Bibliography

1.What are the refractive components of the eye?

The cornea and the lens refract incident light so that it is focused on the fovea, the center of the retina. The cornea contributes approximately 44 diopters (D) compared with only 18 D from the lens. In addition, the anterior chamber depth and axial length of the eye contribute to the refractive status.

2.What are the various types of refractive errors?

•Myopia, or nearsightedness, exists when the refractive elements of the eye place the image in front of the retina.

•Hyperopia, or farsightedness, exists when the image is focused behind the retina.

•Astigmatism usually refers to corneal irregularity that requires unequal power in different meridians to place a single image on the fovea. Lenticular astigmatism (due to the lens) is less common than corneal astigmatism.

•Presbyopia is the natural impairment in accommodation often noted around age 40 years. The power of the corrective “add” or bifocal segment to combat presbyopia increases with age.

3.How is myopia related to age?

Myopia is common among premature infants, less common in full-term infants, and uncommon at 6 months of age, when mild hyperopia is the rule. Myopia becomes most prevalent in adolescence (approximately 25%), peaking by 20 years of age and subsequently leveling off. This information is important for determining the appropriate age to consider refractive surgery.

4.What are the goals of refractive surgery?

Goals vary for each patient. Certain patients desire refractive surgery because of professional or lifestyle issues; examples include athletes and police, fire, and military personnel, who may find glasses or contact lenses hindering or even dangerous. Other patients, such as high myopes, may find spectacle correction inadequate because of image minification or may be intolerant of contact lenses. In general, the goals of refractive surgery are to reduce or eliminate the need for glasses or contact lenses without altering the quality of vision or best-corrected vision.

5.What features characterize a good candidate for refractive surgery? Are there any contraindications?

First, patients considering refractive surgery should be at least 18 to 21 years of age with a stable refraction. Patients with certain ocular conditions (such as severe dry eye or uveitis) or particular systemic diseases (such as autoimmune collagen vascular disease or uncontrolled diabetes) and patients taking medications that impair wound healing are poor candidates. Keratoconus, a condition in which the cornea is irregularly cone-shaped, remains a contraindication for refractive surgery because results are unpredictable. Analysis of corneal curvature with computerized corneal topography should be performed on all patients before surgery because early keratoconus has a prevalence of up to 13% in this population and may be missed by other diagnostic methods.

Second, patients’ motivations and expectations should be explored thoroughly so that unrealistic hopes may be discovered preoperatively. For example, the patient who is constantly cleaning his or her glasses because of “excruciating glare” from dust on the lenses or who desires perfect uncorrected vision is not a good candidate for refractive surgery. A careful discussion of the risks and benefits of surgery is particularly important. Patients may want to try contact lenses before considering surgery. The concept of presbyopia must also be explained; many patients are prepresbyopic and have no understanding that achieving excellent uncorrected vision at distance will require correction for reading at near within a few years.

6.How is corneal topography used in the evaluation of patients undergoing refractive surgery?

Corneal topography is extremely useful for evaluating patients undergoing refractive surgery because it generates precise images of corneal curvature that correspond to a large area of the cornea. This

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Figure 14-2.  Scheimpflug-based corneal topography with corneal changes typically seen in keratoconus.

Figure 14-3.  Scheimpflug-based corneal topography with Belin/Ambrósio enhanced ectasia display.

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Figure 14-4.  Three weeks after four-incision radial keratotomy.

7.What are the major options for the surgical treatment of myopia?

•Radial keratotomy (RK)

•Photorefractive keratectomy (PRK)

•Laser-assisted in situ keratomileusis (LASIK)

•Intracorneal ring segments (Intacs)

•Phakic intraocular lens (IOL) implants

•Clear lens extraction

8.How does RK reduce myopia?

