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Autorefractometry after laser in situ keratomileusis
Autorefractometry after laser in situ keratomileusis Dimitrios S. Siganos, MD, PhD, Corina Popescu, MD, Nikolaos Bessis, DOpt, Georgios Papastergiou, MD
Purpose: To correlate cycloplegic subjective refraction with cycloplegic autorefractometry in eyes that have had laser in situ keratomileusis (LASIK). Setting: Vlemma Eye Institute, Athens, Greece. Methods: Subjective refraction and autorefractometry under cycloplegia were performed in 73 eyes of 46 patients 1, 6, and 12 months after LASIK to correct myopia or myopic astigmatism. The preoperative subjective refraction and autorefractometry under cycloplegia in the same eyes served as controls. Results: A statistically significant difference between subjective refraction and autorefraction was found in the sphere and cylinder at all postoperative times. No statistically significant difference was found in the axis. There was no statistically significant difference in the control eyes. Conclusions: Automated refractometry in eyes that had had LASIK was reliable in the axis only. Retreatments after LASIK should always be based on subjective refraction. J Cataract Refract Surg 2003; 29:133–137 © 2003 ASCRS and ESCRS
L
aser in situ keratomileusis (LASIK) has proven to be an effective treatment of low to moderate myopia, hyperopia, and astigmatism.1 In this refractive technique, introduced in 1991, a thin stromal flap is created with a microkeratome; an excimer laser is then used to recontour the anterior surface of the underlying corneal stroma by removing stromal tissue with extremely high precision.2 A new optical zone with a different refractive power is created, altering the patient’s refractive error. The refractive change produced by a refractive procedure can be quantified by subjective methods such as manifest refraction and by objective methods such as keratometry or automated refraction. The corneal change determined by standard keratometry or in a more detailed way by computerized videokeratography has been used to estimate surgically induced Accepted for publication July 10, 2002. From Vlemma Eye Institute, Athens, Greece. None of the authors has a financial interest in any product mentioned. Reprint requests to D.S. Siganos, MD, PhD, Vlemma Eye Institute, 88 Patission Street, Athens 10434, Greece. E-mail: [email protected]. © 2003 ASCRS and ESCRS Published by Elsevier Science Inc.
refractive changes, including those that occur after photorefractive keratectomy (PRK). Although these values correlate well with manifest refraction values, their precision is low.3–5 Automated refractometry is also used to quantify the refractive outcome of refractive procedures. Recent studies have evaluated the accuracy and precision of automated refractometry in predicting the refractive outcome of PRK.6,7 The automated refractometer tends to underestimate the subjective outcome in the sphere and cylinder. In these studies, however, measurements were performed without cycloplegia, and the authors suggest that despite the automatic fogging included in the refractor, changes in accommodation could be a cause of the low reliability of the automated refractometer. In this study, we evaluated the correlation between manifest refraction and automated refractometry in 46 patients who had LASIK for myopia or myopic astigmatism. We performed subjective and automated refraction under cycloplegia, thus preventing changes in accommodation from influencing the refractive status of the eye. 0886-3350/03/$–see front matter doi:10.1016/S0886-3350(02)01743-1
AUTOREFRACTOMETRY AFTER LASIK
Patients and Methods Forty-six patients (73 eyes) who had LASIK for myopia and myopic astigmatism at our institute participated in the study. The mean age of the patients was 32.14 years ⫾ 9.92 (SD). The mean attempted spherical correction was ⫺6.60 ⫾ 3.65 diopters (D) and the mean attempted cylindrical correction, ⫺1.04 ⫾ 1.09 D. All patients were examined before LASIK and 1, 6, and 12 months after the procedure. The preoperative and postoperative visits included corneal topography, manifest and cycloplegic refractions, automated refraction under cycloplegia, intraocular pressure measurement, pupillometry (Colvard), corneal pachymetry, and a complete ophthalmoscopic examination. Cycloplegia was performed with cyclopentolate 1% (Cyclogyl威) and phenylephrine 5%. One drop of each medication was instilled 2 times with 5 minutes between drops. Cycloplegic refractometry and refraction were performed 30 to 40 minutes after the last drop. Cycloplegic refractometry was always performed before cycloplegic refraction. Subjective refraction was determined using a phorometer and autorefraction, using a Nidek AR600A autorefractor with a measurement range from ⫺18.0 to ⫹23.0 D in sphere and up to ⫾8.0 D in cylinder. Laser in situ keratomileusis was performed under topical anesthesia using the Nidek 5000 excimer laser and the FlapMaker威 microkeratome (Refractive Technologies). In all eyes, the optical zone was 0.5 mm greater than the scotopic pupil and the transition zone was 1.0 mm. Statistical analysis was performed using regression analysis and a paired t test (SPSS software, version 6.0). A P value less than 0.05 was considered statistically significant.
