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Prospective Evaluation of Radial Keratotomy (PERK) Study

Purpose | Background | Description | Patient Eligibility | Recruitment Status | Current Status | Results | Publications | Resource Centers | NEI Representative

Purpose

Background

Approximately 11 million Americans have myopia that can be corrected with eyeglasses or contact lenses. Some of these people may also be candidates for radial keratotomy (RK), a procedure that aims to correct or reduce myopia by surgery that flattens the corneal curvature.

Keratotomy was first performed by surgeons in Europe and the United States in the late 1800s, and the basic optical and mechanical principles of the operation were defined in the 1940s and 1950s by the Japanese doctors T. Sato and K. Akiyama, who used anterior and posterior corneal incisions. The posterior incisions damaged the cornea, and the procedure was modified in the Soviet Union by doctors Fyodorov and V. Durnev to include incisions in only the anterior cornea. Since its introduction into the United States in 1978, numerous ophthalmologists have modified the procedure by introducing technical and surgical improvements such as ultrasonic methods to measure the thickness of the cornea and the use of diamond-bladed micrometer knives to make the incisions.

However, scientific assessment of RK lagged behind growing public and professional interest in the procedure. In 1980, in response to widespread concern about the long-term safety and efficacy of RK, a group of ophthalmic surgeons approached the National Eye Institute with a proposal for a multicenter clinical trial that would evaluate the potential benefits and risks of this procedure.

Description

The Prospective Evaluation of Radial Keratotomy study, involving 435 patients and 99 pilot patients, was a clinical trial designed to evaluate the short- and long-term safety and efficacy of one technique of radial keratotomy. The procedure was evaluated by comparing a patient's refractive error and uncorrected vision before and after surgery. The more myopic eye received surgery first. Patients were required to wait 1 year before having the operation on the second eye.

The surgical technique was standardized, consisting of eight centrifugal radial incisions made manually with a diamond micrometer knife. The diameter of the central, uncut, clear zone was determined by the preoperative spherical equivalent cycloplegic refraction (-2.00 to -3.12 D = 4.0 mm; -3.25 to -4.3 D = 3.5 mm; -4.50 to -8.00 D = 3.0 mm). The blade length, which determined the depth of the incision, was set at 100 percent of the thinnest of four intraoperative ultrasonic corneal thickness readings taken paracentrally at the 3-, 6-, 9-, and 12-o'clock meridians just outside the mark delineating the clear zone. The incisions were made from the edge of the trephine mark to the limbal vascular arcade and were spaced equidistantly around the cornea.

Patients were examined preoperatively and after surgery at 2 weeks, 3 months, 6 months, annually for 5 years, and at 10 years. Examinations in the morning and evening of the same day were done at 3 months, 1 year, 3 years, and 11 years in a subset of the patients to test for diurnal fluctuation of vision and refraction.

The primary outcome variables measured at each visit was the uncorrected and spectacle-corrected visual acuity and the refractive error with the pupil dilated and undilated. The corneal shape was measured with central keratometry and photokeratoscopy. Endothelial function was evaluated using specular microscopy. A slit-lamp microscope examination was made to check for complications from the incisions. Contrast sensitivity was tested in a subset of patients. Patient motivation and satisfaction were studied with psychometric questionnaires at baseline, 1 year, 5-6 years, and 10 years.

Patient Eligibility

All men and women had 2 to 8 diopters of simple myopia and were correctable to 20/20 or better with glasses or contact lenses. All patients had the stability of their myopia documented by previous records. Patients were at least 21 years of age and lived in the metropolitan area of the study centers. Each patient agreed to have surgery on one eye and to wait 1 year for surgery on the other eye. Patients with systemic diseases that might affect corneal wound healing and patients with high corneal astigmatism were excluded from the study.

Patient Recruitment Status

Recruitment began in April 1981 and was completed in October 1983.

Current Status of Study

Completed.

Results

The 10-year followup PERK study results confirmed that radial keratotomy reduced myopia but that the effectiveness of the outcome varied among patients. Of the 427 patients (793 eyes) that underwent radial keratotomy, 374 patients (88 percent; 693 eyes) returned for examination a decade after surgery. Of 675 eyes with refractive data, 38 percent had a refractive error within 0.5 D of emmetropia and 60 percent were within 1.00 D. Uncorrected visual acuity was 20/20 or better in 53 percent of 681 eyes and 20/40 or better in 85 percent. Among 310 patients with bilateral radial keratotomy, 61 percent reported not wearing spectacles or contact lenses for distance vision at 10 years after surgery.

