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.
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.
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.
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.
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.
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 information aids in presurgical planning and postsurgical evaluation. Placido disc-based systems detect reflected images of rings projected onto the cornea. A computer generates a topographic “map” of corneal curvature based on the measured distance between the rings reflected from the cornea ( Fig. 14-1 , A and B ). Optical coherence tomography systems provide high-resolution cross-sectional images of the cornea and are based on the reflection of infrared wavelengths from biological tissues. Scheimpflug-based systems use slit beams and a rotating camera, which maps sections of the cornea ( Fig. 14-2 ). These systems allow for anterior and posterior corneal topography measurement and can also estimate corneal thickness.
Subtle corneal abnormalities, such as early keratoconus (forme fruste) or contact lens-induced corneal warpage, may be detected only by topography. The Oculus Pentacam Scheimpflug system provides the Belin/Ambrósio enhanced ectasia display, which aides in screening of keratoconus and corneal ectasia ( Fig. 14-3 ). In addition, postoperative and preoperative topographic maps may be analyzed to generate “difference” maps that isolate the procedure-induced changes. Computerized corneal topography is also extremely useful for determining the cause of imperfect vision after refractive surgery commonly due to irregular astigmatism.
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
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.
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.
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 ).
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 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.
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 ).
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 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.
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 keratitis
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.
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).
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 achieved in the higher myopia groups for both procedures. Nevertheless, LASIK may be the best corneal technique available for treating higher degrees of myopia.
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.
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 )
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