US Pharm. 2010;35(4):HS20-HS24.
The goal of eliminating refractive errors and allowing individuals to obtain uncorrected optimal visual performance without glasses or contact lenses can be successfully achieved by several surgical procedures. This article will focus primarily on the use of the excimer laser to reshape the cornea with a quick, painless, and popular operation known as LASIK (laser-assisted in situ keratomileusis).
History of Eye Surgery
LASIK: LASIK is a surgical procedure that combines the application of the excimer laser (ultraviolet [UV] 193 nm light energy) under a hinged corneal flap (in situ keratomileusis) to alter the refractive properties of the cornea. The technique was developed in the late 1980s by Ioannis Pallikaris, MD, PhD, and was used in other countries before the first FDA trial in 1989.1-3 LASIK, compared to other refractive surgical procedures, allows for specific sculpting and subtraction of corneal tissue under a protective corneal flap, making possible the correction of a broad range of optical errors including myopia (nearsightedness), hyperopia (farsightedness), and astigmatism (blurring of vision due to irregular shape of the cornea). In contrast to other refractive surgeries and photorefractive keratectomy (PRK), LASIK appears to have several advantages, especially in its speed of recovery and minimal postoperative discomfort.4,5
Other Refractive Surgeries: Besides LASIK, other forms of refractive eye surgery include radial keratotomy (RK), which is used to treat myopia; lamellar surgery, upon which LASIK is based; and PRK, which was the earliest application of the excimer laser to treat myopia.
Presurgical Considerations for LASIK
A wide range of refractive errors including low-to-high myopia, low-to-moderate hyperopia, and large amounts of astigmatism can be easily corrected by LASIK. In addition, many common systemic disorders such as diabetes are no longer a contraindication to surgery. TABLE 1 lists specific criteria for patient selection for the LASIK procedure.6,7
The FDA has approved LASIK in people 18 years and older with stable refractions. Some contraindications include pregnancy and having had eye diseases in the past such as ocular herpes, cataract, glaucoma, and keratoconus. In addition to a thorough preoperative examination and counseling of any patient interested in refractive surgery, some additional testing is warranted to help screen for possible postoperative side effects.
Pupil Examination: The pupil should be measured in both scotopic (dim) and normal (photopic) light. Measurements may be obtained with infrared pupillometers or with a simple pupil card.8 There is a possibility that young patients with large scotopic pupils may be at higher risk for developing symptoms of glare, halos, and nighttime visual disturbances following refractive surgery. Today, with large ablation zones and the use of aberrometers for custom ablations, this is less likely.
Dry Eye Testing: Dryness after LASIK is one of the most common symptoms reported by patients.9 It is believed to result from the cutting of the anterior flap and the loss of cornea sensation that eliminates reflex tearing. Proper preoperative testing evaluating the precorneal tear film, the use of artificial tears before surgery, and discussions about this common condition with the patient should be documented. The prescription eye drops Restasis (cyclosporine ophthalmic emulsion 0.05%) can also be used.
Topography: It is critical for all potential refractive surgery candidates to undergo preoperative cornea mapping (topography) and cornea thickness (pachymetry) measurements to exclude patients at risk who may respond poorly to the surgery.7 Many computerized machines are available to analyze the anterior and posterior curvature of the cornea to aid in selecting and screening good candidates for surgery. These instruments map the surface of the cornea and generate data, which help eliminate patients with abnormal corneas such as seen in keratoconus (a thinning disorder of the cornea that changes it from a round shape to a cone.)
Pachymetry: Pachymetry is a measurement of the thickness of the cornea. The average cornea thickness is approximately 540 microns. Preoperative cornea thickness is an important measure used to determine the stromal thickness that will result from subtracting the flap thickness and the amount of tissue removed by the treatment.10 It is crucial to leave behind enough corneal thickness to maintain cornea integrity so that the cornea does not become ectatic (see Cornea Ectasia and Collagen Cross-Linking). Cornea flap thickness can range from 110 to 180 microns. The minimum residual stromal thickness is recommended as no less than 250 microns.11
Types of LASIK Surgery
The patient is conscious during surgery but can be given mild sedation with an oral benzodiazepine such as diazepam. Anesthetic ophthalmic drops (e.g., proparacaine) are given immediately before the drapes are applied and the speculum inserted. The patient is asked to look directly at a fixation target under the laser (patient cooperation is very helpful).
