- Email: [email protected]
Prevalence of corneal astigmatism before cataract surgery
ARTICLE
Prevalence of corneal astigmatism before cataract surgery Teresa Ferrer-Blasco, PhD, Robert Monte´s-Mico´, PhD, Sofia C. Peixoto-de-Matos, OD, Jose´ M. Gonza´lez-Me´ijome, PhD, Alejandro Cervin˜o, PhD
PURPOSE: To analyze the prevalence and presentation patterns of corneal astigmatism in cataract surgery candidates. SETTING: University of Valencia, Valencia, Spain. METHODS: Refractive and keratometric values were measured before surgery in patients having cataract extraction. Descriptive statistics of refractive and keratometric cylinder data were analyzed and correlated by age ranges. RESULTS: Refractive and keratometric data from 4540 eyes of 2415 patients (mean age 60.59 years G 9.87 [SD]; range 32 to 87 years) differed significantly when the patients were divided into 10-year subsets. There was a trend toward less negative corneal astigmatism values, except the steepest corneal radius and the J45 vector component, in older groups (Kruskal-Wallis, P<.01). In 13.2% of eyes, no corneal astigmatism was present; in 64.4%, corneal astigmatism was between 0.25 and 1.25 diopters (D) and in 22.2%, it was 1.50 D or higher. CONCLUSIONS: Corneal astigmatism less than 1.25 D was present in most cataract surgery candidates; it was higher in about 22%, with slight differences between the various age ranges. This information is useful for intraocular lens (IOL) manufacturers to evaluate which age ranges concentrate the parameters most frequently needed in sphere and cylinder powers and for surgeons to evaluate which IOLs provide the most effective power range. J Cataract Refract Surg 2009; 35:70–75 Q 2009 ASCRS and ESCRS
Expectations and demands of patients require correction of refractive errors after cataract surgery. In addition to spherical refractive errors, astigmatism should be addressed at the time of surgery to achieve the best postsurgery refractive outcomes. Astigmatism can be reduced or eliminated by a variety of surgical techniques, including selective positioning of the phacoemulsification incision, corneal relaxing incisions, limbal relaxing incisions, and excimer laser keratectomy. All these methods have limitations such as the degree of astigmatism to be treated or the long-term mechanical instability, and postoperative outcomes are subject to many variables such as age, magnitude, incision number, depth, and length.1 Toric intraocular lens (IOL) implantation is another option for the correction of corneal astigmatism in cataract patients. Toric IOLs have been used clinically since first described by Shimizu et al.2 in 1994. Previous investigations3–13 have analyzed the results of implantation of several toric IOL models and found this method to be effective in correcting corneal astigmatism. However, the prevalence of different amounts of corneal astigmatism in 70
Q 2009 ASCRS and ESCRS Published by Elsevier Inc.
cataract surgery candidates has not been fully evaluated. Analysis of this would provide valuable information to cataract surgeons and IOL manufacturers. When designing a toric IOL, a critical issue is selecting the range of parameters to optimize the manufacturing process, stock, and clinical application. Currently, we know of no reports of systematic data about the prevalence of the degrees of corneal astigmatism in large population cohorts that could be used to plan large-scale manufacturing. Furthermore, it is well known that corneal astigmatism changes significantly with age.14–19 With increasing numbers of patients having cataract and refractive lens exchange surgeries, it becomes mandatory to analyze such data in different age groups. This study reports the prevalence of corneal astigmatism in a large sample of patients before cataract surgery and describes the distribution in age subsets. This information will allow determination of the most prevalent keratometric conditions so better strategies for IOL manufacturing and clinical prescription can be selected. 0886-3350/09/$dsee front matter doi:10.1016/j.jcrs.2008.09.027
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CORNEAL ASTIGMATISM BEFORE CATARACT SURGERY
PATIENTS AND METHODS This prospective study measured refractive and keratometric data in cataract surgery candidates in the context of presurgical screening. Inclusion criteria included presence of cataract and age between 30 and 90 years. Exclusion criteria included irregular astigmatism, corneal disease, previous corneal or intraocular surgery, and history of ocular inflammation. All eyes were examined before the cataract surgery procedure, including a complete ophthalmological examination, manifest and cycloplegic refractions, slitlamp examination, applanation tonometry, and ophthalmoscopy through dilated pupils. For part of the analysis, the sample was divided into 6 age groups: 40 years or younger, 41 to 50 years, 51 to 60 years, 61 to 70 years, 71 to 80 years, and 81 to 90 years. The analysis of corneal astigmatism was performed using the autokeratorefractometer RM-KR-8800 (Topcon). This system performs an automatic measurement of the central keratometry of the eye (flat axis and steep axis).20,21 Three measurements were performed in each eye. The tenets of the Declaration of Helsinki were followed, and informed consent was obtained from all patients after the nature and possible consequences of the study were explained. Institutional review board approval was also obtained. Data were analyzed using the SPSS statistical package, version 16.0. The Kolmogorov-Smirnov test was used to check normal distribution of variables. This is particularly important because distribution of astigmatic values is skewed toward lower values. Bivariate correlations were evaluated using the Pearson coefficient for normally distributed variables and the Spearman rank correlation coefficient for nonnormally distributed variables. Differences between age groups were analyzed using an analysis of variance for normally distributed variables and the Kruskal-Wallis (KW) test for nonnormally distributed variables. The t test was used with parametric variables and the Wilcoxon signed rank test with nonparametric variables to compare refractive and corneal cylinder for the entire sample and for the age groups separately. The level of significance was established at a Z 0.05.
Submitted: July 25, 2008. Final revision submitted: September 24, 2008. Accepted: September 24, 2008.
