|Year : 2000 | Volume
| Issue : 3 | Page : 213-6
Is pseudophakic astigmatism a desirable goal?
KM Nagpal, C Desai, RH Trivedi, AR Vasavada
Iladevi Cataract and IOL Research Centre, Raghudeep Eye Clinic, Ahmedabad, India
K M Nagpal
Iladevi Cataract and IOL Research Centre, Raghudeep Eye Clinic, Ahmedabad
PURPOSE: To determine whether pseudophakic astigmatism is a desirable goal, and if so, which one is better: against-the-rule (ATR) or with-the-rule (WTR). METHOD: Eyes were included only if they had an uncorrected vision > or = 6/18 and N/18. Three groups, of 40 patients each were evaluated: group 1, pseudophakes with neutral astigmatism; group 2, with ATR and group 3, with WTR astigmatism Unaided distance and near visual acuity was recorded. Statistical analysis was performed using the chi-square test for independence. RESULTS: Unaided distance vision of > or = 6/7.5 was achieved in 19 eyes (47.5%) of group 1 (neutral), 12 eyes (30%) in group 2 (ATR), and 5 eyes (12.5%) in group 3 (WTR) (p = 0.0133, significant). Unaided near vision of > or = N/9 was achieved in 17 eyes (42.5%) in group 1 (neutral), 34 eyes (85%) in group 2 (ATR), and 10 eyes (25%) in group 3 (WTR) (P < 0.001, significant). Group 1 (neutral) fared the best for unaided distance visual acuity. Group 2 (ATR) was better than in group 3 (WTR) for distant vision. Group 2 (ATR) fared the best for unaided near vision. CONCLUSION: ATR astigmatism could be a desirable goal after cataract extraction in selected populations because the largest proportion of these cases achieved good unaided near vision with acceptable distant vision.
Keywords: Astigmatism, complications, etiology, Cataract Extraction, adverse effects, Comparative Study, Humans, Lens Implantation, Intraocular, adverse effects, Prospective Studies,
|How to cite this article:|
Nagpal K M, Desai C, Trivedi R H, Vasavada A R. Is pseudophakic astigmatism a desirable goal?. Indian J Ophthalmol 2000;48:213
|How to cite this URL:|
Nagpal K M, Desai C, Trivedi R H, Vasavada A R. Is pseudophakic astigmatism a desirable goal?. Indian J Ophthalmol [serial online] 2000 [cited 2013 May 21];48:213. Available from: http://www.ijo.in/text.asp?2000/48/3/213/14872
Intraocular lens implantation is the established mode of visual rehabilitation following cataract extraction. However, physiological mechanisms of accommodation cannot be restored with an inelastic, monofocal intraocular lens. The pseudophakic eye can be made emmetropic for a single focus only with a monofocal intraocular lens implantation,. However surgically induced astigmatism has been shown to allow a pseudophakic "pseudo-accommodation". Huber found residual postop erative myopic astigmatism an effective substitute for accommodation, granting independence from spectacles. Trindade et al also suggest the benefit of low, simple, against-the-rule (ATR) astigmatism in pseudophakes for better uncorrected near visual acuity; and Bradbury et al consider with-the-rule (WTR) astigmatism of 1.5 D a desirable goal for postoperative refraction.
In order to address these apparently-conflicting results, we designed a study to compare unaided visual acuity in pseudophakic eyes having neutral, ATR and WTR astigmatism.
| Material and Methods|| |
This is a prospective study conducted between August, 1997 and January, 1998. All the subjects had gone through phacoemulsification 2-6 months before inclusion in the study but had otherwise healthy eyes. Eyes were included only if they had an unaided visual acuity of 6/18 or more for distance, as recorded on the ETDRS chart and N/18 or more for near, recorded on the reduced Snellen's chart in English. Eyes were included only if they had uneventful and uncomplicated intraoperative and postoperative periods. In all cases the pupil was central, normal size and reacting to light. Eyes that required less than one diopter residual spherical correction and less than 2.5 diopter cylider ± 15° on subjective correction were included.
Eyes with more than 2.5 Dcyl acceptance and oblique astigmatism, and eyes with more than 1D sphere were excluded. Patients with posterior capsule plaque or opacification and those with any operative or postoperative complications were excluded. Patients who could not read English charts were also excluded.
