Topographic Analysis and Visual Acuity After Radial Keratotomy

Topographic Analysis and Visual Acuity After Radial Keratotomy

Topographie Analysis and Visual Acuity After Radial Keratotomy Peter J. M c D o n n e l l , M . D . , Jenny Garbus, B.A., and Pedro F. Lopez, M . D . ...

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Topographie Analysis and Visual Acuity After Radial Keratotomy Peter J. M c D o n n e l l , M . D . , Jenny Garbus, B.A., and Pedro F. Lopez, M . D .

Using a computerized corneal topographic mapping system that allows detailed examina­ tion of the corneal curvature in the vicinity of the visual axis, we identified separate regions of differing corneal curvature in three of 11 eyes after radial keratotomy. In these eyes, the excellent uncorrected visual acuity appeared to be inconsistent with the postoperative spherical equivalent as determined by refrac­ tion and corneal curvature by standard keratometer measurements. The distinct regions of corneal curvature appeared to serve as alterna­ tive effective optical zones, thus allowing the patients to have excellent visual acuity. In essence, the cornea became a multifocal lens. Although degradation in the contrast of the image as well as monocular diplopia are possi­ ble, our patients had no significant com­ plaints. QUANTITATIVE ANALYSES of the results of radial keratotomy have documented the lack of direct correlation between the changes in visual acuity, cycloplegic refraction, and keratometric measurement. After radial keratotomy, some individuals may have uncorrected visual acuities that appear to be better than would be predicted based on the change in spherical equivalent, as determined by cycloplegic refraction, and the change in corneal curvature, as determined by keratometry. 1 Herein we describe some of the topographic features of corneas that have undergone radial keratotomy that may help explain the apparent lack of correlation between the parameters of visual

Accepted for publication Sept. 6, 1988. From the Estelle Doheny Eye Institute and the University of Southern California School of Medicine, Los Angeles (Dr. McDonnell and Ms. Garbus) and the Wilmer Ophthalmological Institute, Baltimore (Dr. Lopez). Reprint requests to Peter J. McDonnell, M.D., Estelle Doheny Eye Institute, 1355 San Pablo St., Los Angeles, CA 90033.

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acuity, spherical equivalent, and keratometric change after surgery.

Patients and Methods We examined 11 eyes of six patients after radial keratotomy using the Corneal Modeling System (Computerized Anatomy, Inc., New York, New York).2 This computerized corneal topographic analysis system uses a large number of illuminated rings that are reflected by the cornea to span most of the surface of the cornea from the corneal apex to the corneoscleral Iimbus. The rings are then projected using a digital video system; ring images are digitized and curves fit to describe the shape of the entire corneal surface. This information is then translated into a color-coded topographic map to aid in the visualization of regional changes in corneal curvature. A cursor may then be moved over this map to determine the approximate corneal power at any point. Based on examination of calibrated steel balls, this device has been shown to be as accurate and reproducible as the keratometer. 3 The refractive error, keratometric values, and visual acuities were stable in each patient, and no patient was receiving any ocular medications. Noncycloplegic manifest refractions, retinoscopy, and distance and near visual acuities were recorded. All refractions were performed in the same room using the modified BaileyLovie charts used in the Early Treatment of Diabetic Retinopathy Study. 4 To inhibit accommodation, patients were fogged (by using excessive plus sphere) until the endpoint of the refraction was reached.

Results All 11 eyes examined had fairly round zones of corneal flattening, which were roughly cen-

© A M E R I C A N JOURNAL OF OPHTHALMOLOGY 106:692-695, DECEMBER,

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tered around the corneal apex. These optical zones averaged 6.2 mm in diameter (range, about 4.8 to 7.1 mm). In those eyes in which good visual acuity correlated with almost complete reduction of refractive error, as demonstrated by change in spherical equivalent and keratometric readings, a fairly uniform round zone of flattening was identified (eight eyes). We termed this a single effective optical zone. In those eyes in which marked improvement in visual acuity was obtained, despite the refraction and keratometry indicating substantial residual refractive errors (three eyes), we identified zones of relatively greater flattening within the larger circular area of flattening. These small, effective optical zones appeared to represent areas of the cornea that the patient used to obtain a clear retinal image of distant objects. An illustrative case is that of a 54-year-old myopic man who had worn soft contact lenses for 14 years. For the last two years he had had signs and symptoms of giant papillary conjunctivitis that had not improved with changing lenses and solutions. The patient's uncorrected visual acuity in the right eye was 7/200, correctable to 20/15 with -6.75 -0.75 x 88. Near vision with correction was Jaeger 1. Uncorrected visual acuity in the left eye was 9/200, correctable to 20/15 with -6.00 -0.75 x 120.