Results Preoperatively, there was no statistically significant difference between cycloplegic refractometry and cycloplegic refraction in sphere, cylinder, or axis of astigmatism. One month after LASIK, the automated refractometry readings were statistically significantly different than the cycloplegic refraction measurements; automated refractometry tended to measure a more negative sphere and a higher cylinder (P ⫽ .010 and P ⫽ .008 for sphere and cylinder measurements, respectively) (Figures 1 and 2). The differences in the sphere and cylinder measurements were statistically significant at 6 (P ⫽ .026, P ⫽ .042) and 12 (P ⫽ .045, P ⫽ .017) months (Figures 1 and 2). The observed difference between the subjective refraction and automated refractometry did not change significantly over time. When the differences at the 3 postoperative times were compared, no statistically 134
significant change was found. There was no statistically significant difference between the 2 methods in the axis of astigmatism before and after the procedure and at the 3 postoperative times. The preoperative myopia and astigmatism did not significantly affect the difference between cycloplegic refraction and automated refractometry at the 3 postoperative times.
Discussion Automated refractometry is a rapid, convenient method to assess refractive error with and without cycloplegia. Numerous comparative evaluation studies have analyzed the improvement in accuracy, repeatability, and ease of operation of the various available instruments.8,9 Recently developed handheld instruments have even been used for noncycloplegic screening in 3-year-old kindergarten children.10 When automated refractometry is performed without cycloplegia, the results tend to be inaccurate, most likely because of the suboptimal control of accommodation, particularly in young patients. However, under cycloplegia, the sphere and cylinder results differ little from those obtained with retinoscopy.11,12 Even under cycloplegic conditions, autorefractors tend to measure more negative or less positive refraction values than subjective refraction.8 Previous studies have examined the accuracy of automated refractometry in quantifying the outcome of refractive procedures. Russell and coauthors13 compared objective and subjective refractions after radial keratotomy and found a significant postoperative shift toward myopia in the automated refraction compared with the subjective refraction. Øyo-Szereni and coauthors7 showed that automated refractometry readings tended to vary more from those obtained with subjective methods in PRK-treated eyes. Olsen and coauthors6 also found that automated topography was superior to automated refractometry in predicting spherical equivalent subjective changes in refraction after PRK. In all studies, however, measurements were performed without cycloplegia and changes in accommodation could partly account for the inferior accuracy of automated refractometry after refractive surgery. In our study, we found that results obtained with automated refractometry in unoperated eyes agreed with those obtained with subjective refraction. However,
J CATARACT REFRACT SURG—VOL 29, JANUARY 2003
AUTOREFRACTOMETRY AFTER LASIK
Figure 1. (Siganos) Scattergrams showing the relationship between automated refraction (x-axis) and manifest refraction (y-axis) under cycloplegia for spherical power before LASIK and 3 times postoperatively. A ⫽ preoperative. B ⫽ 1 month post LASIK. C ⫽ 6 months post LASIK. D ⫽ 12 months post LASIK.
postoperatively, automated refractometry was less accurate, tending toward a more negative sphere and a higher cylinder than the subjective measurements. This trend persisted throughout the 1-year follow-up. Automated refractometry was, however, accurate in determining the axis of astigmatism, although there were large standard deviations in measurements preoperatively and postoperatively. Finally, the amount of preoperative myopia
and astigmatism and consequently the ablation depth did not appear to significantly affect the loss of accuracy exhibited by automated refractometry after LASIK. Numerous stromal and epithelial changes occur in the cornea after LASIK as a result of the wound-healing process, and all of them could influence autorefraction. Proteoglycans and newly formed collagen types III and VII are deposited in the stroma after a keratorefractive
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AUTOREFRACTOMETRY AFTER LASIK
Figure 2. (Siganos) Scattergrams showing the relationship between automated refraction (x-axis) and manifest refraction (y-axis) under cycloplegia for cylindrical power before LASIK and 3 times postoperatively. A ⫽ preoperative. B ⫽ 1 month post LASIK. C ⫽ 6 months post LASIK. D ⫽ 12 months post LASIK.
