentering the treatment zone with predictable refractive results. The retreatment parameters used seem adequate for moderate cases of decentration but are probably not optimal for all cases. However, repeated retreatments are possible, and this is surely a good option for difficult cases.

EXAMPLE 6

This 52-year-old female had keratomileusis in 1991. Initial refraction was -13.75 + 1.25 x 95, yielding 20/30 vision. A significant decentration was present postoperatively and visible on topography (left map). Refractive result was -7.75 + 3.25 x 25, resulting in 20/50 vision. In addition to undercorrection, induced astigmatism, and a two-line BCVA loss, the patient complained of marked halos around lights. A retreatment with LASIK was performed 6 years after the initial surgery. In order to recenter the ablation zone, the retreatment was decentered 2 mm from the optical center in the opposite direction from the

first decentered ablation. It was estimated that the decentration of this ablation, by itself, should neutralize some of the induced astigmatism. Therefore, less than half of this astigmatism was included in the retreatment, but the retreatment was calculated to correspond to the spherical equivalent of the undercorrection. The retreatment parameters were -6.75 + 1.50 x 25, delivered with a 2 mm decentration from the optical center at 110. The retreatment diameter was 4.30 mm with a transition zone extending to 7.30 mm. The decision was made not to use a larger diameter ablation in order to limit the ablation thickness. The map on the right shows a marked centration improvement. Final refractive result was -0.50, resulting in 20/30 vision and a marked decrease of halos.

EXAMPLE 7

This 40-year-old engineer had an initial myopic PRK performed with a broadbeam laser for -6.50 + 0.25 x 145 myopia (20/15) in his right eye. The refractive result was -0.50 + 0.75 x 120, resulting in 20/20 vision. The patient had good vision in daylight but he complained of marked halos in dim illumination that made night driving difficult. A moderate decentration (map A) was believed to be the cause of these symptoms.

The retreatment with LASIK consisted of a +1 D hyperopic ablation using a 6 mm diameter zone with a transition zone extending to 9.60 mm. This ablation was decentered 1.50 mm at 60 degrees. Then a -1.25 myopic ablation using a 5.80 mm zone with a transition extending to 8.80 mm was added with a 1.50

mm decentration at 240. Because the refractive spherical equivalent prior to the retreatment was minimally myopic, the myopic correction exceeded the hyperopic correction by -0.25 D. No toric ablation was included in this retreatment. It was supposed that the astigmatism observed after the first surgery was a result of the decentration, and recentering the ablation should by itself correct this astigmatism. The result of this retreatment was a refraction of -0.25 D, yielding 20/20 vision and an almost complete disappearance of halos. Map B shows topographic maps after retreatment. A recentration and an enlargement of the ablation zone, without significant central corneal curvature modification, can be observed on this post-retreatment map.

EXAMPLE 8

This 44-year-old male had myopic LASIK for -5.00 myopia (20/20). The result was a marked decentration as noted on map A. Postoperative refraction was +0.50 + 0.25 x 90, resulting in 20/30 vision. In addition to a decreased vision, the patient was experiencing disturbing glare and halos. Three months after the initial surgery, a retreatment was performed. The LASIK flap was relifted and the retreatment parameters consisted of a +1.50 D

hyperopic ablation within a 5.80 mm zone (transition zone extending to 9.30 mm) with a 1.50 mm decentration at 235, and a myopic ablation of -1 D within a 5.50 mm diameter zone (transition zone extending to 8.5 mm) with a 2 mm decentration at 55 degrees. Refractive results were plano + 0.50 X 100, yielding 20/20 vision. Post-retreatment topography, on map B, confirmed a well-centered ablation zone, which correlated with recovery of the patient’s symptoms.

EXAMPLE 9

This 22-year-old patient had myopic LASIK for -3.50 D (20/15), with resulting decentration. Consequences of this marked decentration were undercorrection, induced astigmatism, monocular diplopia, and a two-line BCVA loss. Postoperative refraction was plano -2.50 x 165 degrees, yielding 20/25vision. The topographical map (map A) shows a marked inferior decentration. It was estimated that treating only the undercorrection would not be sufficient to achieve an adequate centration. The approach chosen for this patient was combined myopic and hyperopic ablations. The retreatment was performed only 1 week after the initial surgery, and the surgical approach chosen was to lift the LASIK flap. The patient was undercorrected (spherical equivalent -1.25); therefore, the myopic ablation value exceeded the hyperopic ablation. Also,