These 10-year examinations indicated that the refractive error had not been stable in these eyes during the postoperative interval. For 310 first operated eyes, the mean refractive error was -0.36 D at 6 months after surgery, and this mean refractive error had changed to +0.51 D at 10 years, for a mean change in a hyperopic direction of +0.87 D between 6 months and 10 years after surgery. The average rate of change was 0.21 D per year between 6 months and 2 years, and +0.06 D per year between 2 and 10 years after surgery. Between 6 months and 10 years, the refractive error of 43 percent of eyes changed in the hyperopic direction by 1.00 D or more. The hyperopic shift was statistically associated with incision length, with smaller clear zone diameters, and larger overall cornea diameters being associated with a greater change in refraction.

Long-term followup revealed no blinding complications. Loss of spectacle-corrected visual acuity of two lines or more on a Snellen chart occurred in 3 percent of all 793 eyes that underwent surgery (Waring GO, Lynn MJ, McDonnell PJ, and the PERK Study Group, 1994).

Stability of Refraction During 11 Years in Eyes With Simple Myopia
This study was undertaken to document normal ranges of variability of refraction and visual acuity in adult myopic eyes over time that can serve as a reference standard for assessing the stability of refractive surgery. Changes in cycloplegic refraction and spectacle-corrected visual acuity were measured during 11 years for the unoperated eye of 62 patients in PERK. Changes were compared for contact lens and non-contact lens wearers.

Spectacle-corrected visual acuities were available for 58 eyes, of which 23 (40 percent) gained or lost one Snellen line and 32 (55 percent) experienced no change. The average change in refraction was an increase in myopia of 0.30 D for the 34 non-contact lens-wearing eyes and 1.12 D for the 28 contact lens wearing eyes. The refractive change was less than 1.00 D for 25 (74 percent) of the non-contact lens wearers, and 11 (39 percent) of the contact lens wearers. There was an increase in myopia of at least 1.00 D for 8 (24 percent) of the non-contact lens wearers, and 16 (57 percent) of the contact lens wearers, the difference being statistically significant (p = 0.007).

Two or more Snellen lines represents a meaningful change in spectacle-corrected visual acuity in operated eyes, and 1.00 D is a meaningful cutoff for stability of refraction. Wearing of contact lenses can confound the results of stability studies (Nizam A, Waring GO, Lynn MJ, and the PERK Study Group, 1996).

Clinical Significance
The goal of the PERK Study was to document the effectiveness, safety, stability of refraction, changes in corneal curvature, and patients' subjective response of a standard technique of radial keratotomy. The 10-year results indicate that the technique of radial keratotomy can effectively reduce but not completely eliminate the need for distance optical correction for myopia; 61 percent of patients reported not wearing spectacles or contact lenses for distance vision at 10 years after surgery. A shift of the refractive error in the hyperopic direction continued during the 10-year period, and this information is of key import to surgeons and potential refractive surgery candidates. The overall study concluded that radial keratotomy can significantly reduce myopia in the range of -2.00 to -8.00 D; that the predictability of refractive outcome was less than desired; that few patients lost spectacle-corrected visual acuity, indicating the operation is reasonably safe; and that patients' acceptance is extremely high, with the majority stating they would have the surgery again.

The study demonstrated that for patients to be free of distance optical correction, a refraction within 0.50 D of emmetropia or a visual acuity of 20/20 in at least one eye was necessary. High patient satisfaction was correlated with freedom from wearing distance spectacles or contact lenses and a visual acuity of 20/20 or better in at least one eye.

There are five reasons why residual myopia of -0.50 to -1.00 D is an advantage after radial keratotomy:

  1. It delays the onset of symptomatic presbyopia.
  2. It may offset the effect of a hyperopic shift.
  3. It allows repeated surgery to more closely approximate emmetropia. Surgery to reduce a hyperopic overcorrection is difficult.
  4. It allows reasonably good uncorrected visual acuity.
  5. It maintains a patient's lifelong habit of being myopic.
    The PERK study serves as a benchmark by which changes and advances in radial keratotomy and other types of refractive surgery can be measured and provides one of the few sources of information about the long-term stability of radial keratotomy.