procedure.14 Changes in epithelial thickness have also been described after LASIK. An increase in epithelial thickness occurs in the postoperative periods, peaking at 3 months and persisting for 12 months.15 The corneal epithelium has a different refractive index than the 136
stroma, and the observed change in its thickness could affect the cornea’s dioptric power.16 A change in the stromal refractive index possibly secondary to a change in stromal hydration during the healing period has also been proposed to explain the
J CATARACT REFRACT SURG—VOL 29, JANUARY 2003
AUTOREFRACTOMETRY AFTER LASIK
difference between changes in refraction and central ocular surface power after LASIK.17 Stromal ablation alters the relationship of the anterior and posterior surfaces of the cornea, as well as the relative composition, and the thickness and curvature of the epithelium and the anterior and posterior stroma. These changes could affect the refractive properties of the cornea after LASIK.3 The different refractive properties of the untreated and treated corneal areas and a steep junction zone between the ablated and unablated cornea could also affect automated refractometry. They should not have influenced automated refractometry readings in our cases as the ablation zone was always greater than the scotopic pupil and a wide transition zone was used in all eyes. In conclusion, automated refractometry was less accurate in eyes that had had LASIK. Although it is a valuable complement to subjective refraction, it should not replace it. Retreatments after LASIK should always be based on subjective refraction.
References 1. Sugar A, Rapuano CJ, Culbertson WW, et al. Laser in situ keratomileusis for myopia and astigmatism: safety and efficacy; a report by the American Academy of Ophthalmology. Ophthalmology 2002; 109:175–187 2. Pallikaris IG, Papatzanaki ME, Siganos DS, Tsilimbaris MK. A corneal flap technique for laser in situ keratomileusis; human studies. Arch Ophthalmol 1991; 109:1699 –1702 3. Hugger P, Kohnen T, La Rosa FA, et al. Comparison of changes in manifest refraction and corneal power after photorefractive keratectomy. Am J Ophthalmol 2000; 129:68 –75 4. Smith RJ, Chang W-K, Maloney RK. The prediction of surgically induced refractive change from corneal topography. Am J Ophthalmol 1998; 125:44 –53 5. Nguyen NX, Langenbucher A, Viestenz A, et al. Correlation among refractive, keratometric and topographic astigmatism after myopic photorefractive keratectomy. Graefes Arch Clin Exp Ophthalmol 2000; 238:642–646
6. Olsen H, Hjortdal JØ, Ehlers N. Comparison of objective methods for quantifying the refractive effect of photo-astigmatic refractive keratectomy using the MEL-60 excimer laser. Acta Ophthalmol Scand 1997; 75:629 – 633 7. Øyo-Szerenyi KD, Wienecke L, Businger U, Schipper I. Autorefraction/autokeratometry and subjective refraction in untreated and photorefractive keratectomytreated eyes. Arch Ophthalmol 1997; 115:157–164 8. Kinge B, Midelfart A, Jacobsen G. Clinical evaluation of the Allergan Humphrey 500 autorefractor and the Nidek AR-1000 autorefractor. Br J Ophthalmol 1996; 80:35–39 9. Wong EK Jr, Patella VM, Pratt MV, et al. Clinical evaluation of the Humphrey automatic refractor. Arch Ophthalmol 1994; 102:870 –875 10. Cordonnier M, Dramaix M. Screening for refractive errors in children: accuracy of the handheld refractor Retinomax to screen for astigmatism. Br J Ophthalmol 1999; 83:157–161 11. Berman M, Nelson P, Caden B. Objective refraction: comparison of retinoscopy and automated techniques. Am J Optom Physiol Opt 1984; 61:204 –209 12. Salvesen S, Køhler M. Automated refraction; a comparative study of automated refraction with the Nidek AR1000 autorefractor and retinoscopy. Acta Ophthalmol 1991; 69:342–346 13. Russell GE, Bergmanson JPG, Barbeito R, Cross WD. Differences between objective and subjective refractions after radial keratometry. Refract Corneal Surg 1992; 8:290 –295 14. Assil KK, Quantock AJ. Wound healing in response to keratorefractive surgery. Surv Ophthalmol 1993; 38:289– 302 15. Spadea L, Fasciani R, Necozione S, Balestrazzi E. Role of the corneal epithelium in refractive changes following laser in situ keratomileusis for high myopia. J Refract Surg 2000; 16:133–139 16. Patel S, Marshall J, Fitzke FW III. Refractive index of the human corneal epithelium and stroma. J Refract Surg 1995; 11:100 –105 17. Patel S, Alio´ JL, Pe´rez-Santonja JJ. A model to explain the difference between changes in refraction and central ocular surface power after laser in situ keratomileusis. J Refract Surg 2000; 16:330 –335