RESULTS The study comprised 4540 eyes in 2415 patients. Patient demographics are shown in Table 1. Figure 1 is a histogram with frequency distribution of corneal astigmatism values for the entire sample showing that 13.2% of eyes did not present corneal astigmatism, 64.6% had corneal astigmatism between 0.25 and 1.25 diopters (D), and 22.2% had astigmatism of 1.50 D or higher. Table 2 presents the descriptive values in the 6 age groups and the statistical significance for comparison among groups. The number of eyes in the 6 age groups was as follows: 40 years or younger, 58 eyes; 41 to 50 years, 714 eyes; 51 to 60 years, 1571 eyes; 61 to 70 years, 1383 eyes; 71 to 80 years, 666 eyes; and 81 to 90 years, 128 eyes. All refractive values showed significant changes between age groups (P!.01, K-W) except the steep keratometric radius (P Z .065, K-W) and the J45 value (P Z .646, K-W). Figure 2 shows the box plot for values of corneal cylinder in the age groups. As shown in Table 2, corneal cylinder tended to become more positive in older groups. A quick inspection of the graph shows that the most homogeneous subsample of corneal astigmatism was in the 61 to 70 year subset. This is probably the most likely population to receive an IOL after cataract surgery. Figure 3 shows the distribution of corneal astigmatism in 0.50 D steps within each age group. There is a slight apparent trend for the graphs to be skewed toward the positive values, as expected from the descriptive data in Table 2 and the trend lines in Figure 2. DISCUSSION Corneal astigmatism can be managed surgically using corneal, relaxing, or limbal incisions and excimer laser
Table 1. Demographic characteristics of patients. Characteristic
From the Optometry Research Group (Ferrer-Blasco, Monte´s-Mico´, Cervin˜o), Optics Department, University of Valencia, Valencia, Spain; the Optica Queiros Lda. Po´voa de Lanhoso (Peixoto-de-Matos), and the Department of Physics (Optometry) (Gonza´lez-Me´ijome), School of Sciences, University of Minho, Braga, Portugal. No author has a financial or proprietary interest in any material or method mentioned. Supported in part by Universitat de Valencia research grants to Robert Monte´s-Mico´ (#UV-AE-20070225#), Jose Manuel Gonza´lez-Me´ijome (#UV-ESTPC-08-2054#), Alejandro Cervin˜o (#UV-AE08-2291#), and Red Tema´tica de Optometrı´a (Ministerio de Ciencia e Innovacio´n SAF2008-01114-E). Corresponding author: Teresa Ferrer-Blasco, PhD, Optics Department, Faculty of Physics, University of Valencia, C/ Dr. Moliner, 50. 46100, Burjassot (Valencia), Spain. E-mail: [email protected].
Number of eyes/patients Mean age (y) G SD Range of age (y) Gender (male/female) Mean corneal astigmatism (D) G SD Range of corneal astigmatism (D) Mean keratometry (D) Flat Steep Range of keratometry (D) Mean sphere (D) G SD Range of sphere (D) Mean cylinder (D) G SD Range of cylinder (D) SD Z standard deviation
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Value 4540/2415 60.59 G 9.87 32 to 87 768/1647 0.90 G 0.93 0.25 to 6.75 43.48 G 1.61 44.08 G 1.59 38 to 48 0.35 G 5.28 16 to C12 0.86 G 0.95 0.25 to –7.00
CORNEAL ASTIGMATISM BEFORE CATARACT SURGERY
55 50 45 40 35 30 25 20 15 10 5 0
7,00 45,66
6,00
13,14
12,10 12,63 7,21 3,63 2,30 1,59 0,88 0,38 0,33 0,00 0,02 0,09 0,02
Corneal Astigmatism (D)
Corneal Astigmatism (D)
Frequency (%)
72
5,00 4,00 3,00 2,00 1,00 0,00
Figure 1. Histogram of the corneal astigmatism (D) distribution in the entire sample (4540 eyes).
keratectomy. The limitations, advantages, and disadvantages have been fully discussed.4 The use of toric IOLs to correct corneal astigmatism is a growing surgical option. The present study attempted to determine the prevalence of different amounts of corneal astigmatism as a function of age, as such data are not available in the literature surveyed by the authors. As previously said, the descriptive analysis provided is valuable information for cataract surgeons and IOL manufacturers. Results revealed that 34.8% of the eyes had corneal astigmatism equal to or higher than 1.00 D. The mean corneal astigmatism (1.02 D) was slightly higher than the value reported by Riley et al.22 (0.89 D) before cataract surgery; the range of values was similar (up to 6.50 D in Riley et al. and up to 6.75 D in the present study). The steepest corneal meridian in the present study was similar to the mean value observed by Riley et al. (44.08 D and 44.13 D, respectively). The results in this study also show a trend toward more hyperopic values in the older groups, agreeing with previous studies conducted by us18,19 and with data from the
<=40
41-50
51-60
61-70
71-80
81-90
Age Group (years) Figure 2. Box plot of values of corneal cylinder (D) in the 6 age groups. Bold lines within boxes represent the median (Q2 or percentile 50%), upper and lower limits of the box represent the first quartile (Q1 or percentile 25%) and third quartile (Q3 or percentile 75%), and bars represent the extreme values (maximum and minimum observations). Asterisks and circles represent outliers.