LASIK involves photoablation (pulses of UV light energy) of the corneal stroma after a cornea flap has been created. The laser energy can be programmed to flatten the central cornea to correct myopic errors (nearsightedness) or steepen the peripheral cornea to correct hyperopic errors (farsightedness). Traditionally, flaps have been produced with mechanical microkeratomes, but more recently femtosecond laser technology has emerged as an alternative for flap creation.12 This procedure uses ultrafast lasers (all-laser LASIK) that precisely photodisrupt tissue by using very short duration energy pulses. The precision of this technique has resulted in wide acceptance of the technology.
Conventional LASIK: Conventional LASIK does not use aberrometry (measurement of the imperfections in the optical system of the eye) when correcting the refractive error.13 Treatment is based upon using the refraction from responses obtained with a phoropter and recent eyeglass prescription. This corrects only low-order aberrations known as sphere and cylinder. The data are manually entered into the laser.
Wavefront-Guided LASIK: Wavefront aberrometry has redefined the diagnosis and treatment of refractive errors in the 21st century. This technology allows the capturing and measurement of the total optical system of the eye. Aberrometers using advanced polynomials are capable of measuring higher-order imperfections smaller than the wavelength of light and using this information to reduce imperfections produced by the application of the excimer laser.14
Studies have shown that precise wavefront-guided ablation minimizes postoperative higher-order aberration, resulting in better contrast sensitivity compared to conventional treatment.15 Patients theoretically have better nighttime vision and less chance of needing an enhancement. This has been promoted as high-definition LASIK utilizing “optical fingerprints” made possible by wavefront aberrometry.
Mechanical Keratome: In this procedure, an automated, precision gear-driven machine with an oscillating blade is used to make a thin (110-160 micron) superior hinged flap on the anterior surface of the cornea. This is done once the eye is immobilized with a low-pressure suction ring. Complications during translation of the keratome are uncommon but can be a major concern. Incomplete flaps are easily handled but prevent the application of the laser until a later time.
Femtosecond Laser (All-Laser LASIK): More recently, femtosecond laser technology (all-laser LASIK) has emerged as an alternative to mechanical flap creation.16 This method precisely photodisrupts tissue with short-duration energy pulses that cleave cornea tissue at a predetermined depth, forming bubbles of water and carbon dioxide at a plane that allows for a smooth interface once the flap is lifted.17 There appears to be no difference between femtosecond and microkeratome flap creation as far as visual outcomes. However, femtosecond flaps can be made thinner and their parameters more precise than conventional methods. There is also an expectation of greater safety to the patient than with mechanical devices.
Photorefractive Keratectomy (PRK): Laser refractive surgery procedures can either be performed by intrastromal ablation (LASIK) or by surface ablation, generally known as PRK. Both use the excimer laser.
Experimental studies evaluating the excimer laser with its UV light energy began in the early 1980s. In the mid 1980s, animal and human applications performed at Louisiana State University in New Orleans demonstrated its potential for correction of refractive errors. Many FDA clinical trials have resulted in the approval of the excimer laser to reduce or eliminate a wide spectrum of refractive errors.13
Surface applications (PRK) have gained a small resurgence in patients with thin corneas and in poor candidates for LASIK who may be at risk for ectasia (a bulging of the cornea). Management of postoperative pain and the application of the chemotherapeutic drug mitomycin C (MMC) to prevent haze and scarring have added to PRK’s safety and popularity.18-20
During the early postoperative period, patients may experience significant tearing, photophobia, blurred vision, and discomfort because of the central corneal abrasion. With the use of the bandage contact lens and nonsteroidal anti-inflammatory drug (NSAID) eye drops (e.g., diclo-fenac sodium 0.1%, ketorolac 0.4%), postoperative pain is usually mild to moderate; however, patients occasionally require systemic analgesia for more severe pain. The contact lens remains on the eye until the epithelial defect is healed (an average of 3-4 days). Antibiotic eye drop therapy (e.g., tobramycin or gatifloxacin 0.3%) is usually continued for 2 to 3 days after the defect has healed, and topical steroid eye drops (e.g., 1% prednisolone) may be continued for up to 3 months postoperatively.
Intraoperative complications related to the use of the microkeratome occur at a rate of less than 1% in experienced hands and usually result in an incomplete or partial primary cut.21-23 When the flap appears less than ideal, it should be replaced, and the procedure can be successfully repeated in 2 to 3 months.
