Ординатура / Офтальмология / Английские материалы / Hyperopia and Presbyopia_Tsubota, Boxer Wachler, Azar_2003

Figure 4 Photograph of epikeratophakia. Note the corneal lens on the top of the cornea.

technique was abandoned. Predictability was not adequate, such that the procedure did not gain popularity. In the specific situation of postoperative hyperopia after radial keratotomy, suturing of the corneal incision is useful for the correction of hyperopia up to 2 diopters. It is believed to stabilize refractive status, thus minimizing corneal shape fluctuation (13,14).

Automated lamellar keratoplasty is another method for the correction of hyperopia. Historically, the idea for this also came from keratomileusis. Steepening of the central cornea was observed to occur with lamellar keratotomy alone. Recently, ectasia of the cornea after myopic LASIK has become a major long-term safety concern. Progressive ectasia in a significant percentage of eyes, another major concern, also renders this technique unattractive. When the cut is deep, more ectasia unavoidably occurs with this procedure. The amount of ectasia depends on the optical zone. When the optical zone is small, the curvature is relatively high. When the optical zone is large, the curvature is low. The nomogram was developed on the basis of this observation (15,16). When the optical zone is 6.6 mm, the correction is 1.0 D; whereas the correction is 6.5 D with an optical zone of 5.0 mm. The cut should be deep—e.g., 65%. The initial results were promising, but the nomogram is not always predictable. With the development of hyperopic LASIK, use of this procedure is now limited (17).

The mechanical corneal contouring device invented by Eiferman and Nordquist is another means of correcting hyperopia (18). The principle is based on the observation that when the peripheral cornea is flattened, the central optical power is increased. The instrument consists of a vacuum chamber and steel blades positioned at orthogonal angles. When a Teflon stopper is added to the blade and the stopper pressed down on the eye, the peripheral cornea bulges, such that the blades can remove more tissue in the periphery than at the central cornea. The clinical results remain unknown.

C. CURRENT SURGICAL CORRECTION OF HYPEROPIA

Photorefractive keratectomy (PRK) for hyperopia is useful for the correction of hyperopia up to 3.0 to 4.0 D; however, healing of the corneal epithelium has effects on the final

result, such as regression and/or haze (19–23). Predictability is still poor for moderate to high myopia. LASIK, which was originally developed for the correction of myopia (24–26), is considered theoretically to be more advantageous for the correction of hyperopia because it is possible to ablate the corneal midperiphery by stromal photorefractive ablation and to prevent strong epithelial regression with an overlying flap (27–29). With the expansion of optical zone treatment, LASIK has now become an acceptable treatment for hyperopia of up to 5 D (30–36). This method is fully discussed in Chapters 7, 13–15.

The Phakic IOL has also been used for the correction of hyperopia as well as for aphakia and high myopia (37–40). The use of posterior chamber phakic IOL, such as the Staar Collamer implantable contact lens (Staar Surgical, AG, Nidau, Switzerland) appears to be promising, although there is a risk of cataract formation. The recent development of very light floating lenses, such as the Medennium (Ciba Vision, Duluth, GA), may be another innovation. The lens is very light, almost floating, and does not touch the patient’s own lens. Iris-claw lenses in phakic eyes, to correct hyperopia, are also promising (Fig. 5), despite the risks of glaucoma and corneal degeneration. Very thin anterior chamber phakic IOLs, angle support lenses such as Nuvita (Bausch & Lomb Surgical, Rochester, NY), and new foldable lenses designed by Baı¨koff (fully discussed in Chapter 11) are other promising technologies. These are discussed in detail in Chapters 11 and 12. Clear lens extraction can produce cystoid macular edema and retinal detachment and is less accurate and predictable for hyperopia below 3.0 D (41,42).

Reshaping the corneal curvature by heating of the peripheral cornea is another major approach for hyperopic correction. Currently, there are three ways to do this. One is laser thermal keratoplasty (LTK) (43–46). This employs a holmium laser technique, called the Sunrise LTK Procedure (Sunrise Technologies International, Inc. Fremont, CA), to heat the corneal collagen in several spots in the periphery. The resulting thermal contraction steepens the central corneal curvature, thus correcting hyperopia. This procedure has received approval from the U.S. Food and Drug Administration (FDA). The treatment range will be up to 2.5 D. The second method is conductive keratoplasty (CK) (Refractec, Inc,

Figure 5 Artisan hyperopia 5 mm, phakic intraocular lens (IOL) for the correction of hyperopia. (Figure provided courtesy of OPHTEC BV, Groningen, The Netherlands.)

Figure 6 Conductive keratoplasty for the correction of hyperopia. (Figure provided courtesy of Refractec, Inc., Irvine, CA, USA.)