Reykjavik Eye Study,17 which showed a hyperopic shift of about C0.3 D after 5 years of follow-up. The authors of th Reykjavik Eye Study also found significant changes in corneal astigmatism in older people, agreeing with the results reported here. Most cataract surgery candidates in the present cohort were women, following a trend in previous studies.22 The values of the blur component from vector decomposition of the refraction were significantly higher than the values obtained recently by FerrerBlasco et al.19 in a population attending an eye clinic, even in the older groups. Only the mean values for women in that study showed mean blur values similar to those reported in the present study. This could be justified by the higher values of astigmatism in some
Table 2. Descriptive statistics (diopters) for the 6 age groups and statistical significance of differences between groups. Manifest refractions in conventional script notation [S (sphere), C (cylinder) f (axis)] were converted to power vector coordinates (M, J0, J45) and overall blurring strength (B) by the following formulas: M Z S C C/2; J0 Z (C/2) cos (2f); J45 Z (C/2) sin (2f); B Z (M2 C J02 C J452)1/2. Age Group (Y)
Sphere
Cylinder
Flat K
Steep K
M
J0
J45
Blur
% 40 41–50 51–60 61–70 71–80 81–90 Total Significance*
2.52 1.31 0.77 0.04 0.59 0.63 0.35 0.004
0.75 1.06 0.85 0.74 0.87 1.08 0.86 0.001
42.77 43.19 43.42 43.61 43.64 43.77 43.48 0.001
43.73 44.14 44.04 44.03 44.21 44.14 44.08 0.059
2.15 1.84 1.20 0.40 0.16 0.12 0.77 0.003
0.26 0.20 0.03 0.14 0.22 0.47 0.04 0.001
0.01 0.01 0.01 0.01 0.01 0.04 0.01 0.646
4.69 6.09 4.36 3.00 2.39 2.73 3.89 0.001
*
Kruskal-Wallis test
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Figure 3. Frequency distribution of corneal astigmatism in 0.50 D steps for the 6 age groups.
of the patients enrolled. Although it is well known that corneal astigmatism changes with age from with the rule to against the rule,14,15,17,23 in the present population the astigmatic changes observed could also be due to changes in the crystalline lens as a consequence of aging, scleroses, and loss of transparency. Despite this, our results agree with the trends found in other studies24 that document lack of significant changes in steep keratometric values, attributing the changes in amount and axis of corneal astigmatism with age to changes in the horizontal meridian (usually the flat meridian in young subjects). Table 3 shows the range of spherical and cylindrical powers of the main toric IOLs currently available. The lowest cylindrical correction is 1.0 D and the highest, 12.0 D. One should consider that there is a difference between the cylindrical power of the toric IOL and the magnitude of corneal astigmatism that it is capable of correcting. At the corneal plane, there is a reduction of about 30% of the cylindrical power (ie, cylindrical power of the toric IOL of 2.0 D and 3.5 D corrects
1.4 D and 2.4 D of corneal astigmatism, respectively). Thus, the lowest and the highest cylindrical powers for which correction can be achieved are 0.7 D and 8.4 D of corneal astigmatism, respectively. To clinically apply the outcomes in this study, surgeons should correlate the proportions reported with the cylinder power nomograms provided by toric IOL manufacturers. For example, using the cylinder power selection nomogram for the Staar toric IOL (Table 4) and assuming astigmatically neutral surgery in most of the eyes in our sample, 64.6% of eyes showing corneal astigmatism up to 1.25 D will not be covered by the options provided and only 17.0% will be corrected with the toric IOL proposed. In addition, 3.2% of patients will be corrected with the toric IOL with the aid of limbal relaxing incisions. Most surgeons who treat astigmatism in cataract patients tend to use limbal or corneal relaxing incisions.25–27 There are some potential downsides to this technique, however. For example, this procedure may be somewhat unpredictable and imprecise. In addition, it is dependent on variable
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CORNEAL ASTIGMATISM BEFORE CATARACT SURGERY
Table 3. Range of spherical and cylindrical powers and corneal astigmatism capable of correcting for the toric IOLs on the market.
Company
Range of Spherical IOL Power (Increment)
Range of Cylindrical IOL Power (Increment)
Range of Corneal Astigmatism Able to Correct
Percentage of Eyes Susceptible to Correction*
AcrySof Toric SN60T3/T4/T5/T6/ T7/T8/T9
C10.0 to C30.0 (0.5)
1.5 to 6.0 (1.0)
1.0 to 4.2
34.2
Torica-S
3.0 to C14.0 (1.0) C15.0 to C25.0 (0.5) C26.0 to 31.0 (1.0)
2.0 to 12.0 (1.0)
1.4 to 8.4
22.3
T-Flex 573T/623T
C6.0 to C26.0 (0.5) 10.0 to C35.0 (0.5)
1.0 to 6.0 (1.0) 1.0 to 11.0 (0.25)
0.7 to 4.2 0.7 to 7.7
40.5 41.2
AA4203TF AA4203TL
C24.0 to C28.5 (0.50) C9.5 to C23.5 (0.50)
2.0 and 3.5 2.0 and 3.5
1.4 and 2.4 1.4 and 2.4
17.1 17.1
Acri.Comfort 643TLC Acri.Comfort 646TLC
0 to C40 10 to C32
1.0 to 12.0 (1.0) 1.0 to 12.0 (1.0)
0.7 to 8.4 0.7 to 8.4
41.2 41.2
Intraocular Lens
Alcon
HumanOptics
Rayner
Staar
Zeiss
IOL Z intraocular lens * Considering only corneal astigmatism in the eyes in the present study (n Z 4540)
healing responses, the skill of the surgeon, and the amount of cylinder that can be corrected is limited. The risks for overcorrection, perforation, and wound gape also need to be considered. Table 3 shows the percentage of eyes in this study that are susceptible to correction (considering corneal astigmatism only). The percentage varied from 17% to 41% as a function of the range of cylindrical power of the different IOLs. Obviously, larger ranges showed a larger percentage. The best ratio percentage versus range of cylinder manufactured was obtained for the Rayner and Alcon IOLs (largest percentage with least range manufactured). However, this proportion seems to change greatly across the age groups as shown in Figure 3, and this should be taken into account by IOL manufacturers and surgeons. For this particular case, Figure 4 shows the proportion of patients within each age group who would be fitted according to the nomogram shown in Table 4 for the Staar toric IOL. This graph shows that about 13% of cataract surgery candidates do not have corneal astigmatism (range from 0% in younger group to 18% in older groups), 65% have corneal astigmatism between 0.25 D to 1.25 D, 15% have astigmatism between 1.50 D and 2.25 D, 4% have corneal astigmatism between 2.50 D and 3.00 D, and 3% have astigmatism equal to or higher than 3.25 D. In conclusion, when choosing a toric IOL to correct astigmatism, a pertinent issue for the surgeon is to determine which IOL covers the larger range of
astigmatic values to cover the overall profile of the pool of candidates to undergo a surgical procedure in each particular clinical population (depending on geographical, socioeconomic, and clinical criteria). The prevalence of corneal astigmatism reported in this study of a large cohort of cataract surgery candidates comprising a wide age range will also help manufacturers match their toric IOL parameter range
Table 4. Nomogram for selection of astigmatic IOL power from values of corneal astigmatism assuming astigmatically neutral surgery for the Staar toric IOL (reproduced from the manufacturer).