Radial keratotomy is a historical method for treatment of myopia and now has largely been replaced by excimer laser procedures. Deep radial incisions cause steepening of the cornea peripherally, which results in secondary flattening of the central cornea. The number, length, and depth of incisions and the size of the clear, central optical zone along with the patient’s age determine the refractive effect. Typically, four incisions are used for low myopia (Fig. 14-4) and eight incisions for moderate myopia. However, some patients had as many as 32 incisions for high myopia.1,2

9.What are the various RK techniques?

•The “American” technique involves making centrifugal incisions (from the center toward the limbus) with an angled diamond knife blade.

•The “Russian” technique uses centripetal incisions (from the limbus toward the center) with a straight vertical diamond knife blade. The Russian technique gives deeper incisions and more refractive effect; however, there is greater danger of entering the optical zone.

Based on statistical analysis of previous cases, standardized nomograms are used to determine the

number of incisions and optical zone size, depending on the patient’s age and desired refractive change.

10.What results have been achieved with RK? What about complications?

Several major investigations have been performed, the most important of which is the Prospective Evaluation of Radial Keratotomy study. This study showed that 60% of treated eyes were within 1 D of emmetropia up to 10 years postoperatively. After 10 years, 53% had at least 20/20 uncorrected vision, and 85% had at least 20/40 vision. However, 43% of eyes had a progressive shift toward hyperopia of at least 1 D after 10 years. This shift was noted to be worse for eyes with the smaller optical zone of 3 mm. Only 3% of patients lost two or more lines of best-corrected visual acuity, and all had 20/30 vision or better. Three of more than 400 patients complained of severe glare or starburst that made night driving impossible. Corneal perforations occurred in 2% of cases; none required a suture for closure. Overall, the best results were achieved in the low-myopia group (−2.00 to

−3.00 D). As with any invasive procedure, infection is a small but real risk (Fig. 14-5).3

11.How does PRK reduce myopia?

PRK involves direct laser treatment of the central corneal stroma. Specifically, the “excited dimer” (excimer) 193-nm UV laser causes flattening of the central cornea through a photoablative/photodecomposition process whereby more tissue is removed centrally than peripherally. Under topical anesthesia, the central corneal epithelium is removed either with a spatula or with the laser. The laser is then used to ablate a precise quantity of stromal tissue with submicrometer accuracy to achieve the

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Figure 14-5.  Infection at radial keratotomy incision.

Figure 14-6.  Mild stromal haze, 3 months after photorefractive keratectomy.

desired refractive effect. PRK is preferred over LASIK in cases of irregular astigmatism, thin corneas, epithelial basement membrane disease, prior corneal surgery, or LASIK complications. In some cases if the LASIK flap cannot be created safely, the procedure may be converted to PRK.

12.What results have been achieved with PRK? What about complications?

A randomized 20-year prospective clinical trial of PRK found the following:

•Slight but significant increase in myopia (0.54 D) after PRK between 1 and 20 years, particularly in those under 40 at the time of treatment and in female patients.

•Corneal power remained unchanged, but axial length increased.

•The procedure was safe, with no long-term sight-threatening complications and with improvements in corrected distance visual acuity and corneal transparency with time.

Residual corneal haze is a known complication of PRK (Figs. 14-6 and 14-7).4

13.What is LASIK?

LASIK stands for laser-assisted in situ keratomileusis. The procedure involves creating a corneal flap to ablate midstromal tissue directly with an excimer laser beam, ultimately flattening the cornea to treat myopia and steepening the cornea to treat hyperopia. Whereas earlier techniques of keratomileusis consisted of removing a corneal cap and resecting stromal tissue manually, technological advancements have revolutionized this procedure into a highly automated process. Contemporary techniques use a femtosecond laser for flap creation and an excimer laser for tissue ablation. After a lid speculum is placed and topical anesthetic is applied, the suction ring is centered on the cornea to stabilize the eye. Historically a mechanical microkeratome blade was

used to create the corneal flap, but currently most surgeons have switched to femtosecond laser for creating the LASIK flap. After the flap is created, the vacuum on the ring is released, and the flap is

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Figure 14-7.  Moderate-to-severe stromal haze, 6 months after photorefractive keratectomy.

then lifted, exposing the bare stromal bed. Next, the excimer laser is applied directly to the stromal tissue. Afterward, the corneal flap is replaced to its original position, typically without sutures, and allowed to heal.