Dry Eyes: There is a high incidence of the development of dry eyes after refractive surgery (up to 36.36%).24,25 This symptom may last up to 6 months after surgery or be permanent. A proper preoperative evaluation for dry eyes is necessary to help prevent this postoperatively. Pharmacologic management of dry eyes includes prescription and nonprescription medications (TABLE 2). Another therapy is punctal occlusion, where a collagen plug is placed in the natural drain of the eye.
Cornea Ectasia and Collagen Cross-Linking: Ectasia (protrusion) can follow LASIK when the cornea takes on an appearance similar to keratoconus. Visual acuity is reduced, and glasses or soft contact lenses can no longer improve vision. Ectasia is a serious complication with certain preoperative risk factors, including abnormal cornea topography and thin corneas.26
By combining a solution of riboflavin and UV light exposure, corneas that have suffered ectasia are being treated to strengthen their weakened status. The procedure is a strengthening of the cross-links of the collagen fibers to allow stability.27,28 Under topical anesthesia, the central area epithelium is removed. The riboflavin is topically applied for a period of minutes followed by irradiation with intense 365-nm UVA light for approximately 30 minutes. The process has been compared to snow blindness that occurs after UV exposures.
Presently, there is a multicenter FDA clinical trial analyzing the safety and efficacy of cross-linking for postoperative ectasia.29 Other clinical studies have shown that it may be just as effective to inject the riboflavin directly into the stroma without removing the epithelium.30 Cornea cross-linking may be helpful in treating keratoconus and in strengthening borderline corneas in patients who are interested in refractive cornea surgery but have suspect or slightly distorted corneas.
Over- and Undercorrections: Residual refractive errors present immediately following LASIK usually result from undercorrections.31-34 Regression can occur months later. Once the prescription stabilizes, retreatment can be performed.34
Glare, Halos, and Starbursts: Some patients complain of halos, starbursts, and a general reduction of qualitative vision under conditions of reduced illumination. These are usually temporary but can persist in a small number of cases. The use of larger optical zones and wavefront technology has reduced the incidence of these symptoms. Some surgeons believe there may be a relationship between the scotopic pupil diameter (size that pupils dilate to a dark room) and nighttime visual disturbances. Some clinicians use topical brimonidine for night vision disturbances.
Dislocated, Wrinkled Flaps: Flap displacement with folds or wrinkles usually occurs within the first 24 hours. These folds are easily fixed by relifting and repositioning of the flap, and respond to treatment days and even weeks after their occurrence.
Epithelial Ingrowth: Epithelial ingrowth manifests itself by small islands of epithelial cells that are seen beneath the flap.35 They usually are found in the midperiphery but may extend centrally and reduce visual acuity. If epithelial ingrowth is extensive and threatens vision, the flap should be lifted and the cells removed. This may need to be repeated with sutures placed in the flap to prevent recurrences. The incidence of ingrowth is reported to be increased following enhancement or follow-up surgery.23
Diffuse Lamellar Keratitis (DLK): This nonspecific intralamellar inflammation appears in the early postoperative period as a granular or sandlike appearance in the flap interface.23 The condition can occur in clusters, affecting several patients on a given surgery day. It is of unclear etiology, and most cases follow uncomplicated LASIK. In rare instances, DLK can result in stromal melting, haze, and loss of best-corrected vision. Management with topical and systemic steroids and irrigation of the stromal bed (material under the surface of the cornea) are usually effective in treating severe cases.
Enhancements are follow-up surgeries that are used to correct residual refractive errors (i.e., over- or under-corrections) or to suit the visual needs of a patient’s changing lifestyle (e.g., monovision correction, a technique that reduces the need for reading glasses or bifocals). The timing of retreatment should be based on a stable refraction.36
Options include recutting a flap, lifting the prior flap, or performing PRK on top of the previous LASIK. With increasing use of MMC for the prevention of haze and the avoidance of epithelial ingrowth, more surgeons are turning to PRK for enhancements.
LASIK surgery for the correction of all kinds of refractive errors offers patients a fast, safe, reproducible, and cost-effective alternative to glasses and contact lenses. Refinements in the delivery systems of the excimer laser and wavefront technology have greatly increased visual outcomes with this commonly performed procedure and have improved the quality of life for many people.37
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