Indian Journal of Ophthalmology

ORIGINAL ARTICLE
Year
: 2001  |  Volume : 49  |  Issue : 1  |  Page : 31--35

Postoperative Astigmatism after Intracapsular Cataract Surgery: Results of a Randomised Controlled Trial in Nepal


Dipak Nag1, Albrecht Hennig2, Allen Foster3, Jennifer R Evans4, Damodar Pradhan2, Gordon J Johnson5, Richard PL Wormald6,  
1 MSc. London School of Hygiene and Tropical Medicine, London, UK
2 MD. Lahan Eye Hospital, Nepal
3 FRCOphth. London School of Hygiene and Tropical Medicine, London, UK
4 MSc. Moorfields Eye Hospital, London, UK
5 FRCOphth. Glaxo Department of Ophthalmic Epidemiology, Institute of Ophthalmology (UCL); and Department of Epidemiology and Institute of Ophthalmology (UCL), London, UK
6 FRCOphth. Moorfields Eye Hospital, London, UK

Correspondence Address:
Dipak Nag
MSc. London School of Hygiene and Tropical Medicine, London, UK




How to cite this article:
Nag D, Hennig A, Foster A, Evans JR, Pradhan D, Johnson GJ, Wormald RP. Postoperative Astigmatism after Intracapsular Cataract Surgery: Results of a Randomised Controlled Trial in Nepal.Indian J Ophthalmol 2001;49:31-35


How to cite this URL:
Nag D, Hennig A, Foster A, Evans JR, Pradhan D, Johnson GJ, Wormald RP. Postoperative Astigmatism after Intracapsular Cataract Surgery: Results of a Randomised Controlled Trial in Nepal. Indian J Ophthalmol [serial online] 2001 [cited 2020 Jun 5 ];49:31-35
Available from: http://www.ijo.in/text.asp?2001/49/1/31/22708


Full Text



Given the fact that uncorrected aphakia after cataract surgery is an important cause of poor outcome,[1,2] it is recommended that there be a move towards the routine use of intraocular lens (IOL) in situations where appropriate skills and facilities can ensure safety and affordability.[3,4]

To investigate the use of anterior chamber intraocular lens (AC IOL) implantation after standard intracapsular cataract extraction (ICCE) as a primary procedure in developing countries, a randomised controlled trial was carried out in Lahan, Nepal. A detailed description of the study has been reported elsewhere.[5] The analysis provided evidence that in rural areas of developing countries, AC IOL can be implanted safely after routine ICCE.

One of the most important complications of cataract surgery is induced astigmatism.[6] It has been found that excessive postoperative astigmatism remains a significant problem delaying visual rehabilitation and limiting the final visual outcome.[7] In this paper we analyse the data from the Lahan study to investigate the occurrence of postoperative astigmatism.

 Materials and Methods



 Participants



Two thousand patients (age 4-64 years) attending Lahan Eye Hospital, Neptal, with bilateral cataract reducing vision to 6/36 or less were randomly allocated to receive ICCE with an ACIOL (ICCE/AC IOL, n=1002) or ICCE with aphakic spectacles (ICCE/AS, n=998) in the first operated eye. The sample size was estimated to be capable of detecting a doubling of the risk of poor visual outcome (defined as visual acuity less than 6/60 in the operated eye at one year of follow up) in either group, at the 5% significance level, with a power of 90%, assuming a 4% prevalence of poor visual outcome and follow up rate of 80%. Follow-up examination of the patients with refraction at the hospital one year after surgery was obtained in 502 patients of the ICCE/AC IOL group and 417 of the ICCE/AS group [Table:1]. Only patients who attended the hospital for follow up underwent refraction, and the analysis was based on the data of these patients.

 Surgical methods



Surgery was done under local anaesthesia with 4.5 x loupe magnification. In both groups the surgical procedure was as follows: a corneal-scleral ab-interno incision with a von Graefe knife, a cryo-extraction of the lens, and peripheral iridectomy at 10 or 2 o'clock positions. In the AC IOL group the horizontal corneal diameter was measured and 1mm added to calculate the diameter of the AC IOL. Three diameters of lens were available: 12.0mm, 12.5mm and 13.0mm, all of +19.0 DSph. The lenses were inserted with the haptics at the 12 and 6 o'clock positions. The corneal section was closed with at least five 8.0 virgin silk sutures. At discharge on day 6, patients who had had ICCE only were given +11.0 DSph spectacles.

 Definitions of terms used in the study



Visual acuity was measured with the Snellen E chart and categorised according to WHO criteria: that is, no visual impairment (6/18 or better), visual impairment (<6/18-6/60), severe visual impairment (<6/60-3/60) and blind (<3/60).Presenting visual acuity was the actual functioning vision that a patient had at the time she or he was examined one year after surgery with or without spectacles or IOL.Best corrected visual acuity was the visual acuity with best correction (with both spherical and cylinder lens following refraction).