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Near vision with correction was Jaeger 1. Keratometry in the right eye was 45.00/45.62 diopters at 70 degrees, for an average keratometry reading of 45.31 diopters. Keratometry in the left eye was 45.37/45.62 diopters at 76 degrees, for an average keratometry reading of 44.50 diopters. Central corneal thickness was R.E.: 0.54 mm and L.E.: 0.51 mm. Opnthalmoscopy showed tilted disks, compatible with the patient's myopia. Corneal topographic imaging of the left cornea demonstrated progressive flattening from the center to the periphery and a central curvature of about 46.3 diopters (Fig. 1). The patient underwent an eight-incision radial keratotomy using a 3.5-mm optical zone in the severely myopic left eye. His uncorrected visual acuity stabilized at 20/25 three months after surgery, and best-corrected visual acuity was -2.25 -0.50 x 90 = 20/15. Uncorrected near vision was Jaeger 1. The uncorrected visual acuity of 20/25 in the left eye, combined with an unaided near vision of Jaeger 1, was surprising given the spherical equivalent of -2.50. However, repeat computerized topographic analysis of the cornea - (Fig. 2) demonstrated an apparent optical zone just inferior to the corneal apex. In this area, which measured 2.1 mm vertically by about 3.8 mm horizontally, the average corneal power was

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Fig. 1 (McDonnell, Garbus, and Lopez). Left, Color-coded topographic map of left cornea before radial keratotomy. Central corneal power is about 46.3 diopters. Right, Diopter power shown in color-coded scale.

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Fig. 2 (McDonnell, Garbus, and Lopez). Left, Color-coded topographic map of left cornea. Cursor within small optical zone with average corneal power of about 39.6 diopters. This optical zone measures 2.1 x 3.8 mm. Right, Larger, roughly spherical optical zone (diameter about 6.0 mm) encompasses smaller zone. Cursor is within this zone and corneal power is 41.5 diopters. Color-coded scale same as in Figure 1.

about 39.6 diopters (Fig. 2, left), representing a decrease of about 6.7 diopters in an eye with a preoperative central curvature of about 46.3 diopters. This was in close agreement with the preoperative spherical equivalent of - 6 . 3 7 diopters, indicating that the patient was effectively piano through this inferior optical zone. This smaller optical zone was encompassed within a larger, roughly spherical, optical zone with a diameter of about 6 mm (Fig. 2, right). This larger optical zone was steeper, with an average corneal power in the center and surrounding the center of the visual axis of about 41.5 diopters. This corresponded to a decrease in corneal curvature of about 4.8 diopters from the preoperative value, and would appear to explain a spherical equivalent of —2.50 as measured postoperatively. When retinoscopy was performed after this topographic analysis, there were two possible endpoints for retinoscopy, one of about —2.50 just above the corneal apex and a separate zone with an endpoint of approximately plano just inferior to the corneal apex. The presence of two different optical zones in the left eye raised the possibility that this patient might experience monocular diplopia. This patient did not spontaneously complain of this problem, but when tested he did report a faint ghost image for letters both at distance and near with the left eye. No such ghost images were detected with the unoperated on right eye.