because of the patient’s young age, a slight overcorrection of +0.50 D was planned. A -2.75 D treatment using a 5.50 mm zone with a transition zone extending to 8.50 mm was delivered with a 2 mm superior decentration from the optical center. Then a +1 D hyperopic treatment using a 6 mm zone with a transition zone extending to 9.60 mm was added with a 2 mm inferior decentration. Although the ametropia present after the initial treatment was induced myopic astigmatism, the retreatment consisted of pure spherical ablations. No toric ablation was included in this retreatment protocol; the correction of the cylinder was achieved by the decentration of these spherical myopic and hyperopic ablations. An excellent result was obtained. The postoperative topography (map B) shows a large and well-centered final ablation zone. The refraction is plano + 0.75 x 15 degrees for a 20/20 vision. This small residual astigmatism showed a 60-degree shift from the astigmatism axis measured before the retreatment.

EXAMPLE 10

This 53-year-old man consulted for a severe treatment decentration after an initial myopic LASIK surgery of -11.50 D (20/25). Postoperative refraction was very inaccurate, due to a marked irregular astigmatism. Best subjective result was +0.50 + 0.75 x 180 for 20/60vision. On the topographical map, a 15.00 D difference was present between corneal curvature measurement taken 2.00 mm superior and 2.00 mm inferior to the optical center (map A).

The astigmatism measurement on this map was 5.82 D. The approach for the retreatment was to lift the flap and deliver a combination of myopic and hyperopic ablations. The retreatment parameters consisted of a 2.50 D hyperopic ablation within a 5.70 mm zone (transition zone extending to 9.30) with a 2.00 mm inferior decentration, and a -1.25 D myopic ablation within a 5.5 mm zone (transition zone extending to 8.50) with a 2.00 mm superior decentration. With no experience in retreating higher parameters (decentration distance or dioptric values), a conservative

approach was preferred to the risk of unpredictable result that could be the consequence of a more aggressive retreatment. A decrease of the corneal irregularity was obtained by this first retreatment (map B); however, the refractive result was unexpected. Refraction measured after the retreatment was -2.50 for 20/50 vision. Before this retreatment, the patient seemed overcorrected by 0.87 D (spherical equivalent). The myopic ablation was -1.25 D and hyperopic ablation was +2.75 D; therefore, a resulting refractive value of -0.67 D was expected instead of the -2.50 D obtained.

Two months after the first retreatment, a second retreatment was performed. A small diameter myopic ablation technique (as described previously to correct surface irregularities) was used. A -4 D myopic ablation within a 4 mm zone (transition zone extending to 6 mm) was performed. This retreatment was delivered with a 1.50 mm superior decentration at 100 degrees. The third topography (map C), taken 20 minutes after this retreatment, showed the persistence of a localized steep zone superior to the optical center. The corneal flap was than immediately lifted again and an additional -3 D ablation was delivered with the same 1.50 mm decentration at 100 degrees, but within a 3 mm zone (transition zone extending to 5 mm). The value of these retreatments exceeded the residual myopia; however, no resulting marked hyperopia was expected. It was learned from previous experience that an off-center ablation within a small diame-

ter gives less refractive effect than the same dioptric value treatment delivered centered and within a standard wider diameter.15 On another map, taken 1 month later (map D), a marked improvement was visible. The refraction after these retreatments was plano + 1.75 x 83, yielding 20/40 vision.

One month after the second retreatment session, a third retreatment was performed. An additional -3 myopic ablation within a small 3 mm diameter was delivered with a 1.50 mm decentration at 100 degrees to flatten the persisting steep zone. Because this ablation was decentered and within a small diameter, it was estimated that the effect on the final refraction should be less than 1 D. Then a +1.00 + 0.50 x 83 hyperopic ablation within a 5.50 mm zone (with a transition extending to 9.10 mm) was added with a 1.00 mm inferior decentration at 270 degrees. The cylinder correction was only +0.50 D, much less than the +1.75 to be corrected. This undercorrection was chosen based on the belief that the astigmatism is secondary to the corneal irregularity and the combined decentered retreatments, by decreasing this irregularity, should correct at least partly this astigmatism. The final result was +0.50 + 0.50 x 80 for 20/25 vision. Topo-graphical map taken after this final retreatment (map E) shows a much more regular and centered ablation. Monocular diplopia disappeared and the four-line BCVA loss was recovered.