Post-operative astigmatism is expressed as the power of the cylinder lens needed for best correction. The power of the cylinder lens was measured in intervals of 0.5 Dioptre Cylinder (DCyl). All cylindrical lens power were converted into minus and categorised as "no astigmatism" (0.0 DCyl.), "mild astigmatism" (-0.5 DCyl.), "moderate astigmatism" (>-0.5 to -1.5 DCyl), "severe astigmatism" (>-1.5 to -3.5 DCyl.), and "very severe astigmatism" (>-3.5 D Cyl.). There was only one patient in each level of mild and very severe astigmatism in the ICCE/AS group. Therefore, "no" and "mild" astigmatism were defined as "acceptable astigmatism", "severe" and "very severe" astigmatism as "large astigmatism" [Table:2].The axis of the astigmatism was categorised as "with the rule" when the horizontal corneal meridian is flatter than the vertical, requiring a minus cylinder at or near the 180o meridian. "Against the rule" astigmatism was the opposite.

 Methods of analysis



The STATA software version-5 computer package was used for the analysis. Characteristics of patients in the two groups who attended the hospital were compared to those who did not attend hospital for follow up using 'z' test for means and chi-square test for proportions as appropriate. Few patients who attended the hospital had missing data on refraction. Characteristics of patients who lacked data on refraction were compared to those who had data by using the Wilcoxon rank-sum test and chi-square test as appropriate. Characteristics of the patients with data on refraction who attended the hospital were compared between the ICCE/AC IOL and the ICCE/AS groups using the chi-square test. Univariate analysis was done to arrive at the proportion of astigmatism within the group. Logistic regression was used to look at the effect of type of surgery on astigmatism. A model was made of postoperative astigmatism with randomised groups, age and gender for logistic regression. The overall effect of AC IOL implantation on astigmatism was computed taking into account age and gender. A p-value of ≤0.05 was considered statistically significant.

 Results



Patients in the ICCE/AS group were less likely to return to hospital for follow up than those in the ICCE/AC IOL group[(417 or 41.8% versus 502 or 50.1%) (chi squared =13.9, d.f. =1, p=<0.01)]. In the ICCE/ACIOL group, age and gender were comparable between the hospital attendees and those who did not attend the hospital. In the ICCE/AS group, those who came for follow up were approximately one year younger than those who did not (p=<0.01). In both groups those who had "impaired presenting visual acuity" were more likely to attend the hospital[(in the ICCE/ACIOL group 308 or 61.3% versus 151 or 36.4% (p=<0.01), and in the ICCE/AS group 176 or 42.2% versus 146 or 29.6%, p=<0.01)]. A total of 10 patients (5 in each group) who were blind and attended the hospital lacked information on refraction. There was no difference in terms of age and gender between those who had missing data and those who had data on refraction. The two groups were similar in age and gender but a significantly higher proportion of patients in the ICCE/AS group had "no visual impairment" at presentation [Table:1].

 Prevalence of astigmatism



The range of the cylinder power was from -7.0 to 0.0 DCyl in the ICCE/ACIOL group (mean -1.98, SD 1.15, Median -2.0, percentiles 25%, -2.5, 75%, -1.0) and -5.0 to 0.0 D Cyl in the ICCE/AS group (mean -1.25, SD 0.71, median -1.5, percentiles 25%, 1.5 and 75%, -1.0). The figure shows the distribution of power of the cylinder lens in the two groups.

[Table:2] shows the different categories of astigmatism in the two groups. Fifty (10.0%) patients in the ICCE/ AC IOL group and 68 (16.3%) in the ICCE/AS group had no astigmatism. A significantly higher proportion in ICCE/AS group did not need astigmatic correction (OR=1.73, 95% CI 1.17-2.57, p=<0.01).

"Acceptable astigmatism" (no and mild astigmatism) was found in 60 (12.0%) patients (95% CI 9.1%-14.9%) in the ICCE/AC IOL group and in 69 (16.5%) patients (95% CI 12.9%-20.1%) in the ICCE/AS group. "Moderate astigmatism" was found in 153 (30.4%) patients (95% CI 26.4-34.6%) and in 288 (69.1%) patients (95% CI 64.6%-73.6%); large astigmatism (severe and very severe astigmatism) was found in 289 (57.6%) patients (95% CI 53.1%-61.6%) and in 60 (14.4%) patients (95% CI 11.0%-17.8%) in the ICCE/AC IOL and the ICCE/AS group respectively. Adjustment of the effect of age and gender made little effect on the odds ratios.

Axis of astigmatism :

Against-the-rule astigmatism was seen in 446 (88.8%) (95%CI 86.0%-91.6%) in the ICCE/AC IOL group and 348 (83.5%) (95%CI 79.9%-87.1%) in the ICCE/AS groups. With-the-rule astigmatism was found in only four patients, three in the ICCE/AC IOL and one in the ICCE/AS group. Three patients, all in the ICCE/AC IOL group had oblique (OBL) astigmatism.

