Stability of refraction during four years after radial keratotomy in the Prospective Evaluation of Radial Keratotomy Study

In Western countries, about 25 percent of adults are nearsighted (myopic). Myopia is caused by an error of refraction in which parallel rays of light passing through the lens of the eye are focused in front of, rather than on, the retina. Myopia is usually corrected by prescription glasses or contact lenses. However, surgical techniques have been developed that may permanently correct myopia by changing the shape of the cornea, the transparent tunic at the front of the iris and lens through which light is refracted. The most commonly used procedure of this type is radial keratotomy. Studies of the long-term effects of surgery have shown a continued increase in the effect of the procedure causing progressive hyperopia (farsightedness). This appears to occur between one to four years after surgery, and amounts to 1.00 or more diopters (D). (A diopter is unit of measure of the refractive power of a lens; a mild to moderate myopia is considered to be between -2.00 and -8.00 diopters.) As part of the Prospective Evaluation of Radial Keratotomy Study, an evaluation was made of the stability of refractive error over a four-year period after radial keratotomy for 341 operated eyes. Compared with preoperative refractions, refraction performed two weeks after surgery showed all eyes to be less myopic; the average change was a decrease of 4.83 D. A loss of 1.00 D or more from the initial effect occurred in 161 eyes (59 percent) between 2 weeks and three months after surgery. Between three months and six months, 95 percent of the eyes maintained stable refraction, and between six months and four years after surgery, 246 eyes (72 percent) sustained a change of less than 1.00 D. Patients with greater preoperative myopia and those having a smaller diameter of the central clear zone were more likely to have an increased effect of surgery. The duration of the increasing effect following surgery is not yet known. (Consumer Summary produced by Reliance Medical Information, Inc.)