EXAMPLE 11

This example shows the only case of “over-retreatment” we observed with this combined myopic and hyperopic retreatment technique. A first retreatment was performed to correct a problem with halos, believed to be the consequence of a moderate decentration. This retreatment resulted with a decentration worse and in the opposite direction from the initial one. Also, it induced one line of visual loss. A second retreatment session succeeded in improving the final outcome.

The first map (A) shows the decentration present after the initial treatment. The myopia was -9.50 + 1.50 x 7 for 20/20 vision. The surgical result was +0.75 + 1.75 x 168, yielding 20/20 vision. The first retreatment parameters consisted of a +1.50 + 1.25 x 168 hyperopic ablation with a 1.5 mm decentration at 30 degrees using a 5.50 mm diameter zone (with a transition zone extending to 9.10 mm) and a -1.00 myopic ablation with a 1.50 mm decentration at 210 degrees using a 5.50 mm diameter zone (with a transition zone extending to 8.50 mm). Result of this retreatment was +0.50 + 2.50 x 155, yielding 20/25 vision. The second map (B), taken after this retreatment, shows a decentration that has shifted 180 degrees

in the opposite direction. A second retreatment session consisted of a decentered hyperopic ablation of +1.00 + 2.00 x 155 within a 5.80 mm diameter zone (transition zone 9.3 mm). Decentration distance was 1.30 mm at 220 degrees. The third map (C) was taken after this second retreatment and shows better centration. Final refractive result was +0.25 + 0.25 x 105, yielding 20/20 vision. In this particular case, the initial retreatment diameters and decentration distances were not much different from those used for the other cases. However, this case had the highest induced hyperopia value prior to the retreatment, and a higher hyperopic astigmatic correction was delivered. Given the moderate decentration, the initial retreatment should have been performed with lesser distance decentration or with lesser dioptric values.

ENLARGEMENT OF

PREVIOUS SMALL DIAMETER ABLATIONS

Small treatment zones can be a cause of halos and glare after an excimer laser refractive procedure. These symptoms are more commonly experienced in lowlight situations. Despite good UCVA in daylight conditions, some patients are substantially incapacitated with night driving.29-33 The reasons for these halos and glare sometimes can be complex. They can result from a small diameter treatment zone or a large pupil diameter in relation to the treatment zone diameter. They also can be the consequence of aberrations due to a large curvature difference between the treated and the nontreated cornea or the consequences of higher-order aberrations.34-37 These symptoms were more common with treatments performed using first-generation broadbeam lasers. These lasers had a smaller treatment diameter and a less regular ablation pattern, as compared to more recent scanning lasers. However, halos can still be encountered, even with the most recent instrumentation. Enlargement of the optical zone can be effective in reducing halos and glare.38,39

With the current laser technology, it is possible to enlarge the ablation zone without significantly altering the refractive effect obtained by the initial surgery. The retreatment technique consists of a combination of myopic and hyperopic ablations, both delivered during the same operative session. These two ablations are delivered on the visual axis. Because their relative power— negative and positive—are nearly equivalent (eg, a -1.00 D ablation and +1.00 D ablation), they should neutralize one another. Consequently, they should not significantly alter the cornea’s refractive power.

We have retreated eight eyes from six patients with this technique. In all cases, the patient’s main presenting complaints were halos, diminished vision in low light situations, and night driving difficulties. In all cases, the initial ablation was regular and well-centered, and any minimal residual ametropia was meas-

ured. Eyes initially treated with PRK were retreated with LASIK. Eyes initially treated with LASIK had the initial flap lifted.

The diopter value of the retreatment, myopic and hyperopic, was between 1 D and 1.50 D. Wide myopic and hyperopic ablation diameters were used. The myopic ablation diameter ranged from 5.50 mm (with a transition zone extending to 8.50 mm) to 6.50 mm (with a transition zone extending to 9.50 mm). The hyperopic ablation diameter ranged from 5.80 mm (with a transition zone extending to 9.30 mm) to 6 mm (with a transition zone extending to 9.60 mm). Factors influencing the choice of ablation diameter were residual corneal thickness and LASIK flap diameter. Smaller diameters were preferred in case of limited residual corneal thickness or smaller LASIK flaps.