14.How has the use of the femtosecond laser in the LASIK procedure helped to improve results versus the microkeratome?

A meta-analysis of multiple studies found that:

•No significant differences were identified between the two groups in regard to a loss of two or more lines of vision or to patients achieving 20/20 vision or better (P = 0.24)

•The femtosecond group had more patients who were within ±0.50 D of target refraction

•Flap thickness was more predictable in the femtosecond group

•The microkeratome group had more epithelial defects

•The femtosecond group had more cases of diffuse lamellar keratitis5

15.What is the range of myopia recommended for correction with LASIK?

LASIK is generally recommended for myopia as low as 1 D and as high as 10 to 12 D, although it is FDA approved for myopia up to 14 D.

16.What are the advantages and disadvantages of LASIK versus RK and PRK?

LASIK offers the advantage of minimal postoperative pain as well as earlier recovery of vision because the epithelium is left essentially intact. There is less chance of corneal scarring and haze than after RK and PRK. The disadvantages of LASIK include the brief intraoperative period of marked visual loss (due to high intraocular pressures generated by the suction ring); the risk of flap irregularities, subluxation, or dislocation (Fig. 14-5); and the expense of the procedure. Additional problems associated with LASIK include irregular astigmatism and the potential for epithelial ingrowth or infection under the flap.

LASIK offers several advantages to the surgeon. Because the technique involves making a flap in the anterior corneal stroma, the risk of corneal perforation associated with RK is virtually nonexistent. The creation of a uniform smooth flap with preservation of the central Bowman’s layer also reduces the subepithelial scarring seen with PRK. The use of the femtosecond laser allows little room for surgeon error. However, its automated aspect also poses disadvantages. The surgeon has limited intraoperative control over creation of the flap and ablation of the stroma. The vacuum ring can be difficult to place on a patient with narrow palpebral fissures or deep orbits. Femtosecond-created flaps can be difficult to lift and tears in the flap can be created. On occasion gas could cause perforation in the flap (vertical gas breakthrough).

17.How do the surgical results of LASIK compare with those of PRK?

Several studies have compared the results of LASIK and PRK in both low-to-moderate and moderate- to-high myopia. Overall, the refractive and visual results are comparable after the first 1 to 3 months. LASIK allows faster visual recovery. Pop and Payette compared the results of LASIK and PRK for the treatment of myopia between −1 and −9 D. They concluded that visual and refractive outcomes were similar at follow-up visits between 1 and 12 months, but LASIK patients were more likely to experience halos. In general, when refractive subgroups are analyzed, less predictable results are

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Figure 14-8.  Dislocated LASIK flap.

achieved in the higher myopia groups for both procedures. Nevertheless, LASIK may be the best corneal technique available for treating higher degrees of myopia.6

18.What is “wavefront?” Are wavefront ablations any better than standard LASIK?

In standard LASIK the spherical and cylindrical aberrations are measured using computerized corneal topography and manifest and cycloplegic refraction. The excimer laser is then programmed based on these data. A wavefront measurement has the ability to measure many more aberrations than just sphere and cylinder. To measure a wavefront an aberrometer shines low-intensity laser light through the pupil. The laser light is then reflected off the retina and through the lens, pupil, and cornea and is distorted by the refractive properties of the eye. This wavefront of light is then used to detect an infinite number of ocular aberrations (evaluated, for example, by Zernike polynomials or Fournier analysis).