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Purpose: To correlate cycloplegic subjective refraction with cycloplegic autorefractometry in eyes that have had laser in situ keratomileusis (LASIK). Setting: Vlemma Eye Institute, Athens, Greece. Methods: Subjective refraction and autorefractometry under cycloplegia were performed in 73 eyes of 46 patients 1, 6, and 12 months after LASIK to correct myopia or myopic astigmatism. The preoperative subjective refraction and autorefractometry under cycloplegia in the same eyes served as controls. Results: A statistically significant difference between subjective refraction and autorefraction was found in the sphere and cylinder at all postoperative times. No statistically significant difference was found in the axis. There was no statistically significant difference in the control eyes. Conclusions: Automated refractometry in eyes that had had LASIK was reliable in the axis only. Retreatments after LASIK should always be based on subjective refraction. J Cataract Refract Surg 2003; 29:133–137 © 2003 ASCRS and ESCRS
L
aser in situ keratomileusis (LASIK) has proven to be an effective treatment of low to moderate myopia, hyperopia, and astigmatism.1 In this refractive technique, introduced in 1991, a thin stromal flap is created with a microkeratome; an excimer laser is then used to recontour the anterior surface of the underlying corneal stroma by removing stromal tissue with extremely high precision.2 A new optical zone with a different refractive power is created, altering the patient’s refractive error. The refractive change produced by a refractive procedure can be quantified by subjective methods such as manifest refraction and by objective methods such as keratometry or automated refraction. The corneal change determined by standard keratometry or in a more detailed way by computerized videokeratography has been used to estimate surgically induced Accepted for publication July 10, 2002. From Vlemma Eye Institute, Athens, Greece. None of the authors has a financial interest in any product mentioned. Reprint requests to D.S. Siganos, MD, PhD, Vlemma Eye Institute, 88 Patission Street, Athens 10434, Greece. E-mail: [email protected]. © 2003 ASCRS and ESCRS Published by Elsevier Science Inc.
refractive changes, including those that occur after photorefractive keratectomy (PRK). Although these values correlate well with manifest refraction values, their precision is low.3–5 Automated refractometry is also used to quantify the refractive outcome of refractive procedures. Recent studies have evaluated the accuracy and precision of automated refractometry in predicting the refractive outcome of PRK.6,7 The automated refractometer tends to underestimate the subjective outcome in the sphere and cylinder. In these studies, however, measurements were performed without cycloplegia, and the authors suggest that despite the automatic fogging included in the refractor, changes in accommodation could be a cause of the low reliability of the automated refractometer. In this study, we evaluated the correlation between manifest refraction and automated refractometry in 46 patients who had LASIK for myopia or myopic astigmatism. We performed subjective and automated refraction under cycloplegia, thus preventing changes in accommodation from influencing the refractive status of the eye. 0886-3350/03/$–see front matter doi:10.1016/S0886-3350(02)01743-1
AUTOREFRACTOMETRY AFTER LASIK
Patients and Methods Forty-six patients (73 eyes) who had LASIK for myopia and myopic astigmatism at our institute participated in the study. The mean age of the patients was 32.14 years ⫾ 9.92 (SD). The mean attempted spherical correction was ⫺6.60 ⫾ 3.65 diopters (D) and the mean attempted cylindrical correction, ⫺1.04 ⫾ 1.09 D. All patients were examined before LASIK and 1, 6, and 12 months after the procedure. The preoperative and postoperative visits included corneal topography, manifest and cycloplegic refractions, automated refraction under cycloplegia, intraocular pressure measurement, pupillometry (Colvard), corneal pachymetry, and a complete ophthalmoscopic examination. Cycloplegia was performed with cyclopentolate 1% (Cyclogyl威) and phenylephrine 5%. One drop of each medication was instilled 2 times with 5 minutes between drops. Cycloplegic refractometry and refraction were performed 30 to 40 minutes after the last drop. Cycloplegic refractometry was always performed before cycloplegic refraction. Subjective refraction was determined using a phorometer and autorefraction, using a Nidek AR600A autorefractor with a measurement range from ⫺18.0 to ⫹23.0 D in sphere and up to ⫾8.0 D in cylinder. Laser in situ keratomileusis was performed under topical anesthesia using the Nidek 5000 excimer laser and the FlapMaker威 microkeratome (Refractive Technologies). In all eyes, the optical zone was 0.5 mm greater than the scotopic pupil and the transition zone was 1.0 mm. Statistical analysis was performed using regression analysis and a paired t test (SPSS software, version 6.0). A P value less than 0.05 was considered statistically significant.