Corneal Astigmatism (D) With the rule* 1.5 D to 2.25 D 2.5 D to 3.00 D 3.25 D or morez Against the rule* 1.5 D to 2.25 D 2.5 D to 3.00 D 3.25 D or morez
IOL Cylinder Power (D)
Expected Cylinder Correction from IOL (D)†
2.0 D 3.5 D 3.5 D CLRIs
1.5 D 2.25 D 2.25 D
2.0 D 3.5 D 3.5 D CLRIs
1.5 D 2.25 D 2.25 D
IOL Z intraocular lens; LRI Z limbal relaxing incision * With the rule, steep corneal meridian between 46 and 134 ; against the rule, steep corneal meridian between 0 to 45 and 135 and 180 † Approximate value of the equivalent IOL cylinder power at the cornea z Combine with astigmatic keratotomy or LRIs
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100
NO ASTIGMATISM [1.50,2.25] [>=3.25]
Frequency (%)
80
[0.25,1.25] [2.50,3.00]
60 40 20 0
<=40
41-50
51-60
61-70
71-80
81-90
Age Group Figure 4. Simulation of frequency of IOL prescription for the patients implanted with the Staar toric IOL according to the manufacturer’s nomogram in Table 4.
to the profile of corneal characteristics within the population. REFERENCES 1. Grabow HB. Intraocular correction of refractive errors. In: Kershner RM, ed, Refractive Keratectomy for Cataract Surgery and the Correction of Astigmatism. Thorofare, NJ, Slack, 1994; 79–115 2. Shimizu K, Misawa A, Suzuki Y. Toric intraocular lenses: correcting astigmatism while controlling axis shift. J Cataract Refract Surg 1994; 20:523–526 3. Grabow HB. Early results with foldable toric IOL implantation. Eur J Implant Refract Surg 1994; 6:177–178 4. Grabow HB. Toric intraocular lens report. Ann Ophthalmol 1997; 29:161–163 5. Werblin TP. Do three-piece PMMA IOLs rotate after implantation in the capsular bag? J Refract Surg 1995; 11:468–471 6. Sun X-Y, Vicary D, Montgomery P, Griffiths M. Toric intraocular lenses for correcting astigmatism in 130 eyes. Ophthalmology 2000; 107:1776–1782; discussion by RM Kershner, 1781–1782 7. Ruhswurm I, Scholz U, Zehetmayer M, Hanselmayer G, Vass C, Skorpik C. Astigmatism correction with a foldable toric intraocular lens in cataract patients. J Cataract Refract Surg 2000; 26:1022–1027 8. Gerten G, Michels A, Olmes A. Torische Intraokularlinsen; klinische Ergebnisse und Rotationsstabilita¨t. [Toric intraocular lenses; clinical results and rotational stability.] Ophthalmologe 2001; 98:715–720 9. Leyland M, Zinicola E, Bloom P, Lee N. Prospective evaluation of a plate haptic toric intraocular lens. Eye 2001; 15:202–205 10. Till JS, Yoder PR Jr, Wilcox TK, Spielman JL. Toric intraocular lens implantation: 100 consecutive cases. J Cataract Refract Surg 2002; 28:295–301 11. Chang DF. Early rotational stability of the longer Staar toric intraocular lens; fifty consecutive cases. J Cataract Refract Surg 2003; 29:935–940 12. De Silva DJ, Ramkissoon YD, Bloom PA. Evaluation of a toric intraocular lens with a Z-haptic. J Cataract Refract Surg 2006; 32:1492–1498
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13. Mendicute J, Irigoyen C, Aramberri J, Ondarra A, Monte´sMico´ R. Foldable toric intraocular lens for astigmatism correction in cataract patients. J Cataract Refract Surg 2008; 34:601–607 14. Hayashi K, Hayashi H, Hayashi F. Topographic analysis of the changes in corneal shape due to aging. Cornea 1995; 14: 527–532 15. Gudmundsdottir E, Jonasson F, Jonsson V, Stefa´nsson E, Sasaki H, Sasaki K. ‘‘With the rule’’ astigmatism is not the rule in the elderly. Reykjavik Eye Study: a population based study of refraction and visual acuity in citizens of Reykjavik 50 years and older. Iceland-Japan Co-Working Study Groups. Acta Ophthalmol Scand 2000; 78:642–646 16. Goto T, Klyce SD, Zheng X, Maeda N, Kuroda T, Ide C. Genderand age-related differences in corneal topography. Cornea 2001; 20:270–276 17. Gudmundsdottir E, Arnarsson A, Jonasson F. Five-year refractive changes in an adult population; Reykjavik Eye Study. Ophthalmology 2005; 112:672–677 18. Monte´s-Mico´ R, Ferrer-Blasco T. Distribution of refractive errors in Spain. Doc Ophthalmol 2000; 101:25–33 19. Ferrer-Blasco T, Gonza´lez-Me´ijome JM, Monte´s-Mico´ R. Agerelated changes in the human visual system and prevalence of refractive conditions in patients attending an eye clinic. J Cataract Refract Surg 2008; 34:424–432 20. Douthwaite W, Pardhan S. Accuracy and