In a wavefront ablation the data collected by the aberrometer are converted into a sphere and cylindrical equivalent (usually with room for physician adjustment) and the customized ablation is carried out. Although the hope for wavefront-guided LASIK and PRK is high, there is no significant clinical evidence that it is better than carefully planned standard LASIK. Some studies have shown a reduction in higher order aberrations after wavefront-guided ablations, while others have shown an increase. As surgical techniques and technology improve, perhaps the clinical results of wavefront LASIK will begin to outshine standard LASIK.

19.Name the important potential complications of LASIK.

Complications are uncommon and are not listed in order of frequency:

•Premature release of suction ring

•Intraoperative flap amputation (microkeratome)

•Postoperative flap dislocation/subluxation (may require suturing of flap into place) (Fig. 14-8)

•Epithelialization of flap-bed interface (causes irregular astigmatism, light scattering, and possibly flap damage) (Fig. 14-9)

•Irregular astigmatism

•Infection

•Diffuse lamellar keratitis (DLK)

•Progressive corneal ectasia7

KEY POINTS: COMMON POTENTIAL CONTRAINDICATIONS TO LASIK

1.Thin cornea.

2.Irregular astigmatism.

3.Keratoconus.

4. Anterior basement membrane dystrophy.

5. Herpes simplex or zoster keratitis.

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Figure 14-9.  Epithelial ingrowth under the LASIK flap.

20.What is diffuse lamellar keratitis? How is it treated?

DLK was originally termed “sands of the Sahara syndrome” because of the clinical appearance of a wavy inflammatory reaction in a LASIK flap interface. It generally appears 1 to 3 days after a primary LASIK procedure or an enhancement. The exact cause is unknown and is most likely multifactorial.

Suspected etiologies include bacterial endotoxins, meibomian secretions, oils from the microkeratome, and excessive laser energy from the IntraLase femtosecond laser. Treatment involves high-dose topical steroids. In severe cases, lifting the flap and irrigating the interface may be helpful.8

21.What is Epi-LASIK? What are the potential advantages?

Epi-LASIK is a modified surface ablation, which uses a keratome and an epithelial separator that creates a plane between the epithelial basement membrane and the Bowman’s membrane. As the “epitheliatome” passes over the eye it creates an epithelial flap on a hinge, very similar to a LASIK flap.

The epithelial flap is then reflected, exposing the surface of the Bowman’s membrane. The excimer laser is then used to alter the shape of the cornea, after which the epithelial flap is repositioned. The advantage of Epi-LASIK is the safety of a surface procedure but with potentially faster visual recovery, less postoperative discomfort, and less haze than PRK. Epi-LASIK has largely been abandoned in favor of LASIK, or the flap is removed entirely as in PRK.

22.What is the femtosecond laser? What are its potential advantages?

Current femtosecond laser technology systems use neodymium:glass 1053-nm (near-infrared) wavelength light. Laser energy is converted into mechanical energy in a process known as photodisruption. The femtosecond laser uses ultrafast pulses of energy to ablate tissue with extreme precision. The ultrashort pulses prevent heat buildup, thus allowing minimal to no damage to surrounding tissues.

In refractive surgery this laser is used to cut a lamellar flap in the cornea. The potential advantages of using the femtosecond laser include greater safety, reproducibility of flap thickness, decreased flap complications, and decreased epithelial defects. Flap striae and interface deposits may also be reduced. Currently available femtosecond lasers for LASIK flap creation include the IntraLase FS and iFS laser systems from Abbott and Femto LDV systems from Ziemer.

In cataract surgery, the femtosecond laser creates a very precise capsulotomy and fragments the lens nucleus. These features may reduce surgical risks associated with traumatic cataracts, zonular instability, and pseudoexfoliation. Primary and secondary corneal cataract incisions can also be created more precisely by the laser. Arcuate incisions that reduce astigmatism can also be created during cataract surgery, taking into account surgically induced astigmatism from cataract incisions. Many of the femtosecond lasers that are used in cataract surgery can also be used to create LASIK flaps.9-12

23.What is progressive corneal ectasia?