Publications

Nizam A, Waring GO, Lynn MJ: Stability of refraction during 11 years in eyes with simple myopia. Invest Ophthalmol Vis Sci 37: S1004, 1996.

McDonnell PJ, Nizam A, Lynn MJ, Waring GO: Morning-to-evening change in refraction, corneal curvature, and visual acuity 11 years after radial keratotomy in the Prospective Evaluation of Radial Keratotomy Study. Ophthalmology 103: 233-239, 1995.

Waring GO, Lynn MJ, McDonnell PJ: Resultados del estudio de evaluacion Prospectiva de la Queratotomia Radial (EPQR) 10 anos despues de la cirugia. Arch Ophthalmol (Edicion Espanola) 6: 99-110, 1995.

Bourque LB, Lynn MJ, Waring GO, Cartwright C: Spectacle and contact lens wearing six years after radial keratotomy in the Prospective Evaluation of Keratotomy Study. Ophthalmology 101: 421-431, 1994.

Waring GO, Lynn MJ, McDonnell PJ: Results of the Prospective Evaluation of Radial Keratotomy (PERK) Study 10 years after surgery. Arch Ophthalmol 112: 1298-1308, 1994.

Nizam A, Waring GO, Lynn MJ, Ward M, Asbell PA, Balyeat H, Cohen E, Culbertson W, Doughman D, Fecko P, McDonald M, Smith R: Stability of refraction and visual acuity during 5 years in eyes with simple myopia. Refract Corneal Surg 8: 439-447, 1992.

Holladay JR, Lynn MJ, Waring GO, Gemmill M, Keehn GC, Fielding B: The relationship of visual acuity, refractive error, and pupil size after radial keratotomy. Arch Ophthalmol 109: 70-76, 1991.

Rowsey JJ, Waring GO, Monlux RD, Balyeat HD, Stevens S, Culbertson W, Barron B, Nelson D, Asbell PA, Smith R, Arentsen J, Cowden J: Corneal topography as a predictor of refractive change in the Prospective Evaluation of Radial Keratotomy (PERK) study. Ophthalmic Surgery 22: 370-380, 1991.

Waring GO, Lynn MJ, Nizam A, Kutner MH, Cowden JW, Culbertson W, laibson PR, McDonald MB, Nelson JD, Obstbaum SA, Rowsey JJ, Salz JJ: Results of the Prospective Evaluation of Radial Keratotomy (PERK) study five years after surgery. Ophthalmology 98: 1164-1176, 1991.

Waring GO, Lynn MJ, Strahlman ER, Kutner MH, Culbertson W, Laibson PR, Lindstrom RD, McDonald MB, Myers WD, Obstbaum SA, Rowsey JJ, Smith RE: Stability of refraction during four years after radial keratotomy in the Prospective Evaluation of Radial Keratotomy study. Am J Ophthalmol 111: 133-144, 1991.

Ginsburg AP, Waring GO, Steinberg EB, Williams PA, Justin N, Deitz JR, Roszka-Duggan VK, Baluvelt K, Bourque L: Contrast sensitivity under photopic conditions in the Prospective Evaluation of Radial Keratotomy (PERK) Study. Refract Corneal Surg 6: 82-91, 1990.

Lynn MJ, Waring GO, Carter JT: Combining refractive error and uncorrected visual acuity to assess the effectiveness of refractive corneal surgery. Refract Corneal Surg 6: 103-109, 1990.

Waring GO, Lynn MJ, Fielding B, Asbell PA, Balyeat HD, Cohen EA, Culberston W, Doughman DJ, Fecko P, McDonald MB, Smith RE, Wilson LB: Results of the Prospective Evaluation of Radial Keratotomy (PERK) study 4 years after surgery for myopia. JAMA 263: 1083-1091, 1990.

Lynn MJ, Waring GO, Nizam A, Kutner MH, Culbertson W, McDonald MB, Meyers WD, Naidoff MA, Nelson JD, Obstbaum SA, Rowsey JJ, Salz JJ: Symmetry of refractive and visual acuity outcome in the Prospective Evaluation of Radial Keratotomy (PERK) Study. Refract Corneal Surg 5: 75-81, 1989.

