Hal D. Balyeat

Results of the Prospective Evaluation of Radial Keratotomy (PERK) Study One Year After Surgery

The Prospective Evaluation of Radial Keratotomy (PERK) study is a nine-center, self-controlled clinical trial of a standardized technique of radial keratotomy in 435 patients who had physiologic myopia with a preoperative refraction between -2.00 and -8.00 diopters. The surgical technique consisted of eight incisions using a diamond micrometer knife with blade length determined by intraoperative ultrasonic pachymetry and the diameter of central clear zone determined by preoperative refraction. At one year after surgery, myopia was reduced in all eyes; 60% were within +/- 1.00 diopter of emmetropia; 30% were undercorrected and 10% were overcorrected by more than 1.00 diopter (range of refraction, -4.25 to +3.38 D). Uncorrected visual acuity was 20/40 or better in 78% of eyes. The operation was most effective in eyes with a refraction between -2.00 and -4.25 diopters. Thirteen percent of patients lost one or two Snellen lines of best corrected visual acuity. However, all but three eyes could be corrected to 20/20. Ten percent of patients increased astigmatism more than 1.00 diopter. Disabling glare was not detected with a clinical glare tester, but three patients reduced their driving at night because of glare. Between six months and one year, the refraction changed by greater than 0.50 diopters in 19% of eyes.

Preliminary Results and Complications of Radial Keratotomy

We preformed radial keratotomy for myopia on 126 eyes of 102 patients. We observed as much as 12 diopters of early postoperative refractive corneal flattening, but a mean of only 5.16 diopters of refractive change can be expected after one year. Among the many complications observed were corneal scarring, an endothelial cell loss of 220 cells/mm2 (6.9%), epithelial ingrowth in 18 eyes (14%), corneal vascularization in one of 126 patients (1%), pain, Cogans corneal dystrophy, overcorrections in 18 patients (14%), anisometropia, perforations of the anterior chamber in 12 patients (10%), iatrogenic astigmatism, corticosteroid glaucoma, night glare in 99 patients (79%), and fluctuating vision in 95 (75%). We studied the following factors to determine if they had predictive value: the patients age, diopters of myopia, corneal diameter, corneal thickness, anterior chamber depth, axial length, scleral rigidity, keratometric readings, and the size of the optical zone. We found that no preoperative factor can be used to estimate precisely the postoperative result. The only consistent variable was the size of the optical zone. The smaller the optical zone, the larger the refractive change. This lack of predictability is the most serious shortcoming of radial keratotomy.

Three-year Results of the Prospective Evaluation of Radial Keratotomy (PERK) Study

The Prospective Evaluation of Radial Keratotomy (PERK) study is a nine-center clinical trial of a standardized technique of radial keratotomy in 435 patients who had simple myopia with a preoperative refractive error between -2.00 and -8.00 diopters (D). We report results for one eye of each patient. The surgical technique consisted of eight incisions using a diamond micrometer knife with the blade length determined by intraoperative ultrasonic pachymetry and the diameter of the central clear zone determined by the preoperative refractive error. At three years after surgery, 58% of eyes had refractive error within one diopter of emmetropia; 26% were undercorrected, and 16% were overcorrected by more than one diopter. Uncorrected visual acuity was 20/40 or better in 76% of eyes. The operation was more effective in eyes with a preoperative refractive error between -2.00 and -4.37 diopters. Between one and three years after surgery, the refractive error changed by 1.00 diopter or more in 12% of eyes, indicating a lack of stability in some eyes.

Radial Keratotomy: Preliminary Report of Complications

We have observed the following complications with radial keratotomy which require further evaluation: missing the visual axis while marking the central cornea, regression of myopic flattening, epithelial defects, recurrent erosions, stromal overgrowth, Cogans map-dot fingerprint corneal dystrophy with Moncreiff iron lines, blood in the incisions, vascular ingrowth, perforation of the anterior chamber, induction of astigmatic errors, epithelial ingrowth, glare complaints and decreased night vision, pain, fluctuating vision from morning to evening, overcorrection and unpredictable results, contact lens refitting difficulties, endothelial cell loss, and corneal scarring. We feel these complications require extensive further study before the relative safety and long-term efficacy of this experimental procedure may be determined.