Corneal ectasia is progressive corneal thinning and steepening with irregular astigmatism that causes poor vision. It is thought to result mainly in eyes with forme fruste keratoconus or from a stromal

bed that is too thin after LASIK. Most surgeons believe that the stromal bed (calculated by taking the central corneal thickness minus the flap thickness minus the laser ablation) should be at least 250 μm to prevent corneal ectasia. However, other surgeons believe that the minimal stromal bed thickness

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should be greater. Long-term follow-up is required to determine the answer. Corneal collagen crosslinking is a treatment option for corneal ectasia, but it is not yet FDA approved.

24.What are intracorneal ring segments?

Intacs are an FDA-approved procedure for the correction of low myopia and mild to moderate keratoconus. This procedure involves the placement of two 150-degree arc segments of polymethylmethacrylate plastic at two-thirds depth in the peripheral cornea. This “tissue addition” results in flattening of the central cornea.13

25.How much myopia do Intacs treat?

Intacs are FDA-approved to treat between −1 and −3 D of nearsightedness in patients with no more than 1 D of astigmatism and who are at least 21 years of age.

26.What are the refractive results of Intacs for myopia?

In U.S. clinical trials at 1 year, 97% of patients had 20/40 vision or better, 74% had 20/20 vision or better, and 53% had 20/16 vision or better without correction.

27.List the potential complications of Intacs.

Complications are not common and are not listed in order of frequency.

•Induced astigmatism

•Fluctuating vision

•Anterior or posterior perforation of the cornea

•Infection

•White deposits along the ring segment

•Extrusion of the ring segment

28.Is the Intacs procedure reversible?

The Intacs can be removed, and most eyes return to their original refractions.

29.What are phakic intraocular lens implants?

These lens implants are placed in the eye without the removal of the patient’s own crystalline lens. There are currently three main types: an anterior chamber lens clipped to the iris, an angle-supported anterior chamber lens, and a posterior chamber lens in the ciliary sulcus (just in front of the crystalline lens). The Artisan or Verisyse iris-clip anterior chamber lens is currently FDA approved for −5 to −20 D of myopia with up to 2.5 D of astigmatism. The Visian implantable collamer lens (ICL) implant (Fig. 14-10) sits in the posterior chamber behind the pupil and is approved for −3 to −20 D of myopia. The toric version of this lens is available outside the United States and can be used to correct up to 6.0 D of astigmatism.14,15

30.What is the effect of the Verisyse phakic IOL implant on endothelial cell count?

The FDA found no significant loss in endothelial cell counts with the Verisyse iris-claw phakic IOL at 2 years after implantation. In a worst case scenario (by adjusting for measurement inaccuracy), 9% of eyes would have been at risk for 10% loss of endothelial cells at 12 months. Eyes at risk were found to have higher preoperative endothelial cell counts. Several authors have reported that the iris-claw lenses do accelerate endothelial cell loss.16,17

31.What are accommodative and multifocal IOLs?

As opposed to the more common single-vision IOLs implanted after cataract surgery, which leave the eye with very little ability to focus, the accommodative IOLs allow the eye to move the implant by

various mechanisms to allow a greater range of focus. The only FDA-approved accommodative IOL as of this writing is the Crystalens by Bausch + Lomb. The optic of the Crystalens is on hinges that allow for vitreous pressure to move the IOL anteriorly and posteriorly. This movement changes the refractive power of the IOL and allows patients greater reading ability. In the FDA 1-year trial, 98% of patients with bilateral implants were 20/25 at distance, 96% could read 20/20 at arm’s length, and 73% could read at near without any assistance from glasses or contact lenses.

Multifocal IOLs simultaneously focus near and distance light. The ReSTOR lens has apodized diffractive changes on the lens surface. The Tecnis lens has its diffractive steps on the posterior surface.

32.Are there any other surgical options for the treatment of myopia or presbyopia?