Rowsey JJ, Monlux R, Balyeat HD, Stevens SX, Gelender H, Holladay J, Krachmer JH, Laibson P, Lindstrom RL, Lynn MJ, mandelbaum S, McDonald MB: Accuracy and reproducibility of KeraScanner analysis in PERK corneal topography. Curr Eye Res 8: 661-674, 1989.

Cartwright C, Lynn M, Waring G, Steinberg E, Bourque L, Poloson D, Williams P, Whiteside J, Garbus J, Cantillo N, Duggan V, Justin N: Relationship of glare to uncorrected visual acuity and cycloplegic refraction one year after radial keratotomy in the Prospective Evaluation of Radial Keratotomy (PERK) Study. J Am Optometric Assoc 59: 36-39, 1988.

McDonnell PJ, Schanzlin DJ: Early changes in refractive error following radial keratotomy. Arch Ophthalmol 106: 212-214, 1988.

Rowsey JJ, Balyeat HD, Monlux R, Holladay J, Waring GO, Lynn MJ: Prospective Evaluation of Radial Keratotomy: Photokeratoscope Corneal Topography. Ophthalmology. Ophthalmology 95: 322-334, 1988.

Santos VR, Waring GO, Lynn MJ, Schanzlin DJ, Cantillo N, Espinal ME, Garbus J, Justin N, Roszka-Duggan V: Morning-to-evening change in refraction, corneal curvature, and visual acuity 2 to 4 years after radial keratotomy in the PERK study. Ophthalmology 95: 1487-1493, 1988.

Serle J, Asbell P, Obstbaum S, Podos S, Anh-Lee N: The evaluation of corneal endothelial permeability in PERK study patients. J Ophthalmol 72: 274-277, 1988.

Cowden JW, Lynn MJ, Waring GO, PERK Study Group: Repeated radial keratotomy in the Prospective Evaluation of Radial Keratotomy (PERK) Study. Am J Ophthalmol 103: 423-432, 1987.

Lynn MJ, Waring GO: Predictability and stability of radial keratotomy. J. Refract Surg 3: 193-196, 1987.

Lynn MJ, Waring GO, Sperduto RD, PERK Study Group: Factors affecting outcome and predictability of radial keratotomy in the PERK Study. Arch Ophthalmol 105: 42-51, 1987.

Santos VR, Waring GO, Lynn MJ, Holladay JT, Sperduto RD: Relationship between refractive error and visual acuity in the Prospective Evaluation of Radial Keratotomy (PERK) study. Arch Ophthalmol 105: 86-92, 1987.

Waring GO, Lynn MJ, Culbertson W, Laibson PR, Lindstrom RD, McDonald MB, Myers WD, Obstbaum SA, Rowsey JJ, Schanzlin DJ: Three-year results of the Prospective Evaluation of Radial Keratotomy (PERK) study. Ophthalmology 94: 1339-1354, 1987.

Atkin A, Asbell P, Justin N, Smith H: Radial keratotomy and glare effects on contrast sensitivity. Documenta Ophthalmologica 62: 129-148, 1986.

Bourque LB, Cosand BB, Drews C, Waring GO, Lynn M, Cartwright C, Prospective Evaluation of Radial Keratotomy (PERK) Study Group: Reported satisfaction, fluctuation of vision, and glare among patients one year after surgery in the Prospective Evaluation of Radial Keratotomy (PERK) study. Arch Ophthalmol 104: 356-363, 1986.

Bourque LB, Cosand BB, Drews C, Waring GO, Lynn M, Cartwright C: Satisfaccion informada, fluctuacion de la vision y resplandor en pacientes al cabo de un ano tras la cirugia en el estudio de la evaluacion prospectiva de la queratotomia radial (EPQR). Arch Ophthalmol 2: 57-64, 1986.

Mandelbaum S, Waring GO, Forster RK, Culbertson WW, Rowsey JJ, Espinal ME: Late development of ulcerative keratitis in radial keratotomy scars. Arch Ophthalmol 104: 1156-1160, 1986.

Schanzlin DJ, Santos VR, Waring GO, Lynn MJ, Bourque L, Cantillo N, Edwards MA, Justin N, Reinig J, Roszka V: Diurnal change in refraction, corneal curvature, visual acuity, and intraocular pressure after radial keratotomy in the PERK study. Ophthalmology 93: 167-175, 1986.