Prospective Evaluation of Radial Keratotomy: Photokeratoscope Corneal Topography

Preoperative and postoperative corneascope photographs of 368 myopic patients undergoing radial keratotomy in the Prospective Evaluation of Radial Keratotomy (PERK) study were optically scanned and digitized. A high-resolution scanning system was developed in order to quantify the preoperative and postoperative corneal shape accurately. Careful analysis of the 72 data points in the nine representative rings demonstrated that corneal topography is best represented by radius of curvature from the center to the periphery. The normal myopic cornea flattens approximately +0.28 mm from the center to the periphery, demonstrating the corneas aspheric nature. More highly myopic patients in the PERK population (-4.50 to -8.00 diopters [D]) demonstrated corneas that are 0.08 to 0.10 mm steeper than the less myopic population (-2.00 to -3.12 D). Optical zone, patient age, and gender are all correlated to changes in corneal topography after radial keratotomy. In more myopic populations, men have corneas which are flatter than those of women by 0.09 to 0.11 mm in all rings represented on corneoscopy. Highly myopic males also experience more corneal flattening after 3.0-mm optical zone radial keratotomy. Regardless of the optical zone used in radial keratotomy, the resulting corneal topography flattens in all rings. However, the ratio of millimeters of radius of curvature change to diopters of correction is consistent for each ring. The dioptric change observed after radial keratotomy corresponds closely with the millimeters of flattening at the respective rings being examined. The central rings flatten 0.166-mm radius of curvature per diopter of refractive alteration obtained. The largest degree of corneal flattening occurs centrally, 0.72 mm, in the more highly myopic patients who underwent 3-mm optical zone radial keratotomy. The use of smaller optical zones in radial keratotomy produces larger changes in the radius of curvature and, consequently, in the amount of refraction than when larger optical zones are used. When compared with younger patients, older patients with 3.0, 3.5, and 4.0 optical zone radial keratotomies experience more central and peripheral corneal flattening. This study of the corneal topography of the myopic population demonstrates that the refractive change resulting from radial keratotomy is related to alterations in corneal topography. The use of similar modifications of the corneal surface may be effective for newer refractive surgical procedures.

Predicting the Results of Radial Keratotomy

Radial keratotomy on 251 patient eyes was performed. Patients traversing the one-year gate after surgery have demonstrated a refractive change best predicted by the optical zone size, the number of incisions, the corneal dissymmetry, and the age of the patient. One year after surgery, a 16-incision radial keratotomy with 3.0-mm optical zone produced 4.66 diopters of refractive change if the limbus was traversed (N = 13), and 5.18 diopters of refractive change if all incisions remained in clear cornea (N = 39). An eight-incision, 3.0-mm optical zone radial keratotomy produced 5.21 diopters of refractive change at one year (N = 37). A 3.5-mm optical zone, eight-incision radial keratotomy produced 3.66 diopters of change at one year (N = 6). A 4.0-mm optical zone produced 2.67 diopters of change with 16 incisions (N = 8), and 3.45 diopters of change with eight incisions at one year (N = 9). Although older patients appear to have significantly greater refractive change with radial keratotomy at the three-month gate, this difference is not yet confirmed at one year after surgery. The shape of the cornea before surgery contributes to the predictability of the refractive results when the horizontal and vertical keratometer measurements are analyzed. Corneal dissymmetry produces greater refractive results with steeper horizontal K when against-the-rule astigmatism is present before surgery.

Nylon Suture Toxicity after Cataract Surge

A cluster of symptoms and signs that appear to be related to wound closure developed in 10 of 105 consecutive patients (9.5%) who underwent uncomplicated planned extracapsular cataract extraction (ECCE) with posterior chamber intraocular lens (PC IOL) implants. These signs and symptoms included foreign body sensation, conjunctival injection and infiltrates localized to the scleral wound, and scleral excavation underlying the running 10-0 nylon suture possibly resulting from localized scleral edema. The clinical presentation ranged from 1 to 6 weeks. Conjunctival stains demonstrated eosinophils and polymorphonuclear leukocytes in some cases. Gram stains, conjunctival cultures, and results of suture toxicology studies were negative. The authors believe that these findings represent a previously unreported complication after ECCE: acute inflammation primarily localized to the conjunctiva and sclera adjacent to the cataract wound. The etiology of this postoperative complication is yet to be determined.