 Visual acuity with best correction



[Table:3] shows the visual acuity with best correction; 460 (91.6%) patients (95% CI 89.1%-94.1%) in the ICCE/AC IOL group and 401 (96.2%) (95% CI 94.3-98.1) in the ICCE/AS group had 'no visual impairment' (6/18 or better). (odds ratio 2.28, 95% CI 1.26-4.13, p=<0.01). Adjustment of the effect of age and gender had little effect on the odds ratio (OR=2.24, 95% CI 1.23-4.05, p=<0.01).

 Discussion



An important limitation of this study is the poor followup at hospital at one year - 502 (50.1%) in the AC IOL group and 417 (41.8%) in the spectacle group. Follow up is difficult to achieve in rural areas of developing countries. In this trial 1827 or 91.4% patients were followed up at one year, but approximately half came to the hospital and half were examined in their own villages, where accurate and objective refraction was not possible.

Lower attendance of the ICCE/AS patients in hospital may be for a variety of reasons including: (a) the ICCE/AS group achieved better visual rehabilitation (visual acuity 6/18 or better) compared to the ICCE/AC IOL group; (b) in the ICCE/AS patients had fewer surgery-related complications compared to the ICCE/AC IOL patients; and (c) the ICCE/AS patients had lost their spectacles by one year, had poorer vision and were therefore less able to return for follow up.[5]

In both groups patients who had "impaired presenting visual acuity" (<6/18-6/60) attended the hospital in significantly higher proportion than those examined in the village. This may have led to a biased overestimate of the prevalence of astigmatism in both groups.

The proportion of patients who attended the hospital was significantly lower in the ICCE/AS group compared to the ICCE/AC IOL group, which may have caused an underestimate of the odds ratio calculated between the two groups. It is unlikely that patients who lacked refractive data would have made any difference between the groups.

 Astigmatism



There are no clear definitions for acceptable, moderate and large astigmatism in the literature.[8-10] In this study, acceptable astigmatism was defined as -0.5 to 0.0 D Cyl, assuming that this range would not hamper the patient's vision. The prevalence of astigmatism beyond the acceptable range was high in both groups. This is likely to relate to surgical technique.[11,12]

"Large astigmatism" defined as >-1.5 D Cyl, was approximately four times higher in the ICCE/AC IOL group. Also, the 'presenting visual acuity' was significantly poorer in the ICCE/AC IOL group, but it was possible to improve with refraction, suggesting that the ICCE/AC IOL group does have more uncorrected refractive error. This is confirmed by the finding of greater number of patients with large astigmatism in the ICCE/AC IOL group.

Although the surgical technique was similar in both groups, the IOL itself may cause additional astigmatism due to tension or tilting.[13] The diameter of the AC IOL can influence astigmatism. One study[14] has demonstrated that operative measurement of corneal diameter is not particularly accurate. The authors explained that the diameter of the IOL comes in 0.5mm increments; however, the anterior chamber diameter does not conform to this stepwise increment, hence the IOL may be a fraction of a millimetre too small or too large. In the latter situation the corneo-scleral ring is stretched, inducing astigmatism. Consequently, if the lens remains vertical the corneal curvature will be flattened in the 90 meridian, inducing "against-the-rule" astigmatism. This suggests that the sutures at 90 and to each side need to be a little tighter. Moreover, the 8.0 virgin silk sutures might not keep the wound as tight for as long a period as other suture material. After one year of follow up most of the patients had against-the-rule astigmatism in both the ICCE/AC IOL and the ICCE/AS groups.

A significantly higher proportion of patients in the ICCE/AS group did not need cylinder correction. Refraction in an aphakic eye is more difficult than in a pseudophakic eye. This higher proportion in the ICCE/AS of those who required only spherical lens may be due to measurement bias and not an actual difference in prevalence of astigmatism.

 Visual acuity



With best correction, 460 (91.6%) in the ICCE/ACIOL and 401 (96.2%) in the ICCE/AS group of patients reviewed at the hospital had no visual impairment. Of 42 patients with <6/18 best corrected visual acuity in the ACIOL group, 34 (81.0%) had greater than 1.5 DCyl; this compares with 5 of 16 patients (31.3%) in the aphakic spectacle group [Table:3].

In the original report[5] the authors stated that uveitis and/or secondary glaucoma were significantly higher in the ICCE/AC IOL group than in the ICCE/AS group.The higher number of patients with "impaired vision" after full correction may be a reflection of more postoperative complications in the ICCE/AC IOL group compared to the ICCE/AS group.

In conclusion, this study suggests that postoperative astigmatism is an important complication of cataract surgery with an AC IOL, being seen more frequently than following cataract surgery without an AC IOL. It is important that attention is paid to the correct size of AC IOL and to the suture material and the technique.

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