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* PURPOSE: The normal human cornea flattens peripherally. The amount of flattening, or asphericity, has traditionally been calculated from multiple keratometric measurements. We devised a mathematical technique for determining asphericity from computed corneal topography. We then determined whether asphericity affects the refractive outcome of radial keratotomy. * METHODS: One eye each of 41 patients who underwent four- or eight-incision radial keratotomy and preoperative computed corneal topography was identified retrospectively and analyzed. The asphericity, P, of each cornea was calculated by fitting Baker's equation ([y.sup.2] = [2r.sub.0]x - [Px.sub.2]) to each meridian of the topographic map. For each patient, we calculated the difference between the refractive outcome in diopters for radial keratotomy and the prediction of a quadratic least-squares best-fit model involving optical zone size and age. * RESULTS: Asphericity could be calculated from the topographic maps in all 41 patients and ranged from 0.33 to 1.28, with mean [+ or -] S.D. of 0.82 [+ or -] 0.21. Asphericity varied among the meridians of a cornea, with an average standard deviation among meridians of 0.17. No statistical correlation was found between calculated asphericity and refractive outcome. * CONCLUSIONS: Corneal asphericity can be calculated from corneal topographic maps. Asphericity is not constant in the different meridians of a normal cornea. Corneal asphericity is not useful in predicting the refractive outcome of radial keratotomy.

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.)

It has been reported that uncorrected farsightedness, or hypermetropia, (reduced ability to see objects that are close to the eyes) can cause esotropia, the inward turning of one eye relative to the other. This may be due to excessive accommodation, the process of adjustment of the eye to variations in distance. The authors had observed patients with uncorrected farsightedness who made a great effort of accommodation to obtain 20/20 vision, yet had little or no esotropia. A study was undertaken of nine children with no history of esotropia or any kind of strabismus (crossed eyes), no previous correction of farsightedness, no anisometropia (difference of refractive powers of the eyes), and minimal or no esotropia. There was a control group of 30 children with esotropia corrected by wearing glasses. These two groups were compared for clinical characteristics. In addition to the usual eye examination, information was gathered on stimulus accommodation convergence/accommodation (AC/A) ratio, near point of accommodation, and stereoacuity (accuracy of three-dimensional vision). The farsighted patients without esotropia had a low AC/C ratio compared with the patients who had esotropia corrected by glasses. This finding explains why esotropic deviations may not be found in some farsighted patients without vision correction. Both groups of patients showed defective or absent three-dimensional vision. It is recommended that optical correction be performed in children under the age of four even when there is no evidence of esotropia. (Consumer Summary produced by Reliance Medical Information, Inc.)

The cause of myopia, or nearsightedness, is unknown. A relationship between the development of myopia and a lack of patterned visual stimulation in infants has been demonstrated. It is suggested that the lack of a clear image on the retina deregulates the ocular growth during development, and this leads to myopia. Eleven children were studied who had sustained a hemorrhage into the vitreous of the eye. This vitreous hemorrhage causes opacification (blockage of light) of the media of the eye. In 10 of the 11 cases, the posterior pole of the eye was obscured and the reception of visual images on the retina were compromised. Seven of the children developed the vitreous hemorrhage before the age of one, and six of these patients developed myopia in the affected eye. The child who did not develop myopia did not have a vitreous hemorrhage which obscured the posterior pole of the eye. The four children over the age of two and a half did not develop myopia. The degree of myopia correlated with the age of onset and the length of time that the media was opacified. It appears that two and a half years is the age after which vitreous hemorrhage does not cause myopia. The incidence of myopia is also associated with premature birth or low birth weight. Once vitreous hemorrhage is identified, the child should be followed closely, and appropriate corrective eye glasses and patching therapy is recommended. (Consumer Summary produced by Reliance Medical Information, Inc.)

As patients who have undergone radial keratotomy (several radial incisions into the cornea) grow older, more of them will undergo cataract surgery. It is not known how their corneas will respond to additional surgery. All patients have errors in refraction, a type of visual impairment, following keratotomy, usually immediately after surgery, but there is partial improvement after about three months. In some patients improvement may not occur for up to five years; corneal healing from keratotomy may require several years. Four cases describe refractive complications following cataract surgery in patients who had undergone radial keratotomy several years earlier. These patients had cataract surgery with insertion of a posterior chamber intraocular lens. All patients had some initial hyperopia (farsightedness) caused by flattening of the cornea. This condition partially regressed and an average of 0.42 diopter of flattening persisted (remained mildly farsighted). The amount of flattening of the cornea decreased as the interval between keratotomy and cataract surgery increased. After keratotomy the topography of the corneal curvature is changed, with the center being flatter than the edges. It was observed that measurements of the curvature of the cornea, necessary for selection of the proper intraocular lens (to be inserted during cataract surgery), were less accurate when measured with a keratometer. A new system of measurement is recommended.