repeatability of the Topcon K-3500 autokeratometer on calibrated convex surfaces. Cornea 1995; 14:253–257 21. Pardhan S, Beesley J. Measurement of corneal curvature in young and older normal subjects. J Refract Surg 1999; 15:469–474 22. Riley AF, Grupcheva CN, Malik TY, Craig JP, McGhee CNJ. The Auckland Cataract Study: demographic, corneal topographic and ocular biometric parameters. Clin Exp Ophthalmol 2001; 29:381–386 23. Asano K, Nomura H, Iwano M, Ando F, Niino N, Shimokata H, Miyake Y. Relationship between astigmatism and aging in middle-aged and elderly Japanese. Jpn J Ophthalmol 2005; 49:127–133 24. Lam AKC, Chan CCY, Lee MH, Wong KM. The aging effect on corneal curvature and the validity of Javal’s rule in Hong Kong Chinese. Curr Eye Res 1999; 18:83–90 25. Wang L, Misra M, Koch DD. Peripheral corneal relaxing incisions combined with cataract surgery. J Cataract Refract Surg 2003; 29:712–722 26. Budak K, Friedman NJ, Koch DD. Limbal relaxing incisions with cataract surgery. J Cataract Refract Surg 1998; 24: 503–508 27. Mu¨ller-Jensen K, Fischer P, Siepe U. Limbal relaxing incisions to correct astigmatism in clear corneal cataract surgery. J Refract Surg 1999; 15:586–589
J CATARACT REFRACT SURG - VOL 35, JANUARY 2009
First author: Teresa Ferrer-Blasco, PhD Optics Department, Faculty of Physics, University of Valencia, Burjassot (Valencia), Spain
Prevalence of corneal astigmatism before cataract surgery Teresa Ferrer-Blasco, PhD, Robert Monte´s-Mico´, PhD, Sofia C. Peixoto-de-Matos, OD, Jose´ M. Gonza´lez-Me´ijome, PhD, Alejandro Cervin˜o, PhD
PURPOSE: To analyze the prevalence and presentation patterns of corneal astigmatism in cataract surgery candidates. SETTING: University of Valencia, Valencia, Spain. METHODS: Refractive and keratometric values were measured before surgery in patients having cataract extraction. Descriptive statistics of refractive and keratometric cylinder data were analyzed and correlated by age ranges. RESULTS: Refractive and keratometric data from 4540 eyes of 2415 patients (mean age 60.59 years G 9.87 [SD]; range 32 to 87 years) differed significantly when the patients were divided into 10-year subsets. There was a trend toward less negative corneal astigmatism values, except the steepest corneal radius and the J45 vector component, in older groups (Kruskal-Wallis, P<.01). In 13.2% of eyes, no corneal astigmatism was present; in 64.4%, corneal astigmatism was between 0.25 and 1.25 diopters (D) and in 22.2%, it was 1.50 D or higher. CONCLUSIONS: Corneal astigmatism less than 1.25 D was present in most cataract surgery candidates; it was higher in about 22%, with slight differences between the various age ranges. This information is useful for intraocular lens (IOL) manufacturers to evaluate which age ranges concentrate the parameters most frequently needed in sphere and cylinder powers and for surgeons to evaluate which IOLs provide the most effective power range. J Cataract Refract Surg 2009; 35:70–75 Q 2009 ASCRS and ESCRS
Expectations and demands of patients require correction of refractive errors after cataract surgery. In addition to spherical refractive errors, astigmatism should be addressed at the time of surgery to achieve the best postsurgery refractive outcomes. Astigmatism can be reduced or eliminated by a variety of surgical techniques, including selective positioning of the phacoemulsification incision, corneal relaxing incisions, limbal relaxing incisions, and excimer laser keratectomy. All these methods have limitations such as the degree of astigmatism to be treated or the long-term mechanical instability, and postoperative outcomes are subject to many variables such as age, magnitude, incision number, depth, and length.1 Toric intraocular lens (IOL) implantation is another option for the correction of corneal astigmatism in cataract patients. Toric IOLs have been used clinically since first described by Shimizu et al.2 in 1994. Previous investigations3–13 have analyzed the results of implantation of several toric IOL models and found this method to be effective in correcting corneal astigmatism. However, the prevalence of different amounts of corneal astigmatism in 70
Q 2009 ASCRS and ESCRS Published by Elsevier Inc.