Irvine, CA) (Fig. 6). This method uses a radiofrequency generator as the energy source instead of a holmium:YAG laser. The energy is delivered through a microtip inserted deep into the stroma. The procedure is considered to minimize regression relative to LTK because the energy is applied deep in the cornea, thereby creating an affected spot that is uniform in depth. The CK was approved by the FDA in April 2002. Diode laser treatment is a third approach (47–49). This procedure uses a 1.8-U diode laser as an energy source (Rodenstock, Munich, Germany). The application is similar to CK in that the probe is in contact with the peripheral cornea. The diode laser has not yet obtained FDA approval. All three technologies are described in detail in Chapters 8 and 9.

The ICS (Intrastromal Corneal Segments) or “Hyperopia Segments” is a variation of the INTACS Prescription Inserts (Addition Technology, Inc., Fremont, CA) under investigation in the United States, Europe, Brazil, Mexico, Singapore, and the U.K. While INTACS inserts correct for myopia by flattening the central portion of the cornea, the ICS is designed to correct hyperopia by steepening the anterior corneal curvature by the insertion of the ring materials at the limbal area, instead of inserting at the 7-mm central zone as for myopic correction. The ICS may also be used for hyperopia concurrent with astigmatism or hyperopic astigmatism. Clinical investigations have been initiated in both Mexico and Europe for the treatment of hyperopia using the ICS clinical product. The results are most encouraging, with stability achieved around the Month 3 exam and hyperopic corrections of up to 4.63 D (based MRSE) at Month 6 (n 43) and slightly less than 3.0 diopters of hyperopic correction (2.75 D) at the Month 12 exam. Manifest Refraction stability is demonstrated through the Month 12 time point. Clinical trials in Europe are ongoing.

Corneal implants have long been an attractive idea, but lack of suitable materials has inhibited the development of this technology. New materials have again made it attractive. Historically, Barraquer, who inserted glass materials into the corneal stroma in animals, developed the intracorneal technique. There was always a loss of transparency, with vascularization and extrusion of lenses. It was not known at that time that nutrients such as

amino acids and glucose come from the aqueous humor and that oxygen is supplied from the ambient air. The glass lens interfered with the exchange of these components through the corneal stroma. Then, fenestrated intracorneal polysulfone lenses were developed to enhance the exchange of metabolic components. However, opacification was associated with these fenestration techniques. This was a major obstacle to the implementation of this technology. The recent development of a small-diameter lens made of a hydrogel copolymer (Chiron Corporation, Emeryville, CA), with a water content of 45%, for correction of presbyopia, has paved a new pathway for this technology (53). Since the lens is small in diameter (1.8–2.2 mm), nutrient diffusion is not impaired (54). Lindstrom suggested the usefulness of this technology for a select group of patients (53). Anamed, Inc., recently developed novel biocompatible, clear, and permeable hydrogel materials with substantially higher permeability than typical hydrogels with similar water content. The refractive index of the material is essentially identical to that of corneal tissue (1.376 D), and has a water content exceeding 70%. The lens diameter ranges from 4.5 to 6.0 mm, depending on the diopter correction needed. The meniscus design includes a central thickness of 50 m and an edge thickness of 20 m. A preliminary animal study showing the safety and efficacy of this lens and a limited clinical trial were both performed by Dr. Stephen G. Slade, M.D., Director of the Laser Center of Houston, Texas. He reported that there was no haze formation except in one patient with minimal haze ( 1), which later resolved. No lines of best-corrected visual acuity were lost and the refractive correction was accurate. A flap is made with a microkeratome and the lens is placed in the center of the pupil. Since making a flap with a microkeratome is now a standard technique in refractive surgery, this technique is relatively simple for most surgeons currently performing LASIK. In LASIK, it is necessary for the correction of hyperopia to remove twice as much tissue as the same myopic diopter correction with certain regression, such that the intracorneal approach is reasonable. The FDA approved Phase I of the clinical trial for the PermaVision intracorneal lens, involving 10 eyes, which began in November 2001 (Fig. 7). An interim clinical analysis has been completed and submitted to the FDA, along with a

Figure 7 PermaVision intracorneal lens for the correction of hyperopia. (Figure provided courtesy of Anamed Inc., Lake Forest, CA, USA.)

request to move into Phase II of the clinical protocol. As of November 2002, Anamed is awaiting FDA approval to begin Phase II. This is considered to be a promising technology. However, the long-term complications—such as corneal stromal and epithelial thinning as well as endothelial change—must be evaluated in terms of safety.

D.CURRENT MEDICAL AND SURGICAL CORRECTION OF PRESBYOPIA

The major means of correction are simple glasses or simple contact lenses (55). The development of bifocal glasses provided the first convenience, allowing the use of only one pair of glasses throughout the day. Bifocal contact lenses are another popular method for the correction of presbyopia (56). According to the 1999 contact lens spectrum reader profile survey, 21.5% were fit with monovision, 9% with soft multifocals, 3% with rigid gas permeable (RGP) multifocals, and 5% with single-vision contact lenses and reading glasses. The remainder had spectacles only. A major disadvantage of this method is compromised visual quality (57). Success depends on the patient having a realistic expectation. Monovision contact lenses are also used (58,59). The increasing prevalence of dry eye in the elderly might be an obstacle to the application of this technique for many patients. Since the introduction of disposable bifocal contact lenses (Vistakon’s Acuvue Contact Lenses, Jacksonville, FL) in 1999 (60), use of bifocal contact lenses for the correction of presbyopia has been increasing. With further development of materials and designs from companies such as Ciba Vision and Bausch & Lomb, Inc., bifocal contact lenses have apparently become the major corrective method for presbyopia.