Although our experience is limited, this technique seems very effective; it achieved good results in most treated cases. In six out of eight cases, a moderate or marked decrease of the symptoms was achieved, and these patients were satisfied with the retreatment result. Only one patient (two eyes) did not notice significant change. In all cases, topographical maps showed a larger ablation zone after retreatment. The improvement was less noticeable in cases in which the initial treatment was for the correction of higher myopia. It is possible that for these cases, higher retreatment values would be more effective and lead to a better result; however, more aggressive retreatment parameters possibly have a higher risk of inducing irregular astigmatism. We do not have experience using this retreatment technique with dioptric value combinations higher than 1.50 D. Another consideration is that in cases of initial deep ablations, residual corneal thickness is often limited. Therefore, in order to respect a safe minimal residual corneal thickness, it is often not possible to use more aggressive retreatment parameters. Thus, this above-men- tioned technique has more limitations for the patients who were initially in the higher myopic group, and these patients are precisely those types who are more at risk for experiencing symptoms after an initial surgery and who would benefit more from this technique. Furthermore, in some cases, it is possible that other higher-order aberrations can also contribute to the halo symptoms and yet are not corrected by an enlargement of the treatment zone.40-44

This technique is safe and has predictable refractive outcomes. In our experience, no visual loss or other complication was observed. Results show that after the retreatment all eight eyes were in a -0.50 to +0.50 range (spherical equivalent). Induced astigmatism did not exceed 0.50 D in any case.

In conclusion, enlargement of previous small diameter myopic ablations, without altering the refractive effect of the initial surgery, is possible with a combination of large diameter myopic and hyperopic treatments. Although our experience is limited, this technique seems to be effective and safe in selected cases.

EXAMPLE 12

This example shows the topographical maps before and after a retreatment performed to enlarge a previous small diameter myopic ablation zone. Four years earlier, this 24-year-old female had PRK performed within 4 and 5 mm diameter zones for -5.75 D myopia. The refractive result was -0.25 + 0.75 x 155, yielding 20/20 vision. She had good vision in daylight conditions, but with dim light conditions she complained of halos, decreased vision, and marked difficulties with driving. Retreatment consisted of an enlargement of the ablation zone with a combination

of +1.25 D hyperopic treatment and -1.75 D myopic treatment. Both ablations were delivered on the visual axis. An additional plano +0.75 x 155 astigmatism treatment was included as a retreatment of residual astigmatism. A final +0.25 refractive result was planned. The hyperopic treatment zone diameter was 5.80 mm with a transition zone extending to 9.30 mm, and the myopic treatment zone diameter was 5.50 mm with a transition zone extending to 8.50 mm. As noted on map B, a marked enlargement of the optical zone was achieved by this retreatment. The patient experienced an almost complete disappearance of her symptoms. Final refraction was +0.25 D, yielding 20/15 vision.

EXAMPLE 13

These topographical maps illustrate another example of enlargement of the treatment zone. This 26-year-old patient was complaining of halos at night and marked night-driving difficulties. These symptoms were not corrected by the use of glasses for a small residual myopic astigmatism of -1.00 + 0.75 x 88 (20/20). In the retreatment protocol, in addition to a -1.00 + 0.75 x 88 ablation for the correction of this small residual myopic astigmatism, a combination of -1.25 D and +1.00 D ablations was included to enlarge the treatment zone. The myopic treatment exceeded the

hyperopic treatment by 0.25 D because a final refraction of +0.25 was planned. The hyperopic treatment zone diameter was 6 mm with a transition zone extending to 9.50 mm, and the myopic treatment zone diameter was 6 mm with a transition zone extending to 9 mm. As noted on map B, a marked enlargement of the treatment zone was present after the retreatment. Final refraction was plano + 0.25 x 50, resulting in 20/20 vision. A very significant diminution of halos under low light conditions was experienced by the patient, although some mild residual symptoms were still experienced.

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If the term super vision is taken literally to mean vision of a quality or performance exceeding the norm, then one could argue that by definition, any technique correcting presbyopia achieves super vision since presbyopia is a part of the normal aging process. More ambitiously, the quest for the correction of presbyopia ideally will lead to a technique that will restore the normal accommodative function of the eye: a continuously variable and active focusing system.

Customization of the visual correction of presbyopia adds another dimension to the problem at hand. In addition to restoring accommodation, a wavefront-guided customized correction of presbyopia could simultaneously correct ametropia and high- er-order aberrations and perhaps optimize the focusing range and visual quality according to the patient’s individual needs. Comparing the visual outcomes of current clinical techniques for the correction of presbyopia with these ideal goals clearly indicates that the quest for the customized correction of presbyopia is very much in its infancy.