cataract surgery candidates has not been fully evaluated. Analysis of this would provide valuable information to cataract surgeons and IOL manufacturers. When designing a toric IOL, a critical issue is selecting the range of parameters to optimize the manufacturing process, stock, and clinical application. Currently, we know of no reports of systematic data about the prevalence of the degrees of corneal astigmatism in large population cohorts that could be used to plan large-scale manufacturing. Furthermore, it is well known that corneal astigmatism changes significantly with age.14–19 With increasing numbers of patients having cataract and refractive lens exchange surgeries, it becomes mandatory to analyze such data in different age groups. This study reports the prevalence of corneal astigmatism in a large sample of patients before cataract surgery and describes the distribution in age subsets. This information will allow determination of the most prevalent keratometric conditions so better strategies for IOL manufacturing and clinical prescription can be selected. 0886-3350/09/$dsee front matter doi:10.1016/j.jcrs.2008.09.027
71
CORNEAL ASTIGMATISM BEFORE CATARACT SURGERY
PATIENTS AND METHODS This prospective study measured refractive and keratometric data in cataract surgery candidates in the context of presurgical screening. Inclusion criteria included presence of cataract and age between 30 and 90 years. Exclusion criteria included irregular astigmatism, corneal disease, previous corneal or intraocular surgery, and history of ocular inflammation. All eyes were examined before the cataract surgery procedure, including a complete ophthalmological examination, manifest and cycloplegic refractions, slitlamp examination, applanation tonometry, and ophthalmoscopy through dilated pupils. For part of the analysis, the sample was divided into 6 age groups: 40 years or younger, 41 to 50 years, 51 to 60 years, 61 to 70 years, 71 to 80 years, and 81 to 90 years. The analysis of corneal astigmatism was performed using the autokeratorefractometer RM-KR-8800 (Topcon). This system performs an automatic measurement of the central keratometry of the eye (flat axis and steep axis).20,21 Three measurements were performed in each eye. The tenets of the Declaration of Helsinki were followed, and informed consent was obtained from all patients after the nature and possible consequences of the study were explained. Institutional review board approval was also obtained. Data were analyzed using the SPSS statistical package, version 16.0. The Kolmogorov-Smirnov test was used to check normal distribution of variables. This is particularly important because distribution of astigmatic values is skewed toward lower values. Bivariate correlations were evaluated using the Pearson coefficient for normally distributed variables and the Spearman rank correlation coefficient for nonnormally distributed variables. Differences between age groups were analyzed using an analysis of variance for normally distributed variables and the Kruskal-Wallis (KW) test for nonnormally distributed variables. The t test was used with parametric variables and the Wilcoxon signed rank test with nonparametric variables to compare refractive and corneal cylinder for the entire sample and for the age groups separately. The level of significance was established at a Z 0.05.
Submitted: July 25, 2008. Final revision submitted: September 24, 2008. Accepted: September 24, 2008.
RESULTS The study comprised 4540 eyes in 2415 patients. Patient demographics are shown in Table 1. Figure 1 is a histogram with frequency distribution of corneal astigmatism values for the entire sample showing that 13.2% of eyes did not present corneal astigmatism, 64.6% had corneal astigmatism between 0.25 and 1.25 diopters (D), and 22.2% had astigmatism of 1.50 D or higher. Table 2 presents the descriptive values in the 6 age groups and the statistical significance for comparison among groups. The number of eyes in the 6 age groups was as follows: 40 years or younger, 58 eyes; 41 to 50 years, 714 eyes; 51 to 60 years, 1571 eyes; 61 to 70 years, 1383 eyes; 71 to 80 years, 666 eyes; and 81 to 90 years, 128 eyes. All refractive values showed significant changes between age groups (P!.01, K-W) except the steep keratometric radius (P Z .065, K-W) and the J45 value (P Z .646, K-W). Figure 2 shows the box plot for values of corneal cylinder in the age groups. As shown in Table 2, corneal cylinder tended to become more positive in older groups. A quick inspection of the graph shows that the most homogeneous subsample of corneal astigmatism was in the 61 to 70 year subset. This is probably the most likely population to receive an IOL after cataract surgery. Figure 3 shows the distribution of corneal astigmatism in 0.50 D steps within each age group. There is a slight apparent trend for the graphs to be skewed toward the positive values, as expected from the descriptive data in Table 2 and the trend lines in Figure 2. DISCUSSION Corneal astigmatism can be managed surgically using corneal, relaxing, or limbal incisions and excimer laser
Table 1. Demographic characteristics of patients. Characteristic
From the Optometry Research Group (Ferrer-Blasco, Monte´s-Mico´, Cervin˜o), Optics Department, University of Valencia, Valencia, Spain; the Optica Queiros Lda. Po´voa de Lanhoso (Peixoto-de-Matos), and the Department of Physics (Optometry) (Gonza´lez-Me´ijome), School of Sciences, University of Minho, Braga, Portugal. No author has a financial or proprietary interest in any material or method mentioned. Supported in part by Universitat de Valencia research grants to Robert Monte´s-Mico´ (#UV-AE-20070225#), Jose Manuel Gonza´lez-Me´ijome (#UV-ESTPC-08-2054#), Alejandro Cervin˜o (#UV-AE08-2291#), and Red Tema´tica de Optometrı´a (Ministerio de Ciencia e Innovacio´n SAF2008-01114-E). Corresponding author: Teresa Ferrer-Blasco, PhD, Optics Department, Faculty of Physics, University of Valencia, C/ Dr. Moliner, 50. 46100, Burjassot (Valencia), Spain. E-mail: [email protected].