The intraocular lens with multifocal optics is another method for correcting presbyopia. This method is based on a theory termed “the simultaneous vision principle,” whereby separate images of near and distant objects are formed and, if the power difference between the two optical systems is more than 3.0 D, the images are dissimilar enough for the brain to interpret them as separate. The brain therefore selects the highly focused image and suppresses the other. This IOL can be achieved with two distinct optical elements (bifocal IOL) (61) or by means of diffractive optics (62), in which concentric diffractive zones are applied to the posterior surface of the implant in order to focus light from near objects. Both types of IOLs require central fixation and are relatively successful in younger patients. Monovision intraocular lenses are also the choice for presbyopic correction (61).

The IOL with real accommodative power has long been studied by Japanese and other researchers. The gel technology reached a certain level, using monkey eyes, in which the lens capsule was filled with soft gel. However, clinical application has not yet begun. Recently, a hinged haptic accommodative lens was developed and has attracted considerable attention. The proper functioning of the lens is dependent on movement of the remaining lens capsule, contracted by the ciliary muscle. When the lens capsule expands, the lens changes position and focuses. There are now several companies working on this technology (Fig. 8).

Like the multifocal intraocular lens, LASIK can also offer the multifocal effect by means of changing the corneal shape. It was first observed that regional variation in corneal curvature in the eyes of patients 45 years of age or older sometimes provides good near vision without correction (63). The regional variation in corneal power apparently explained how the multifocal lens effect could be achieved. Thus, intentional multifocal LASIK is a potential technology, which is still under investigation (64). Another practical method is monovision LASIK. Monovision is defined as providing optical correction of

Figure 8 Finite-element computer simulation of accommodative intraocular lens (IOL), a flexible micro-optic with accommodative features. (Figure provided courtesy of Human Optics AG, Erlangen, Germany.)

one eye for distance vision and of the other eye for near vision. This is usually achieved with contact lenses or intraocular lenses after cataract extraction, but can be achieved by LASIK as well (65). The ideal diopter difference necessary for both distant and near vision has not yet been determined. Since most patients with presbyopia undergoing LASIK still have some accommodative ability, there are several components that should be evaluated and determined for mass application of LASIK monovision. This is fully discussed by Azar in Chapter 18.

The concept of anterior ciliary sclerotomy (ACS) is a new challenge in the treatment of presbyopia (66). This surgery is based on the theory that the lens is ectodermal in origin and constantly grows throughout life, gradually filling the eye and leaving no space for accommodation (67,68). Loss of lens elasticity might contribute to the mechanism of presbyopia, and this theory raises the possibility that reduced space is the cause of the reduced accommodative power of the lens. Thus, somehow expanding the globe by ciliary sclerotomy can provide space for the ciliary body and lens for accommodation. Along this line, the original anterior sclerotomy as well as Fukasaku incisional surgeries have been developed (69). Since there is regression of the results due to wound healing, Fukasaku recently developed a method of inserting silicon plugs for the maintenance of the incision (69). Furthermore, the erbium:YAG laser has also been applied to making a wide scleral incision that may not heal quickly, thus maintaining the effect (Figs. 9 and 10). I have personal experience of two patients who had previously undergone LASIK. Both were Japanese males, aged 58 and 48 years. Both had 1.0 far vision without correction and near vision of 0.3 without correction, and both were having difficulty with reading. I applied the laser to a limbal-scleral area 4.0 mm in length. A total of 8 lasers were applied in a radial configuration. One day after surgery, both patients had 0.6 to 0.7 near

A B

C

Figure 9 (A) Scleral relaxation by laser. The eight incisions at the sclera. The incision is 4 mm in length and 0.5 mm from the limbus. The parallel incisions are 2 mm apart. (B) Possible mechanism of scleral relaxation for the treatment of presbyopia. (C) Scleral incision by Erbium:YAG laser. Optics are applied directly to the sclera where the conjunctiva is opened.

Figure 10 Slit-lamp view of the sclera incision by Erbium:YAG. The arrow indicates the incision fully covered by the conjunctiva.

vision without correction, reporting that they could read the newspaper without glasses. The technique must be evaluated in regard to long-term safety and efficacy, but the results appear to be promising. Recently, Schachar et al. proposed a new surgical treatment using a scleral expansion ring based on the same theory (Fig. 11) (67). Since several negative reports have been published on this theory and surgery (70,71), this area is discussed in Chapters 3, 20, and 21.

Figure 11 Slit-lamp view of the Schachar scleral band. Note that the scleral band is visible and slightly elevated.

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