Because the crystalline lens adds about 18 D of power to the optical system, clear lens extraction may be used in patients with a comparable level of myopia. However, performing intraocular surgery for a purely refractive goal is controversial. In addition, highly myopic eyes carry a moderate risk of retinal detachment, which is increased after lens extraction.

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CHAPTER 14  REFRACTIVE SURGERY  141

Figure 14-10.  Visian ICL implant.

The SMILE procedure developed by Zeiss utilizes the VisuMax femtosecond laser to create a thin disc of tissue (lenticule) inside the corneal stroma, which is then extracted through a small incision. Unlike LASIK, this procedure does not require a flap. It is intended for treatment of myopia up to −10.00 D. This procedure is not yet FDA approved in the United States.

Kamra and Raindrop are corneal inlays that offer surgical options for presbyopia. The Kamra inlay (Fig. 14-11) works on the same principle as a camera aperture by increasing the depth of focus. The opening in the inlay allows only focused light into the eye, allowing one to see near, far, and everything in-between. The Raindrop inlay (Fig. 14-12) is designed to gently reshape the anterior cornea, providing near and intermediate vision in the nondominant eye. At this time neither the Kamra nor the Raindrop is FDA approved in the United States.

33.What are the treatment options for astigmatism?

The correction of astigmatism is slightly more forgiving than the correction of myopia. A patient with 3.00 D of astigmatism is usually quite pleased with a postoperative residual of 1.25 D of cylinder correction because reasonably good vision results. Each of the procedures for myopia has adaptations to address astigmatism alone or simultaneously with myopia. Astigmatic keratotomy refers to making transverse (straight) or arcuate astigmatic cuts in the midperiphery

of the steep corneal meridian. We have learned that crossing of transverse and radial incisions is problematic. Epithelial ingrowth into the stroma, healing difficulties, and significant scarring may result. Excimer laser photo-astigmatic refractive keratotomy uses a cylindrical ablation pattern rather than spherical ablation to remove tissue in a chosen meridian (astigmatic correction). If compound myopic astigmatism is present, a combination of spherical and cylindrical patterns results in correction of both myopia and astigmatism. Similar astigmatic corrections have been achieved with LASIK. Whichever procedure is employed, the axis of the astigmatism should

be marked with the patient seated, because it may shift when the patient reclines. Corneal

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Figure 14-11.  Kamra corneal inlay.

Figure 14-12.  Raindrop corneal inlay.

astigmatism can also be addressed during cataract surgery with a toric intraocular lens implanted after crystalline lens extraction.

34.What can be done about astigmatism after a corneal transplant?

There are several options. First, selective removal of sutures in steep meridians may improve astigmatism. A rigid gas-permeable contact lens may be especially effective in alleviating irregular astigmatism. However, many patients do not tolerate or desire contact lenses after corneal transplant surgery. Once all sutures are out and the refraction is stable, arcuate relaxing incisions may be performed in the donor cornea along the steep meridian to reduce astigmatism. An alternative technique involves using a blade to open the wound partially and relax several clock hours of the graft–host junction as opposed to creating incisions in the donor tissue. The femtosecond laser can also be used to create arcuate keratotomy incisions. As described above, the excimer laser also has been used to correct post-corneal transplant astigmatism. Relaxing incisions combined with compression sutures (across the graft–host interface) have been used successfully to correct astigmatism of 5 to 10 D by causing steepening of the cornea in the sutured median (Fig. 14-13). For astigmatism greater than 10 D, a wedge resection (of corneal tissue followed by sutured closure of the wound) may be performed in the flat meridian.

35.A 40-year-old Olympic ski coach desires refractive surgery so that he may see distance clearly. His refraction is −3.00 × −2.00 at 180 in both eyes. The surgeon performs radial incisions for 3.00 D of myopia and transverse incisions to flatten the steep meridian by 2.00 D at 90 degrees. Is the patient happy?