Number of eyes/patients Mean age (y) G SD Range of age (y) Gender (male/female) Mean corneal astigmatism (D) G SD Range of corneal astigmatism (D) Mean keratometry (D) Flat Steep Range of keratometry (D) Mean sphere (D) G SD Range of sphere (D) Mean cylinder (D) G SD Range of cylinder (D) SD Z standard deviation
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Value 4540/2415 60.59 G 9.87 32 to 87 768/1647 0.90 G 0.93 0.25 to 6.75 43.48 G 1.61 44.08 G 1.59 38 to 48 0.35 G 5.28 16 to C12 0.86 G 0.95 0.25 to –7.00
CORNEAL ASTIGMATISM BEFORE CATARACT SURGERY
55 50 45 40 35 30 25 20 15 10 5 0
7,00 45,66
6,00
13,14
12,10 12,63 7,21 3,63 2,30 1,59 0,88 0,38 0,33 0,00 0,02 0,09 0,02
Corneal Astigmatism (D)
Corneal Astigmatism (D)
Frequency (%)
72
5,00 4,00 3,00 2,00 1,00 0,00
Figure 1. Histogram of the corneal astigmatism (D) distribution in the entire sample (4540 eyes).
keratectomy. The limitations, advantages, and disadvantages have been fully discussed.4 The use of toric IOLs to correct corneal astigmatism is a growing surgical option. The present study attempted to determine the prevalence of different amounts of corneal astigmatism as a function of age, as such data are not available in the literature surveyed by the authors. As previously said, the descriptive analysis provided is valuable information for cataract surgeons and IOL manufacturers. Results revealed that 34.8% of the eyes had corneal astigmatism equal to or higher than 1.00 D. The mean corneal astigmatism (1.02 D) was slightly higher than the value reported by Riley et al.22 (0.89 D) before cataract surgery; the range of values was similar (up to 6.50 D in Riley et al. and up to 6.75 D in the present study). The steepest corneal meridian in the present study was similar to the mean value observed by Riley et al. (44.08 D and 44.13 D, respectively). The results in this study also show a trend toward more hyperopic values in the older groups, agreeing with previous studies conducted by us18,19 and with data from the
<=40
41-50
51-60
61-70
71-80
81-90
Age Group (years) Figure 2. Box plot of values of corneal cylinder (D) in the 6 age groups. Bold lines within boxes represent the median (Q2 or percentile 50%), upper and lower limits of the box represent the first quartile (Q1 or percentile 25%) and third quartile (Q3 or percentile 75%), and bars represent the extreme values (maximum and minimum observations). Asterisks and circles represent outliers.
Reykjavik Eye Study,17 which showed a hyperopic shift of about C0.3 D after 5 years of follow-up. The authors of th Reykjavik Eye Study also found significant changes in corneal astigmatism in older people, agreeing with the results reported here. Most cataract surgery candidates in the present cohort were women, following a trend in previous studies.22 The values of the blur component from vector decomposition of the refraction were significantly higher than the values obtained recently by FerrerBlasco et al.19 in a population attending an eye clinic, even in the older groups. Only the mean values for women in that study showed mean blur values similar to those reported in the present study. This could be justified by the higher values of astigmatism in some
Table 2. Descriptive statistics (diopters) for the 6 age groups and statistical significance of differences between groups. Manifest refractions in conventional script notation [S (sphere), C (cylinder) f (axis)] were converted to power vector coordinates (M, J0, J45) and overall blurring strength (B) by the following formulas: M Z S C C/2; J0 Z (C/2) cos (2f); J45 Z (C/2) sin (2f); B Z (M2 C J02 C J452)1/2. Age Group (Y)
Sphere
Cylinder
Flat K
Steep K
M
J0
J45
Blur
% 40 41–50 51–60 61–70 71–80 81–90 Total Significance*
2.52 1.31 0.77 0.04 0.59 0.63 0.35 0.004
0.75 1.06 0.85 0.74 0.87 1.08 0.86 0.001
42.77 43.19 43.42 43.61 43.64 43.77 43.48 0.001
43.73 44.14 44.04 44.03 44.21 44.14 44.08 0.059
2.15 1.84 1.20 0.40 0.16 0.12 0.77 0.003
0.26 0.20 0.03 0.14 0.22 0.47 0.04 0.001
0.01 0.01 0.01 0.01 0.01 0.04 0.01 0.646
4.69 6.09 4.36 3.00 2.39 2.73 3.89 0.001
*
Kruskal-Wallis test
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Figure 3. Frequency distribution of corneal astigmatism in 0.50 D steps for the 6 age groups.
of the patients enrolled. Although it is well known that corneal astigmatism changes with age from with the rule to against the rule,14,15,17,23 in the present population the astigmatic changes observed could also be due to changes in the crystalline lens as a consequence of aging, scleroses, and loss of transparency. Despite this, our results agree with the trends found in other studies24 that document lack of significant changes in steep keratometric values, attributing the changes in amount and axis of corneal astigmatism with age to changes in the horizontal meridian (usually the flat meridian in young subjects). Table 3 shows the range of spherical and cylindrical powers of the main toric IOLs currently available. The lowest cylindrical correction is 1.0 D and the highest, 12.0 D. One should consider that there is a difference between the cylindrical power of the toric IOL and the magnitude of corneal astigmatism that it is capable of correcting. At the corneal plane, there is a reduction of about 30% of the cylindrical power (ie, cylindrical power of the toric IOL of 2.0 D and 3.5 D corrects
1.4 D and 2.4 D of corneal astigmatism, respectively). Thus, the lowest and the highest cylindrical powers for which correction can be achieved are 0.7 D and 8.4 D of corneal astigmatism, respectively. To clinically apply the outcomes in this study, surgeons should correlate the proportions reported with the cylinder power nomograms provided by toric IOL manufacturers. For example, using the cylinder power selection nomogram for the Staar toric IOL (Table 4) and assuming astigmatically neutral surgery in most of the eyes in our sample, 64.6% of eyes showing corneal astigmatism up to 1.25 D will not be covered by the options provided and only 17.0% will be corrected with the toric IOL proposed. In addition, 3.2% of patients will be corrected with the toric IOL with the aid of limbal relaxing incisions. Most surgeons who treat astigmatism in cataract patients tend to use limbal or corneal relaxing incisions.25–27 There are some potential downsides to this technique, however. For example, this procedure may be somewhat unpredictable and imprecise. In addition, it is dependent on variable
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CORNEAL ASTIGMATISM BEFORE CATARACT SURGERY
Table 3. Range of spherical and cylindrical powers and corneal astigmatism capable of correcting for the toric IOLs on the market.
