Cataract surgery is a commonly performed ocular procedure worldwide. With technological advancements in cataract removal, expectations for better quality of vision are rising. It not only addresses the dependency on the spectacle, but also improves quality of life. With the development of premium intraocular lenses (IOL) and small incisions to remove cataracts, cataract surgery has become a refractive surgery. Spherical error can be adjusted in the IOL to be implanted, but astigmatism requires special attention. European studies mention that one-third of patients undergoing cataract surgery have corneal astigmatism of ≥1 D; 22% have >1.5 D; and 8% have >2.0 D astigmatism.1–3 A MEDLINE search retrieved only 2 studies in Indian populations: one by Prasher and Sandhu in urban population4 and the other by Moulick et al5 in western Indian population.
Data from rural population of central India are lacking regarding the prevalence of astigmatism in patients presenting for cataract surgery. Therefore, the present study was designed to determine an estimate for astigmatism and the need for toric IOL in rural population of India.
This prospective, observational study was performed from January 2017 to May 2018 in a tertiary eye care center serving rural population in central India. Written informed consent was taken from all patients, and the study was conducted in accordance with the tenets of the Declaration of Helsinki. The ethical committee of the hospital approved the study.
Consecutive patients presenting for cataract surgery during the study period were included in the study. The data collected include age, sex, and keratometry (K). A single trained observer performed keratometry with auto refractokeratometer (Topcon KR- 800, Topcon Medical Systems, INC. Oakland, NJ). This keratometer performs K readings in the central 3.2 mm of the cornea. Both flat (K1, angle of flat axis and K2, angle of steep axis) and steep axis measurements were determined. At least 3 measurements were taken for each eye and average of 3 was considered for the study purpose. A-scan biometry (Axis-II PR Biometer, Quantel Medicals, France) was used for axial length measurement and IOL power calculation.
The exclusion criteria included corneal opacity, pterygium, a previous history of glaucoma, corneal, retinal, lid and squint surgery, corneal trauma, inflammatory eye diseases, and keratoconus. Patients with systemic diseases like diabetes and collagen vascular diseases were excluded from the study.
With the rule astigmatism (WTR) was designated when the steepest meridian of the surface of the cornea was within 90 ± 30 degrees. Against the rule astigmatism (ATR) was designated when the steepest meridian was within 180 ± 30, and oblique mean astigmatism was designated when the astigmatism was neither ATR nor WTR, and the steepest meridian was between 120 and 150 degree or 30 and 60 degree. Axial length was defined as the distance between the anterior surface of the cornea and the fovea.6
The data were entered in an Excel sheet [Software version 14.1.0 (110310)/2011] (Microsoft Corporation, Redmond, WA), and statistical analysis was performed with SPSS version 13.0 (SPSS Inc, Chicago, IL). Chi-square test and t test were used for categorical and continuous variables, respectively. Values are expressed as mean ± standard deviation (SD) and percentage where appropriate. A P value <0.05 was considered statistically significant.
A total of 1000 eyes from 880 patients with cataracts were studied, comprising 560 males (63.63%) and 320 (36.36%) females. The mean age of the patients was 65.1 ± 10.12. Most patients (n = 360) were in the age group of 50 to 59 years. The age distribution of astigmatism is shown in Table 1. The mean astigmatism (on Keratometry) was 0.89 ± 0.63 D (range 0.13–9.01 D). The mean corneal astigmatism was 0.96 ± 0.86 in males and 0.85 ± 0.83 in females (P = 0.12). We found that 179 eyes (17.9%) had astigmatism from 1.0 to 1.49 D, 34 eyes (3.4%) had astigmatism between 2.00 and 2.5 D, and 20 eyes (2%) had astigmatism >3 D. Overall 325 eyes (32.5%) had astigmatism >1.00 D. The range of astigmatism and number of eyes are depicted in Table 2. The magnitude of corneal astigmatism increased with age (P = 0.001). ATR was seen in 446 eyes (44.6%), and was followed by oblique astigmatism in 320 eyes (32%) and WTR in 207 eyes (20.7%). No astigmatism was seen in 27 eyes (2.7%). ATR increased with age and reached a maximum between 60 and 69 years. Ages and types of astigmatism are presented in Table 3. Male eyes had flatter corneal curvature (K1 = 41.22 ± 1.52 D and K2 = 42.22 ± 1.50 D) than female eyes (K1 = 43.22 ± 1.32 and K2 = 44.23 D) (P = 0.02). The mean axial length was 25.59 mm. There was no significant difference in axial length in males (25.00 ± 1.12 mm) and females (24.98 ± 1.10 mm) (P = 0.12). The mean IOL power was 21.05 ± 1.0 D. There was no significant difference between IOL power in males (20.55 ± 0.2) and females (20.67 ± 0.2) (P = 0.15).