Villasenor RA, Santos VR, Cox CK, Harris DF, Lynn M, Waring GO: Comparison of ultrasonic corneal thickness measurements before and during surgery in the Prospective Evaluation of Radial Keratotomy (PERK) study. Ophthalmology 93: 327-330, 1986.

Nelson JD, Williams P, Lindstrom RL, Doughman DJ: Map-finger-dot changes in the corneal epithelial basement membrane following radial keratotomy. Ophthalmology 92: 199-205, 1985.

Novak AF, Lindstrom RL, Williams PA, Everson M: Corneal pachymetry prior to radial keratotomy: a comparison of techniques. J Refract Surg 1: 151-153, 1985.

Waring GO, Lynn MJ, Gelender H, Laibson PR, Lindstrom RL, Myers WD, Obstbaum SA, Rowsey JJ, McDonald MB, Schanzlin DJ, Sperduto RD, Bourque LB: Results of the Prospective Evaluation of Radial Keratotomy (PERK) study one year after surgery. Ophthalmology 92: 177-198, 307, 1985.

Bourque LB, Rubenstein R, Cosand B, Waring GO, Moffitt S, Gelender H, Laibson PR, Lindstrom RL, McDonald M, Myers WD, Obstbaum SA, Rowsey JJ: Psychosocial characteristics of candidates for the Prospective Evaluation of Radial Keratotomy (PERK) Study. Arch Ophthalmol 102: 1187-1192, 1984.

Steinberg EB, Wilson LA, Waring GO, Lynn MJ, Coles WH: Stellate iron lines in the corneal epithelium after radial keratotomy. Am J Ophthalmol 98: 416-421, 1984.

Steinberg EB, Waring GO: Comparison of the two methods of marking the visual axis on the cornea during radial keratotomy. Am J Ophthalmol 96: 605-608, 1983.

Waring GO, Moffitt SD, Gelender H, Laibson PR, Lindstrom RL, Myers WD, Obstbaum SA, Rowsey JJ, Safir A, Schanzlin DJ, Bourque LB: Rationale for and design of the National Eye Institute Prospective Evaluation of Radial Keratotomy (PERK) study. Ophthalmology 90: 40-58, 1983.

Resource Centers

Co-Chairmen's Offices
Peter J. McDonnell, M.D.
Doheny Eye Institute
University of Southern California
1450 San Pablo Street
Los Angeles, CA 90033
Telephone: (213) 342-6426
Fax: (213) 342-6440

George O. Waring III, M.D.
Department of Ophthalmology
Emory University School of Medicine
1327 Clifton Road, N.E.
Atlanta, GA 30322
Telephone: (404) 248-3244
Fax: (404) 248-5145

Clinical Coordinating Center
Katherine K. Lindstrom, M.P.H
Department of Ophthalmology
Emory University
1327 Clifton Road, N.E.
Atlanta, GA 30322
Telephone: (404) 248-4381
Fax: (404) 248-5145

Statistical Coordinating Center
Michael J. Lynn, M.S.
Division of Biostatistics
Emory University
1599 Clifton Road, N.E.
Atlanta, GA 30329
Telephone: (404) 727-7695
Fax: (404) 727-8737

Michael H. Kutner, Ph.D.
Division of Biostatistics
Emory University
1599 Clifton Road, N.E.
Atlanta, GA 30329
Telephone: (404) 727-7695
Fax: (404) 727-8737

Portia Griffin, B.A.
Division of Biostatistics
Emory University
1599 Clifton Road, N.E.
Atlanta, GA 30329
Telephone: (404) 727-7695
Fax: (404) 727-8737

Psychometric Testing Center
Linda B. Bourque, Ph.D.
School of Public Health
University of California
10833 LeConte Avenue
Los Angeles, CA 90024-1772
Telephone: (310) 825-4053
Fax: (310) 825-8440

NEI Representative

Donald F. Everett, M.A.
National Eye Institute
National Institutes of Health
Suite 1300
5635 Fishers Lane MSC 9300
Bethesda, MD 20892-9300
USA
Telephone: (301) 451-2020
Fax: (301) 402-0528
Email: deverett@nei.nih.gov

Last Updated: 10/21/99



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