All surgeries were done through a temporal clear corneal tunnel. After the anterior capsulorhexis, hydrodissection and rotation of the nucleus, and in-the-bag phacoemuslification was carried out. Step by step chop in situ and lateral separation technique was adopted. AcrySofR (CMA 30 BA, Alcon Laboratories Inc. Fort worth, Texas.) foldable intraocular lens was implanted in the bag.
We designed this study in terms of three age and gender matched groups. We enrolled appropriate eyes in each groups till we obtained 40 cases.
Group 1: Neutral [<0.5Dcyl at any axis],
Group 2: Against the rule (ATR) astigmatism [0.5 to 2.5 Dcyl (± 150)],
Group 3: With the rule (WTR) astigmatism [0.5 to 2.5 Dcyl (±15°)].
Unaided visual acuity was tested on the ETDRS chart in English alphabet at 4 meters for distance and on the reduced Snellen's chart in English at 33 cm for near before dilatation. In addition to refraction assessment on an autorefractometer, each eye underwent streak retinoscopy and subjective acceptance test. The best-corrected visual acuity was noted for distance and near vision. All the cylinders quoted are those obtained by the best-corrected manifest refraction. All refractions were transposed to a minus cylinder form. ETDRS values were converted into Snellen's equivalent.
In addition to recording unaided visual acuity for distant and near vision, all the eyes were also tested with the aid of the spherical component to negate the effect of the spherical component on the visual acuity. These eyes were astigmatically unaided, so as to evaluate the effect of astigmatism only. Statistical analysis was performed using the chi-square test for independence.
| Results|| |
One hundred and nine eyes achieved best corrected visual acuity 6/6 or better, 7 eyes achieved 6/7.5, and 4 eyes achieved 6/9. All eyes achieved best corrected near vision N/6.
[Table - 1] shows the amount of spherical component based upon the best corrected manifest refraction in the three groups. There was no statistically significant intergroup difference (p=0.216). The amount of cylindrical component between ATR and WTR groups was comparable (p=0.23).
Nineteen eyes (47.5%) in group (neutral), 12 eyes (30.6%) in group 2 (ATR) and 5 eyes (12.5%) in group 3 (WTR) had unaided visual acuity ≥ 6/7.5 [Table - 3]. Eyes in group 1 (neutral) did significantly better for unaided distant vision (p=0.0133). However better performance in group 2 over group 3 was not statistically significant (p=0.1568).
Seventeen (42.5%) eyes in group 1 34 (85%) eyes in group 2, 10 (25%) of eyes in group 3 could read N/9 or better on the reduced Snellen's chart [Table - 3]. Thus the effect of ATR astigmatism upon the uncorrected near vision was highly significant (p<0.001, significant).
[Table - 4] shows the results of distant and near visual acuity respectively, after only the spherical component was corrected. These results are totally in accordance with the findings unaided visual acuity performance.
| Discussion|| |
In an astigmatic eye, a point object is focussed in the form of two focal lines corresponding to the two principal meridians, separated by a focal Interval. In simple myopic astigmatism, the more myopic meridian produces a focal line in front of the retina, while the emmetropic meridian forms a focal line upon the retina. According to Sturm's hypothesis, two focal lines always move together, posteriorly when the object is brought closer to the eye and anteriorly as the object is moved further away.
The theory of Sturm's hypothesis has been used to explain 'astigmatic accommodation". In order to obtain a clear vision, it is sufficient that one of the focal lines falls upon the retina. Thus for a distant object, a sharp focal line is obtained upon the retina corresponding to the emmetropic meridian [Figure:1a]. When the same object is brought closer to the eye, the focal line obtained from the myopic meridian lies upon the retina because of the posterior movement of the two focal lines [Figure:1b]. This amounts to the possibility of a far point and a near point of reduced blur resulting in an increased depth of focus.
The orientation of the focal line depends upon the meridian by which it is obtained. Thus, the vertical meridian focuses the rays into a horizontal line and the horizontal meridian gives rise to a vertical focal line. In WTR astigmatism, the myopic meridian lies at 90° while the emmetrophic meridian this lies at 180°. Thus when looking at a point object at distance, a vertical focal line is obtained upon the retina and a horizontal focal line in front of it. Conversely, in ATR astigmatism, the myopic meridian is at 180° and the emmetropica meridian is at 90°. Thus for a distant point object, a sharp horizontal focal line is obtained on the retina and a vertical focal line in from of it [Figure:1a].