Technological advancements in the calculation of IOL power during cataract surgery have reduced postoperative spherical refractive error. However, residual preoperative corneal cylindrical power affects postoperative visual outcomes. Various methods have been designed to manage corneal astigmatism. These include limbal relaxing incisions,7 opposite clear corneal incision,8 femtosecond laser-assisted astigmatic keratectomy,9 and laser refractive procedures.10 These methods have various limitations, such as unpredictability in correction of astigmatism, applicability only in low to moderate astigmatism (0.5–1.5 D), regression of astigmatism, infection risk, and high equipment cost (Excimer and femtosecond laser). Therefore, the need for toric IOL implantation to correct astigmatism during cataract surgery is growing. The present study was performed in a rural Indian population to determine the frequency of preoperative corneal astigmatism in patients presenting for cataract surgery.
Analysis of data showed that 32.5% of the eyes having corneal astigmatism is less than previously reported authors: 40.63% by Prashar and Sandhu,4 48.3% by Mohammadi et al,11 34.8% by Ferrer-Blasco et al,3 41.74% by De Bernardo et al,12 45.45% by Guan et al,13 42% by Day et al,14 41.3% by Curragh and Hassett,15 47.27% by Yuan et al,16 66.9% by Isyaku et al,17 and 33.9% by Miyake et al.18 The prevalence of astigmatism in our case series was >27.95% reported by Hoffmann and Hütz.2 Variability in the prevalence of corneal astigmatism in different populations could be due to the different instruments used to determine the astigmatism, environmental and genetic factors, and inclusion criteria. The detailed analysis of the reasons for the low frequency of astigmatism is beyond the scope of this study.
In the present case series, 1.51 to 2.99 D astigmatism represented 12.6% of all cases. This value is less than those reported by Yuan et al (23.37%),16 Ferrer-Blasco et al (19%),3 Mohammadi et al (18.78%),11 Hoffmann and Hütz (31.75%),2 Miyake et al (13%),18 Moulick et al (18.8%),5 and Curragh and Hassett (19%).15 A 1.5 to 3.00 D range of astigmatism is important because 20% of patients undergoing cataract surgery for age related cataracts fall into this range of astigmatism.19 These data may be helpful to IOL manufacturers, and hospital and insurance companies for planning strategies.
The mean corneal astigmatism in the current study was 0.89 ± 0.63 D (range 0.13–9.01 D). A higher mean astigmatism has been reported in previous studies: 0.98 ± 0.78 by Hoffmann and Hütz,2 1.15 ± 0.84 by Yuan et al,16 1.12 ± 1.37 by Mohammadi et al,11 1.04 ± 1.04 by Prashar and Sandhu,4 1.01 ± 0.69 by Chen et al,20 1.03 ± 0.73 by Khan and Muhtaseb,21 1.07 ± 0.73 by Guan et al,13 and 0.90 ± 0.93 by Ferrer-Blasco et al.3 The mean corneal astigmatism was marginally higher in males than females (males 0.96 D and females 0.85 D; P = 0.12). This finding may be because males outnumbered females in our case series (males 560, 63.63%, and females 320, 36.36%). Yuan et al16 have found higher rates of corneal astigmatism in females (1.19 D) than males (1.11 D). Mohammadi et al11 have also reported higher mean corneal astigmatism in females (1.13 D) than males (1.11 D).
Most patients (n = 360, 36%) were in the age group of 50 to 59 years, which was followed by 60 to 69 years (n = 330, 33%). The mean age of the patients was 65.1 ± 10.12 years. Studies have reported variable mean ages. A higher mean age than that in our study has been reported by Yuan et al (69.80 ± 11.15),16 Chen et al (70.56 ± 9.55),20 Khan and Muhtaseb (75.54 ± 0.71),21 Guan et al (72.27 ± 11.59),13 De Bernardo et al (71.89 ± 10.19),12 and Lekhanont et al (68.21 ± 9.19).22 A lower mean age has been reported by Mohammadi et al (64.92 ± 11.48),11 Parashar et al (59.54 ± 10.96),4 Ferrer-Blasco et al (60.59 ± 9.87)3 and Moulick et al (61 ± 10).5 The mean age is higher in our study due to late presentation of patients for cataract surgery in the rural population of India. Socioeconomic status in the rural population is another factor contributing to the higher mean age.