Results Preoperatively, there was no statistically significant difference between cycloplegic refractometry and cycloplegic refraction in sphere, cylinder, or axis of astigmatism. One month after LASIK, the automated refractometry readings were statistically significantly different than the cycloplegic refraction measurements; automated refractometry tended to measure a more negative sphere and a higher cylinder (P ⫽ .010 and P ⫽ .008 for sphere and cylinder measurements, respectively) (Figures 1 and 2). The differences in the sphere and cylinder measurements were statistically significant at 6 (P ⫽ .026, P ⫽ .042) and 12 (P ⫽ .045, P ⫽ .017) months (Figures 1 and 2). The observed difference between the subjective refraction and automated refractometry did not change significantly over time. When the differences at the 3 postoperative times were compared, no statistically 134
significant change was found. There was no statistically significant difference between the 2 methods in the axis of astigmatism before and after the procedure and at the 3 postoperative times. The preoperative myopia and astigmatism did not significantly affect the difference between cycloplegic refraction and automated refractometry at the 3 postoperative times.
Discussion Automated refractometry is a rapid, convenient method to assess refractive error with and without cycloplegia. Numerous comparative evaluation studies have analyzed the improvement in accuracy, repeatability, and ease of operation of the various available instruments.8,9 Recently developed handheld instruments have even been used for noncycloplegic screening in 3-year-old kindergarten children.10 When automated refractometry is performed without cycloplegia, the results tend to be inaccurate, most likely because of the suboptimal control of accommodation, particularly in young patients. However, under cycloplegia, the sphere and cylinder results differ little from those obtained with retinoscopy.11,12 Even under cycloplegic conditions, autorefractors tend to measure more negative or less positive refraction values than subjective refraction.8 Previous studies have examined the accuracy of automated refractometry in quantifying the outcome of refractive procedures. Russell and coauthors13 compared objective and subjective refractions after radial keratotomy and found a significant postoperative shift toward myopia in the automated refraction compared with the subjective refraction. Øyo-Szereni and coauthors7 showed that automated refractometry readings tended to vary more from those obtained with subjective methods in PRK-treated eyes. Olsen and coauthors6 also found that automated topography was superior to automated refractometry in predicting spherical equivalent subjective changes in refraction after PRK. In all studies, however, measurements were performed without cycloplegia and changes in accommodation could partly account for the inferior accuracy of automated refractometry after refractive surgery. In our study, we found that results obtained with automated refractometry in unoperated eyes agreed with those obtained with subjective refraction. However,
J CATARACT REFRACT SURG—VOL 29, JANUARY 2003
AUTOREFRACTOMETRY AFTER LASIK
Figure 1. (Siganos) Scattergrams showing the relationship between automated refraction (x-axis) and manifest refraction (y-axis) under cycloplegia for spherical power before LASIK and 3 times postoperatively. A ⫽ preoperative. B ⫽ 1 month post LASIK. C ⫽ 6 months post LASIK. D ⫽ 12 months post LASIK.
postoperatively, automated refractometry was less accurate, tending toward a more negative sphere and a higher cylinder than the subjective measurements. This trend persisted throughout the 1-year follow-up. Automated refractometry was, however, accurate in determining the axis of astigmatism, although there were large standard deviations in measurements preoperatively and postoperatively. Finally, the amount of preoperative myopia
and astigmatism and consequently the ablation depth did not appear to significantly affect the loss of accuracy exhibited by automated refractometry after LASIK. Numerous stromal and epithelial changes occur in the cornea after LASIK as a result of the wound-healing process, and all of them could influence autorefraction. Proteoglycans and newly formed collagen types III and VII are deposited in the stroma after a keratorefractive
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AUTOREFRACTOMETRY AFTER LASIK
Figure 2. (Siganos) Scattergrams showing the relationship between automated refraction (x-axis) and manifest refraction (y-axis) under cycloplegia for cylindrical power before LASIK and 3 times postoperatively. A ⫽ preoperative. B ⫽ 1 month post LASIK. C ⫽ 6 months post LASIK. D ⫽ 12 months post LASIK.