Company
Range of Spherical IOL Power (Increment)
Range of Cylindrical IOL Power (Increment)
Range of Corneal Astigmatism Able to Correct
Percentage of Eyes Susceptible to Correction*
AcrySof Toric SN60T3/T4/T5/T6/ T7/T8/T9
C10.0 to C30.0 (0.5)
1.5 to 6.0 (1.0)
1.0 to 4.2
34.2
Torica-S
3.0 to C14.0 (1.0) C15.0 to C25.0 (0.5) C26.0 to 31.0 (1.0)
2.0 to 12.0 (1.0)
1.4 to 8.4
22.3
T-Flex 573T/623T
C6.0 to C26.0 (0.5) 10.0 to C35.0 (0.5)
1.0 to 6.0 (1.0) 1.0 to 11.0 (0.25)
0.7 to 4.2 0.7 to 7.7
40.5 41.2
AA4203TF AA4203TL
C24.0 to C28.5 (0.50) C9.5 to C23.5 (0.50)
2.0 and 3.5 2.0 and 3.5
1.4 and 2.4 1.4 and 2.4
17.1 17.1
Acri.Comfort 643TLC Acri.Comfort 646TLC
0 to C40 10 to C32
1.0 to 12.0 (1.0) 1.0 to 12.0 (1.0)
0.7 to 8.4 0.7 to 8.4
41.2 41.2
Intraocular Lens
Alcon
HumanOptics
Rayner
Staar
Zeiss
IOL Z intraocular lens * Considering only corneal astigmatism in the eyes in the present study (n Z 4540)
healing responses, the skill of the surgeon, and the amount of cylinder that can be corrected is limited. The risks for overcorrection, perforation, and wound gape also need to be considered. Table 3 shows the percentage of eyes in this study that are susceptible to correction (considering corneal astigmatism only). The percentage varied from 17% to 41% as a function of the range of cylindrical power of the different IOLs. Obviously, larger ranges showed a larger percentage. The best ratio percentage versus range of cylinder manufactured was obtained for the Rayner and Alcon IOLs (largest percentage with least range manufactured). However, this proportion seems to change greatly across the age groups as shown in Figure 3, and this should be taken into account by IOL manufacturers and surgeons. For this particular case, Figure 4 shows the proportion of patients within each age group who would be fitted according to the nomogram shown in Table 4 for the Staar toric IOL. This graph shows that about 13% of cataract surgery candidates do not have corneal astigmatism (range from 0% in younger group to 18% in older groups), 65% have corneal astigmatism between 0.25 D to 1.25 D, 15% have astigmatism between 1.50 D and 2.25 D, 4% have corneal astigmatism between 2.50 D and 3.00 D, and 3% have astigmatism equal to or higher than 3.25 D. In conclusion, when choosing a toric IOL to correct astigmatism, a pertinent issue for the surgeon is to determine which IOL covers the larger range of
astigmatic values to cover the overall profile of the pool of candidates to undergo a surgical procedure in each particular clinical population (depending on geographical, socioeconomic, and clinical criteria). The prevalence of corneal astigmatism reported in this study of a large cohort of cataract surgery candidates comprising a wide age range will also help manufacturers match their toric IOL parameter range
Table 4. Nomogram for selection of astigmatic IOL power from values of corneal astigmatism assuming astigmatically neutral surgery for the Staar toric IOL (reproduced from the manufacturer).
Corneal Astigmatism (D) With the rule* 1.5 D to 2.25 D 2.5 D to 3.00 D 3.25 D or morez Against the rule* 1.5 D to 2.25 D 2.5 D to 3.00 D 3.25 D or morez
IOL Cylinder Power (D)
Expected Cylinder Correction from IOL (D)†
2.0 D 3.5 D 3.5 D CLRIs
1.5 D 2.25 D 2.25 D
2.0 D 3.5 D 3.5 D CLRIs
1.5 D 2.25 D 2.25 D
IOL Z intraocular lens; LRI Z limbal relaxing incision * With the rule, steep corneal meridian between 46 and 134 ; against the rule, steep corneal meridian between 0 to 45 and 135 and 180 † Approximate value of the equivalent IOL cylinder power at the cornea z Combine with astigmatic keratotomy or LRIs
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100
NO ASTIGMATISM [1.50,2.25] [>=3.25]
Frequency (%)
80
[0.25,1.25] [2.50,3.00]
60 40 20 0
<=40
41-50
51-60
61-70
71-80
81-90
Age Group Figure 4. Simulation of frequency of IOL prescription for the patients implanted with the Staar toric IOL according to the manufacturer’s nomogram in Table 4.
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First author: Teresa Ferrer-Blasco, PhD Optics Department, Faculty of Physics, University of Valencia, Burjassot (Valencia), Spain