The accommodation response of the eye involves intricate dynamic optomechanical interactions. Developing successful technology to restore lost accommodation in the presbyope, as well as to customize the correction, poses formidable challenges. Compared to wavefront-guided customized correction of myopia, hyperopia, or even astigmatism, the dynamic aspect of accommodation adds an additional dimension and level of complexity to the issues at hand. Currently, the first and fundamental hurdle is the absence of a definite and complete description of the mechanism of accommodation and its degradation leading to presbyopia. Today, the quest for the customized correction of presbyopia still relies on the acceptance of either a century-old theory some regard as dogma, or on other quasi-theories ill-supported by objective data relating to accommodation and presbyopia.

response is still not fully understood, but most experimental studies1 support the principles of Helmholtz’s theory of accommodation.2 According to this theory, the ciliary muscles of the eye contract in response to a stimulus to accommodation bringing about a relaxation of the zonules suspending the crystalline lens. This relaxation of the zonules permits the elastic crystalline lens to adopt a more curved shape, thereby increasing its refractive power (Figure 41-1).

A number of alternate or modified theories of the accommodation mechanism have been proposed. In his “catenary theory,” Coleman suggests that the vitreous plays an active role in the accommodation mechanism by providing support to the lens.3-5 According to this theory, ciliary muscle contraction during accommodation causes a forward movement of the choroid and vitreous. Together with relaxation of the zonules and vitreous support, this motion induces an anterior protrusion of the lens accompanied by steepening of the lens surfaces. More recently, Schachar proposed a controversial theory of accommodation in which relaxation of the ciliary muscle relieves tension in the anterior and posterior zonules, but increases the tension of equatorial zonules. According to the Schachar theory, this tension increases the lens equatorial diameter and steepens the central optical zone of the lens.6

The paucity of solid information relating to the accommodation mechanism extends to presbyopia. There is currently no definitive explanation for the age-related changes in the eye leading to presbyopia. Most likely, presbyopia results from a combination of factors affecting several of the anatomical structures involved in the accommodation response.7 However, there is strong evidence that a substantial amount of the accommodative loss with presbyopia can be explained by the inability of the aging crystalline lens to change shape7,8 in concordance with Helmholtz’s theory.

ACCOMMODATION AND PRESBYOPIA

Accommodation can be defined as the ability of the eye to change its refractive power in response to a stimulus. Accommodation provides the eye with a continuously variable focusing ability, which affords a clearly focused image to the eye regardless of the reading distance. The accommodation mechanism is a complex process that involves the lens, lens capsule, zonules, ciliary muscles, ciliary body, choroid, and vitreous mounted on the mechanical framework of the cornea and sclera. The contribution of each of these elements to the accommodation

According to Helmholtz’s theory,9 the age-related loss of accommodation is caused by a gradual loss of elasticity of the crystalline lens with age, which prevents the lens from changing shape. Substantial evidence also suggests that in addition to a change in the mechanical properties of the crystalline lens, there are accompanying changes in the gradient distribution and magnitude of the refractive index of the crystalline lens with presbyopia.10-12

354 Chapter 41

Figure 41-1. Helmholtz’s theory of accommodation and presbyopia.

According to Helmholtz’s theory,9 the age-related loss of accommodation is caused by a gradual loss of elasticity of the crystalline lens with age, which prevents the lens from changing shape. Although the ciliary muscles and zonules remain intact and operational during near vision, the more rigid crystalline lens of the presbyopic eye is unable to change shape to facilitate an increase in refractive power. Substantial evidence also suggests that in addition to a change in the mechanical properties of the crystalline lens, there are accompanying changes in the gradient distribution and magnitude of the refractive index of the crystalline lens with presbyopia.10-12

Of the global population of 6 billion, there are currently over 1.3 billion presbyopes in the United States. More than 40% of the population will be presbyopic over the next decade.

Regardless of the cause, the effect to presbyopes is a gradual decrease in their amplitude of accommodation with age (Figure 41-2), eventually leading to the loss of the ability of the eye to focus on near objects. With symptoms commencing around the age of 40, virtually everyone requires optical aids for reading and other close work by the age of 50. Of the global population of 6 billion, there are currently over 1.3 billion presbyopes. In the United States, due to improved health care and life expectancy, the percentage of presbyopes is higher, at around 35%. The US Bureau of Census’s International Database (IDB)15 predicts that over the next 10 years (by the year 2012), over 40% of people in the United States will be in the presbyopic age group. Thus, presbyopia is an extensive issue in vision correction and vision care delivery. The enormity of the presbyopia population and the associated size of the potential market for vision correction products in this sector are driving many developments in this area.