The patient is unhappy because of residual myopia. He now knows more about the “coupling effect” than his surgeon. When one incision causes corneal flattening in one meridian, there is a compensatory steepening of the unincised corneal meridian 90 degrees away. In the case above, the coupling effect of the incised and unincised meridians (90 degrees apart) should have been anticipated.

Radial incisions must be used to correct the 3.00 D of spherical myopia as well as the approximate 1.00 D of steepening induced by the transverse incisions. In general, short incisions tend to cause less steepening of the unincised meridian than longer incisions.

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Figure 14-13.  Treatment for post-corneal transplant astigmatism. Compression sutures were placed in the flat meridians (1:00 to 3:00 and 6:30 to 8:00), and relaxing incisions were performed in the graft wound 90 degrees away.

36.What about procedures for hyperopia?

Of the available options, none is as effective or reliable as the procedures for myopia.

For low levels of hyperopia, holmium laser thermokeratoplasty has been used with some success. This procedure is FDA approved for the “temporary reduction” of hyperopia in patients 40 years of age or older with between +0.75 and +2.50 D of manifest spherical equivalent with −0.75 D of astigmatism. Eight (or 16) peripheral laser spots are placed in a ring (or two), each spot causing shrinkage

of the stromal collagen and resulting in steepening of the central cornea. Problems to be resolved include regression of effect and induced astigmatism.

Conductive keratoplasty (CK) involves the use of low-energy radio-frequency energy delivered to the cornea with a guarded needle in a ring pattern around the midperiphery of the cornea. The heat generated causes collagen shrinkage, allowing the central cornea to steepen. CK was FDA approved in 2002 for the treatment of hyperopia in patients 40 years of age and older with a manifest refraction between +0.75 and +3.25 D. The procedure is effective for low to moderate hyperopia, but the trend is for regression over several years. CK has also recently been FDA approved to treat presbyopia in people over 40.

Hyperopic excimer laser PRK also has been approved by the FDA to treat hyperopia between +1 and +6 D. Treatment of hyperopic astigmatism also has been approved by the FDA in patients with +0.5 to +5.0 D of sphere with refractive astigmatism of +0.5 to +4.0 D and a maximal manifest refraction spherical equivalent of +6.0 D at the spectacle plane. The laser is used to create a large, donut-shaped ablation that requires a generous epithelial defect (often 9 mm or more).

Hyperopic LASIK treatments are currently FDA approved to treat up to 6 D of hyperopia with up to 6 D of astigmatism. Performing the laser ablation under a corneal flap has the theoretical advantage of decreased haze (ablation performed deep to the Bowman’s layer) and faster healing response (no large epithelial defect).

Phakic intraocular lenses can treat hyperopia as well as myopia; however, they are not currently FDA approved to do so.

Clear lens extraction is a technique already familiar to most surgeons. Phacoemulsification is performed with implantation of one or two IOLs as required by the degree of hyperopia. However, accommodation is completely eliminated by the procedure. Moreover, the risks of intraocular surgery, including endophthalmitis, are difficult to justify in eyes without organic disease.

37.What are the effects of refractive surgical procedures on corneal endothelial cells?

Although endothelial cell loss was an early concern in RK, studies using specular microscopy have demonstrated only a small, nonprogressive loss of endothelial cells. After excimer laser treatment of myopia, studies in animals and humans suggest a small, insignificant loss of endothelial cells that diminishes over time. Certainly it is much more of a concern with intraocular surgery, especially phakic IOLs. Ongoing studies are important. As the treated population grows older, patients eventually will require cataract surgery. There are already case reports of a renewed hyperopic shift in post-RK patients undergoing cataract surgery. Is corneal decompensation in Fuchs’ endothelial dystrophy accelerated by previous refractive surgery? Many questions remain unanswered. Effects of the laser itself, the inflammatory response, and the toxicity of topically applied drugs may contribute to endothelial cell loss and require further study.