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Corneal Surgery for Correction of Refractive Errors
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Am Fam Physician. 2000 Jun 15;61(12):3703-3704.
All of the currently available refractive corneal surgical procedures reshape the cornea to redirect light rays on the retina. Whether myopia, hyperopia or astigmatism is present, surgical correction attempts to redirect light rays to accommodate the refractive error. Diseases of the eye such as retinal detachment and angle-closure glaucoma are unaffected by the surgery. Consultants from the Medical Letter on Drugs and Therapeutics reviewed the statistics on refractive corneal surgery, including efficacy and adverse effects.
The laser in-situ keratomileusis (LASIK) procedure uses an excimer laser to make a thin flap in the cornea and etch away a predetermined pattern of tissue beneath the flap. The flap is then repositioned without suturing. The surgery is usually done in both eyes in a single procedure. Following the procedure, most patients experience minimal discomfort and recover vision within 48 hours. Five to 30 percent of patients require a repeat procedure within three to six months of the first surgery. Up to 60 percent of myopic patients achieve 20/20 uncorrected vision, while more than 85 percent achieve 20/40 uncorrected vision, which is legally acceptable for driving. Although about 5 percent of patients lose two or more lines of best spectacle-corrected vision, less than 0.5 percent are left with best spectacle-corrected vision of less than 20/40. Some patients still need reading glasses as they age. Less than 1 percent of patients require flap revision. The risk of a vision-threatening infection is one to five per 10,000 procedures. Dry eyes and night vision problems may continue long-term.
Photorefractive keratectomy (PRK) is similar to LASIK but does not involve use of a corneal incision or a flap. The laser ablates the corneal epithelium, and the defect heals by re-epithelialization. Overall results are similar to those of LASIK; however, considerable postoperative discomfort may occur. Other disadvantages include prolonged recovery of vision, difficulty performing the corrective procedures and central haziness. As with LASIK, glare and halos around lights can be problematic. PRK is usually done separately on the two eyes, with an interval of four to six weeks between procedures.
Intracorneal rings are implanted into the peripheral cornea to flatten the anterior curvature of the central cornea and correct low degrees of myopia. The rings are made of a polymethylmethacrylate (PMMA). About 66 percent of patients achieve 20/20 vision or better. An advantage of the intracorneal rings is that they can be removed and the eye can be restored to its preoperative refractive error. A different size ring can be inserted, making the procedure adjustable to changing refraction. The risk of central corneal scarring that can occur following the other refractive surgeries is generally eliminated in this case. Common complications include mild postoperative pain and occasional problems with night glare. Perforation of the anterior chamber has been reported, as well as infection and astigmatism, requiring removal of the rings.
Phakic intraocular lenses that could be inserted without removing the natural lens are under investigation for correction of high myopia and hyperopia. Radial keratotomy, which involves making deep incisions in the cornea, is rarely performed today.
The consultants conclude that LASIK achieves 20/20 vision or better in about 60 percent of patients with less-than-severe myopia, and more than 90 percent of these persons do not need a further corrective lens. Results are less predictable in patients with astigmatism or high levels of myopia. The results of PRK are similar to those of LASIK, but there is more postoperative discomfort in the former method. Synthetic intracorneal rings can be adjusted as the refraction changes. The long-term effects of the vision-corrective surgeries is unknown. Because none of the surgeries corrects loss of accommodation, reading glasses will probably be needed as aging occurs.
Medical Letter Consultants. Corneal surgery for correction of refractive errors. Med Lett Drugs Ther. December 17, 1999;41(1068):122–3.
Copyright © 2000 by the American Academy of Family Physicians.
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