Figure 41-2. The decrease in amplitude of accommodation with age causes nearest unaided reading distance to increase. Some form of reading correction is required for virtually everyone by the age of 50. Graph shows combined data from Duane (1912)13 (in red), data from The Ayreshire Group with standard deviation (in black), and textbook curve (in green) as reported in Borish (1949).14

THE CORRECTION OF PRESBYOPIA

Editor’s note:

A number of strategies have been developed to treat presbyopia, including diffractive optics, multifocal lenses, and monovision. They have resulted in mixed success with tradeoffs in distance and near vision. A promising strategy is the removal of the human lens and replacement with a flexible gel that allows accommodative movement. This option presents a different set of technical and biologic challenges for success.

S. MacRae, MD

A number of techniques for the correction of presbyopia are currently available or under development. These include what may be called the conventional correction devices currently regularly prescribed, such as spectacles, contact lenses, or intraocular lenses (IOLs), as well as more recent innovations such as laser surgery of the cornea or lens and scleral surgery. Except for scleral surgery, these devices and techniques rely on a variety of optical designs or treatment approaches, including monovision, bifocal, multifocal, progressive, or accommodative corrections. Table 41-1 categorizes these devices and techniques according to how a change in refractive power is effected from distance to near viewing. It can be seen, generally, that all conventional devices compromise on some aspect of optical performance for the near power. Consequently, all of these devices suffer some form of visual performance reduction, as will be discussed further. Only those techniques that can deliver continuously variable power from distance to near precisely when needed truly restore the accommodative ability of the prepresbyopic eyes. These techniques include a few surgical techniques currently under development and will be discussed in later sections.

Spectacles and Contact Lenses

The most basic technique to correct (or perhaps more appropriately “relieve”) presbyopia is the wear of simple reading glasses or bior multifocal spectacles.16 While simple and inexpensive, these optical devices possess only discrete power steps. There are distances at which the eye cannot achieve sharp focus—the optical “dead spots”—resulting in blurred vision at certain viewing distances. Limited options exist for continuously variable focusing. Progressive aspheric spectacle lenses (PALs) overcome some of the limitations of the discretely powered devices and have taken over as the primary means for correcting presbyopia. However, these types of spectacles sacrifice the field of view in order to provide continuous power, while their complex optical surfaces necessarily introduce significant distortion and other aberrations. Clinically, these disadvantages are tolerated by the majority of wearers, as evidenced by the popularity of this mode of correction. However, they fall short of the full field of view, continuously variable system of natural accommodation.

Contact lens options available for correction of presbyopia include the simultaneous vision and the alternating vision bifocal/multifocals and can be either soft or rigid gas permeable (RGP).17 The most common of these are the simultaneous vision soft contact lenses, of which over five individual models and types are currently available around the world. Simultaneous

vision contact lenses (and IOLs, discussed later in this chapter) divide their optic zone into a near and a distance focusing portion. Typically, these portions consist of a circular central optic zone and an annular peripheral optic zone (termed “concentric” bifocals) although other configurations have been attempted. In such an arrangement, light from both the distance and the near visual object are admitted simultaneously to the eye (Figure 41- 3). The result is a superimposition of a blurred distance (or near) image over a sharp near (or distance) image that produces visual symptoms relating to contrast loss as well as ghosting and halos.18,19 A variation on the concentric simultaneous vision contact lens are designs that make use of diffractive optics to provide a near and distance image simultaneously, and with similar visual outcome.

An alternative exists to the simultaneous vision bifocal contact lenses that could potentially offer good field of view at full aperture with near power and only when required. This is the alternating vision, or “translating,” bifocal modality. In these types of contact lenses, the optic zones are situated such that only the distance optic zone lies substantially over the pupil during distance gaze. During near viewing, when the wearer’s gaze tends to be depressed, the lens is translated upwards due to its interaction with the eyelids. This puts the near optic zone in position directly over the pupil. Due to the alternating nature of this design approach, translating bifocals can potentially offer excellent vision at both distance and near.20 However,