procedure.14 Changes in epithelial thickness have also been described after LASIK. An increase in epithelial thickness occurs in the postoperative periods, peaking at 3 months and persisting for 12 months.15 The corneal epithelium has a different refractive index than the 136
stroma, and the observed change in its thickness could affect the cornea’s dioptric power.16 A change in the stromal refractive index possibly secondary to a change in stromal hydration during the healing period has also been proposed to explain the
J CATARACT REFRACT SURG—VOL 29, JANUARY 2003
AUTOREFRACTOMETRY AFTER LASIK
difference between changes in refraction and central ocular surface power after LASIK.17 Stromal ablation alters the relationship of the anterior and posterior surfaces of the cornea, as well as the relative composition, and the thickness and curvature of the epithelium and the anterior and posterior stroma. These changes could affect the refractive properties of the cornea after LASIK.3 The different refractive properties of the untreated and treated corneal areas and a steep junction zone between the ablated and unablated cornea could also affect automated refractometry. They should not have influenced automated refractometry readings in our cases as the ablation zone was always greater than the scotopic pupil and a wide transition zone was used in all eyes. In conclusion, automated refractometry was less accurate in eyes that had had LASIK. Although it is a valuable complement to subjective refraction, it should not replace it. Retreatments after LASIK should always be based on subjective refraction.
References 1. Sugar A, Rapuano CJ, Culbertson WW, et al. Laser in situ keratomileusis for myopia and astigmatism: safety and efficacy; a report by the American Academy of Ophthalmology. Ophthalmology 2002; 109:175–187 2. Pallikaris IG, Papatzanaki ME, Siganos DS, Tsilimbaris MK. A corneal flap technique for laser in situ keratomileusis; human studies. Arch Ophthalmol 1991; 109:1699 –1702 3. Hugger P, Kohnen T, La Rosa FA, et al. Comparison of changes in manifest refraction and corneal power after photorefractive keratectomy. Am J Ophthalmol 2000; 129:68 –75 4. Smith RJ, Chang W-K, Maloney RK. The prediction of surgically induced refractive change from corneal topography. Am J Ophthalmol 1998; 125:44 –53 5. Nguyen NX, Langenbucher A, Viestenz A, et al. Correlation among refractive, keratometric and topographic astigmatism after myopic photorefractive keratectomy. Graefes Arch Clin Exp Ophthalmol 2000; 238:642–646
6. Olsen H, Hjortdal JØ, Ehlers N. Comparison of objective methods for quantifying the refractive effect of photo-astigmatic refractive keratectomy using the MEL-60 excimer laser. Acta Ophthalmol Scand 1997; 75:629 – 633 7. Øyo-Szerenyi KD, Wienecke L, Businger U, Schipper I. Autorefraction/autokeratometry and subjective refraction in untreated and photorefractive keratectomytreated eyes. Arch Ophthalmol 1997; 115:157–164 8. Kinge B, Midelfart A, Jacobsen G. Clinical evaluation of the Allergan Humphrey 500 autorefractor and the Nidek AR-1000 autorefractor. Br J Ophthalmol 1996; 80:35–39 9. Wong EK Jr, Patella VM, Pratt MV, et al. Clinical evaluation of the Humphrey automatic refractor. Arch Ophthalmol 1994; 102:870 –875 10. Cordonnier M, Dramaix M. Screening for refractive errors in children: accuracy of the handheld refractor Retinomax to screen for astigmatism. Br J Ophthalmol 1999; 83:157–161 11. Berman M, Nelson P, Caden B. Objective refraction: comparison of retinoscopy and automated techniques. Am J Optom Physiol Opt 1984; 61:204 –209 12. Salvesen S, Køhler M. Automated refraction; a comparative study of automated refraction with the Nidek AR1000 autorefractor and retinoscopy. Acta Ophthalmol 1991; 69:342–346 13. Russell GE, Bergmanson JPG, Barbeito R, Cross WD. Differences between objective and subjective refractions after radial keratometry. Refract Corneal Surg 1992; 8:290 –295 14. Assil KK, Quantock AJ. Wound healing in response to keratorefractive surgery. Surv Ophthalmol 1993; 38:289– 302 15. Spadea L, Fasciani R, Necozione S, Balestrazzi E. Role of the corneal epithelium in refractive changes following laser in situ keratomileusis for high myopia. J Refract Surg 2000; 16:133–139 16. Patel S, Marshall J, Fitzke FW III. Refractive index of the human corneal epithelium and stroma. J Refract Surg 1995; 11:100 –105 17. Patel S, Alio´ JL, Pe´rez-Santonja JJ. A model to explain the difference between changes in refraction and central ocular surface power after laser in situ keratomileusis. J Refract Surg 2000; 16:330 –335
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