A pterygium-induced refractive change often leads to visual impairment. These changes are localized and cannot be measured accurately either by refraction or keratometry. In 140 eyes in which refractions were recorded there was poor correlation between the magnitude of refractive cylinder and topographic cylinder. This can be due to the hemi-astigmatic nature of the induced changes. During manifest refraction patient deals with two images, one from the more spherical temporal cornea and one from the flatter nasal cornea. The patient preferentially views the more spherical image and therefore the corneal changes are incompletely reflected in the refraction.
Keratometry measures only the central cornea and peripheral cornea is ignored and hence the results can be erroneous in eyes with pterygium. Computerized videokeratography remains the best tool for evaluating the corneal surface changes induced by pterygium.
Pterygium was seen to have a considerable effect on topographic indices in the present series. Flattening was seen in the horizontal meridian, which was associated with astigmatism. The exact mechanism of flattening is not clear. It is thought to be caused by the formation of tear meniscus between the corneal apex and the elevated pterygium, causing an apparent flattening of the normal corneal curvature.10
Lin and Stern found a significant correlation between the pterygium size and corneal astigmatism; they reported pterygium to induce significant degrees of astigmatism once it exceeded> 45% of the radius.5
Tomidokoro et al
. evaluated the percentage extension of pterygium on cornea and found larger pterygia to adversely affect astigmatism, asymmetry and irregularity of the cornea.7
In the current study pterygia were divided into grades depending on the extension of pterygium on the cornea. Grade II or larger pterygium was associated with increase in astigmatism, asymmetry and irregularity. The ACP reduced significantly in Grade II or larger pterygium. Hence, for patients with pterygium requiring cataract surgery, decision of surgery should be taken depending on the grade of pterygium; in cases with Grade I, atrophic and non-progressive pterygium one can consider cataract surgery directly. However, pterygium Grade II or larger significantly affects the refractive component of cornea which can lead to erroneous intraocular lens power calculation and post-cataract refractive surprise. Hence in cases with pterygium Grade II or larger, a stepwise approach should be followed; pterygium excision should be performed prior to cataract surgery. By time-course analysis, cornea has been shown to stabilize one month after pterygium surgery. Hence, cataract surgery or refractive surgery if considered should be performed at least one month after pterygium surgery.7
Simultaneous cataract and pterygium surgery should not be done in cases with large pterygium as one may have an unexpected refractive surprise postoperatively.
Stern and Lin reported improvement in topographic indices in 16 eyes; they reported corneal astigmatism to reduce from 5.93±2.46D to 1.92±1.68D.6
Tomidokoro et al
. analyzed 119 eyes and reported increase in corneal spherical power from 43±1.18 to 45.2±1.6D.7
Yagmur et al
. studied the effect of pterygium excision in 30 eyes and found topographic astigmatism to reduce from 4.65±3.02 to 2.33±2.26D.9
In the current study all the topographic parameters were seen to improve significantly following pterygium excision.
The relationship between the preoperative refractive cylinder and postoperative refractive cylinder (Pearson′s correlation coefficient, r =. 2986) was expressed by the following equation:
Postoperative refractive cylinder = 0.283+0.266 x preoperative refractive cylinder.
Computerized videokeratography remains the best tool in evaluating pterygium-associated corneal changes. The corneal changes are seen to improve significantly following pterygium excision.