Stability of refraction and visual acuity during 5 years in eyes with simple myopia

BACKGROUND Normal ranges of variability of refraction and visual acuity in adult myopic eyes are needed as a reference standard for assessing the stability of refractive corneal surgery. METHODS We measured the changes in spectacle-corrected visual acuity and cycloplegic refraction during 5 years for the unoperated eye of 82 patients aged 21 to 57 years in the Prospective Evaluation of Radial Keratotomy Study. The changes were compared for contact lens and non-contact lens wearers. We also compared the 5-year cycloplegic and manifest refractions for these unoperated eyes. RESULTS Of 77 eyes, 44% gained or lost one Snellen line and 48% experienced no change in spectacle-corrected visual acuity between baseline and 5 years. Only one eye (1%) lost two lines, and 7% gained two lines. The refractive change was less than 1.00 D for 84% of the 37 non-contact lens wearing eyes. Only 13% became more myopic by at least 1.00 D (maximum increase in myopia, 2.00 D), and 3% became less myopic by 1.00 D. Of 45 contact lens wearing eyes, 38% became more myopic by at least 1.00 D. The 5-year manifest refraction was 0.50 D to 1.50 D more myopic than the cycloplegic refraction for 37% of eyes. CONCLUSIONS We recommend using two or more Snellen lines as the standard for a meaningful change in spectacle-corrected visual acuity in operated eyes, and 1.00 D as a meaningful cutoff for stability of refraction. The wearing of contact lenses can confound the results of stability studies. The difference between the cycloplegic and manifest refractions suggests that the cycloplegic refraction should be used in planning for refractive surgery.

Corneal topography as a predictor of refractive change in the prospective evaluation of radial keratotomy (PERK) study.

The first operated eyes of 435 patients undergoing radial keratotomy in the Prospective Evaluation of Radial Keratotomy (PERK) study were evaluated by photokeratography to document the preoperative and postoperative corneal shape. We determined by regression analysis and analysis of variance that the corneal shape preoperatively improved the prediction of the corneal shape 6 months postoperatively in the 3-mm-clear-zone population. The reduction of myopia in all 435 eyes ranged from 1.25 to 9.75 diopters. We studied the effect of the preoperative corneal shape on this variability in the outcome of the surgery using rings 2 and 7 on photokeratography and corneal diameter. In the 3-mm-clear-zone group, eyes with flat prolate corneas had a greater reduction in myopia (4.65 D); those with steeper, more spherical corneas had less reduction in myopia (3.48 D). In addition, eyes with a 3-mm clear zone and flat central corneas alone (8.0 mm = 42.19 D) flattened approximately 0.75 D more than those with steep central corneas (7.0 mm = 48.21 D). In the 3.5-mm and 4.0-mm clear zone groups, the change in corneal curvature was not related to the preoperative curvature. A stepwise regression analysis of the 151 eyes in the 3.0-mm-clear-zone population demonstrated the following predictive equation for radial keratotomy; change in cycloplegic refraction = -14.55 + [-2.097 x average ring-2 radius] + [3.605 x average ring-7 radius] + [0.69 x horizontal corneal diameter] + [0.079 x age] + [-0.379 x spherical equivalent cycloplegic refraction]. There was a 1.17-D observed difference in the effect of radial keratotomy between those eyes with a steep/steep corneal topography (7.2% of the 3.0-mm-clear-zone population) and the flat/flat topography (29% of the 3.0-mm-clear-zone PERK population). A knowledge of corneal topography provides an additional tool for understanding the operative variability of radial keratotomy.

Accuracy and reproducibility of KeraScanner analysis in PERK corneal topography

The measurement of the corneal radius of curvature centrally and in the corneal periphery is exceedingly difficult because of variables in photography and data acquisition. We present a technique of Automated image scanning of corneal photographs which provides a more accurate and reproducible analysis of the paracentral corneal contour. We analyzed the sources of error of corneal topography measurement. CorneaScope photographs of calibration balls were generated by each Prospective Evaluation of Radial Keratotomy (PERK) Surgical Center. The 90% confidence intervals for individual ring-radial positions on the 8.00 and 10.00 mm balls were 7.91 to 8.09 ram and 9.89 to 10.11 mm respectively. Our isopter method of analysis for clinical comparison averages all-eight radii of curvature for each CorneaScope (R) ring. This isopter technique reduces the corresponding 90% confidence intervals to 7.96 to 8.03 and 9.95 to 10.06 ram. The measurement variability of a 10 mm calibration ball decreased in the following or...