A line consists of multiple points. Therefore, in astigmatism, it is perceived as a succession of strokes fused into a blurred image. If the orientation of the line is the same as that of the emmetropic focal line, an overlap of all the strokes occurs producing a sharply defined image, with only the uppermost and the lowermost stroke extending beyond, giving it a tailed appearance. Thus vertical lines in WTR and horizontal lines in ATR astigmatism appear sharper for distance. Also, in accordance with Sturm's thesis. The horizontal lines become sharper in WTR and vertical lines in ATR astigmatism for near vision. This is known as astigmatism paradox.
Most object and certainly the printed matter are most decipherable when the vertical component is clear. Trindade et al used this principle to support their observations of good uncorrected near visual acuity in subjects with ATR astigmatism. An average English letter is best recognized when the vertical component of the letter is sharper. This is because vertical strokes of the letters of the English alphabet help in their recognition. Also, in printed matter there is usually less space between the letters on a line than between the lines themselves. Therefore, if the horizontal components of the letters are sharper, as in WTR astigmatism for near, the letters run together, becoming less distinct. Conversely, when the vertical lines are sharper, as in ATR astigmatism for near vision, the letters appear more separate and thus more distinct.
For unaided distant visual acuity the neutral group performed significantly better, as expected. The superior unaided near visual acuity in subjects of ATR astigmatism in our study is in accordance with the results obtained by Trindade et al. In their report, 90% in the ATR group could read J1-2 level without correction while 80% in the WTR group could read J5-6 without correction. They found no statistically significant difference between ATR and WTR groups. However, they did not consider the neutral group. Morever, the study examined only 10 eyes in each group.
The results obtained after correcting the spherical correction only were similar to those obtained without spherical correction. We found that the effect of astigmatism is not affected by the spherical component. We had not asked for a subjective response from the patients about their near vision and dependence on spectacles. However, Huber et al have reported that pseudophakic patients with ATR usually do not need to depend on spectacles except in situations such as driving or prolonged reading.
A limitation of this study is that we did not take into consideration the size of the pupil when we examined the eyes. Pupil size could influence the outcome. A small pupil provides a better uncorrected visual acuity, irrespective of the type of the refractive error. Also, we did not study eyes that had an astigmatism of more than 2.5D. Thus we cannot comment upon how a larger cylinder affects the uncorrected vision. The other limitation of the study is that visual acuity for both distance and near was checked only in the English alphabet. Other alphabets that do not have such a strongly prominent vertical character may yield different results.
Thus, it can be concluded that astigmatically neutral subjects fare better for unaided distance vision is concerned. We also observed better uncorrected distance acuity in ATR cases compared to WTR, although it was not significant. However, for unaided near visual acuity, ATR astigmatism was better than its neutral and WTR counterparts. It is obvious that pseudophakic ATR has beneficial effect on unaided near visual acuity Hence, it should be a desirable goal where the patient's occupation or lifestyle demands a greater use of near vision. A typical situation is that of a person with restricted motility.
In future, with possible introduction of multifocal and accommodative intraocular lenses, this discussion may not appear important enough. For now, however, it is heartening to know that postoperative astigmatism can be utilized to give a pseudophake better uncorrected visual acuity.
| Acknowledgement|| |
We are thankful to Dr.A.N. Setalvad for providing statistical assistance.
| References|| |
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|2.||Bradbury JA, Hillman JS, Cassells-Brown A. Optimal postoperative refraction for good unaided near and distance vision with monofocal intraocular lenses. Br J Ophthalmol 1992;76:302 [PUBMED] |
|3.||Vasavada AR, Singh R. Step-by-step chop in situ and lateral separation of very dense cataracts. J Cataract Refract Surg 1998;24:156-59. |
|4.||Abrahms D. Duke-Elder's Practice of Refraction, 10th edition. Churchill: Livingstone; 1993. pp 65-70. |
|5.||Datiles MB, Gncayco T. Low myopia with low astigmatic correction gives cataract surgery patients good depth of focus. Ophthalmology 1990;97:922-26. |
|6.||Huber C. Myopic astigmatism, a substitute for accommodation in pseudophakia. Doc Ophthalmol 1981;52:123-78. [PUBMED] |
[Figure - 1], [Figure - 2]
[Table - 1], [Table - 2], [Table - 3], [Table - 4]
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