Discussion Patients with a residual myopic refractive error after radial keratotomy may have better visual acuity than do individuals with a similar refractive error who have not had surgery. Santos and associates 1 and Waring and associates 5 attributed the greater change in spherical equivalent compared to the change in keratometry to induction of greater asphericity in the cornea. These authors suggested that patients who have undergone radial keratotomy may have better visual acuity than expected based on their refractive error because of the increased spherical aberration of the paracentral cornea, which produces a larger blur circle on the retina. 1 Our explanation of the surprisingly good visual acuity obtained in some patients after radial keratotomy is based on results obtained from a device that allows quantitative evaluation of the corneal curvature of not only the midperipheral but also the paracentral cornea. The Corneal Modeling System has a large number of rings that traverse almost the entire cornea, allowing a much greater percentage of the corneal area to be examined topographically.2 As illustrated in one eye of a patient who underwent radial keratotomy for a preoperative spherical equivalent of —6.37 diopters, an effective optical zone was identifiable just inferior to the corneal apex in which there had been

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a 6.7-diopter change in central corneal power postoperatively. The spherical equivalent of -2.50 could also be explained by the presence of an effective optical zone involving the center and the area superior to the corneal apex, in which the average keratotomy reading had changed about 4.8 diopters. Although the patient described herein did not volunteer that the quality of vision in his left eye was less than that in his right eye, he did report, when questioned in the examining room, that he had a faint ghost image for letters at both distance and near with his left eye (bifocal cornea) and that this ghost image was definitely not present when those letters were examined with the right eye. In essence, this result of surgery set the stage for degradation in the contrast of the image as well as the development of monocular diplopia. Binder 6 reported complaints of monocular double vision, ghost images, or degradation of the visual image as complications of refractive surgery. He hypothesized that these occurred when radial keratotomy incisions were extended into the area of the cornea overlying the pupil. He also noted that these problems were a result of decentration of the corneal apex in keratomileusis or decentration of the lenticule in keratophakia. We believe that the ghost image observed by our patient probably represented the blurred image produced by the separate effective optical zones of his bifocal cornea. Although the ghost image was a fairly subtle finding that did not disturb our patient, the development of a multifocal cornea may cause more serious problems in other patients. We interpret the topographic changes noted in some patients to be analogous to some concentric bifocal contact lenses, an analogy first suggested by Santos and associates. 1 With bifocal contact lenses, some visual compromise is inevitable because of image degradation from the other optical zone with decrease in contrast of the image. Most patients are able to detect monocular diplopia, probably because of a prismatic effect at the junction of the two optical

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zones. 7 These findings are similar to those reported by our three patients with multifocal corneas. With the development of tools for examining corneal topography in greater detail, further understanding will be gained of the topographic alterations induced in the corneas as a result of refractive surgery. It may then be possible to intervene surgically in patients with undesired effective optical zone changes, such as a multifocal cornea, in order to produce a single, relatively large optical zone with corresponding improvement in the quality of the retinal image.

References 1. Santos, V. R., Waring, G. O., Ill, Lynn, M. J., Holladay, J. T., Sperduto, R. D., and the PERK study group: Relationship between refractive error and visual acuity in the prospective evaluation of radial keratotomy (PERK) study. Arch. Ophthalmol. 105:86, 1987. 2. Gormley, D. J., Gersten, M., Kopiin, R. S., and Lubkin, V.: Corneal modeling. Cornea 7:30, 1988. 3. Hannush, S. B., and Waring, G. O.: Computer assisted corneal topography. Accuracy and reproducibility with three instruments. ARVO Abstracts. Supplement to Invest. Ophthalmol. Vis. Sci. Philadelphia, J. B. Lippincott, 1988, p. 389. 4. Bailey, I. L., and Lovie, J. E.: New design principles for visual acuity letter charts. Am. J. Optom. Physiol. Opt. 53:740, 1976. 5. Waring, G. O., Ill, Lynn, M. J., Culbertson, W., Laibson, P. R., Lindstrom, R. D., McDonald, M. B., Myers, W. D., Obstbaum, S. A., Rowsey, J. J., Schanzlin, D. J., and the PERK study group: Threeyear results of the prospective evaluation of radial keratotomy (PERK) study. Ophthalmology 94:1339, 1987. 6. Binder, P. S.: Optical problems following refractive surgery. Ophthalmology 93:739, 1986. 7. Erickson, P., and Robboy, M.: Performance characteristics of a hydrophilic concentric bifocal contact lens. Am. J. Optom. Physiol. Opt. 62:702, 1985.