ATR was seen in 44.6% of cases. The higher percentage of ATR in the present case series than those of WTR (20.7%) has been noted in previous studies by Prasher and Sandhu (ATR 51.88%, WTR 28.34%),4 Yuan et al (ATR 52.41%, WTR 30.36%),16 Isyaku et al (ATR 56.66%, WTR 41.78%),17 Moulick et al (ATR 51.1%, WTR 27.8%),5 Guan et al (ATR 50%, WTR 31.82%),13 and Mohmmadi et al (ATR 46.8%, WTR 36.9%).11 However, De Bernardo et al (ATR 39%, WTR 44%)12 and Hoffman and Hütz (ATR 34.4%, WTR 46.8%) have noted a higher percentage of WTR than ATR.2 ATR increased with age, a finding consistent with those from other studies.2,3,4,23,24 The exact cause for the increase of ATR with age is not known. Possible factors may include pressure from the eyelid and physiological changes in the corneal structure with age.
The oblique astigmatism in the present study was 32%, which is higher than that reported in most previous studies: 17.22% by Yuan et al,16 23.9% by Prasher and Sandhu,4 18.18% by Guan et al,13 18.9% by Hoffmann and Hütz,2 16.2% by Mohammadi et al,11 21.1% by Moulick et al,5 and 15% by Riley et al.25 Detailed analysis of the high frequency of oblique astigmatism in the present case series was beyond the scope of the study but may have been due to genetic factors.
The cornea in females was significantly steeper than in males (P
= 0.02), in agreement with findings from Hoffman and Hütz, Chen et al, and Prasher and Sandhu.2,20,4 The literature shows steep cornea in females is compensated by a shorter axial length.26,27 In our study, we found that the average axial length in females (24.98 ± 1.10 mm) was shorter when compared with males (25.00 ± 1.12 mm) (P = 0.12). Another proposed reason for this difference is the differences in the levels of sex hormones, which might affect the elasticity of the cornea.28 For the same reason, no significant difference was noted in IOL power between males (20.55 ± 0.2) and females (20.67 ± 0.2).
The current study is the first to report the frequency of corneal astigmatism before cataract surgery in rural population in central India. The location was a hospital-based tertiary care center serving a large population of the district. The data collected in a prospective manner over a period of 17 months may be helpful to establish the need for toric IOL in the population. A PubMed search revealed 2 studies in Indian populations: Prasher and Sandhu4 studied the prevalence of corneal astigmatism in urban population of India in a retrospective manner, and another study from western India by Moulick et al5 was a prospective case series of 1-year duration, which studied only 223 patients.
The limitation of the present case series was the use of an auto refractokeratometer for the measurement of astigmatism because of the lack of availability of an optical biometer. Auto refractokeratometers measure astigmatism of the anterior surface of the cornea. For the minimum postoperative residual corneal astigmatism, consideration of the posterior corneal curvature is imperative. Koch et al have suggested the role of posterior corneal curvature in toric lens calculation; in eyes with WTR astigmatism, the total corneal astigmatism is overestimated by 0.5 to 0.6 D, whereas for ATR astigmatism, it is underestimated by 0.2 to 0.3D.29 The Barrett toric lens calculator considers the posterior corneal surface for total corneal astigmatism.
Another limitation of the study is that it was a hospital-based study, which did not include community individuals. Therefore, extrapolation of data should be done with caution.
To summarize, corneal astigmatism of ≥1.00 D was found in 32.5% in the studied Indian rural population. These eyes should benefit from toric IOL implantation. ATR was seen in 44.6% cases and was found to increase with age. The case series indicated the highest percentage of patients with oblique astigmatism in the population, a finding that requires further study. The frequency of toric IOL reported in this study may help manufacturers tailor their IOLs to the needs of the market.
The authors acknowledge Dr. Avinash Turankar, Associate Professor, Department of Pharmacology, Government Medical College Nagpur, Maharashtra, India, for statistical assistance.
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