Pterygium-induced corneal refractive changes.

To study the effect of pterygium on corneal topography, a retrospective analysis of 151 eyes with primary pterygia was done. All cases underwent videokeratography preoperatively and one month postoperatively. Statistical analysis of average corneal power (ACP), corneal astigmatism, surface regularity index (SRI) and surface asymmetry index (SAI) was done before and one month after surgery. Topographic indices were compared statistically for various grades of pterygia. Increase in the grade of pterygia had a significant effect on topographic indices. Corneal astigmatism reduced from 4.40+/-3.64 diopter (D) to 1.55+/-1.63D ( P value < 0.001) following surgery. The regularity of corneal surface improved and asymmetry of the cornea reduced one month after surgery. Pterygium leads to significant changes in corneal refractive status, which increase with the increase in the grade of pterygia and improve following pterygium excision.

Pterygium is commonly seen in India, a part of the "pterygium belt" described by Cameron.1 The indications for pterygium surgery are (a) visual impairment; (b) recurrent inflammation; (c) motility restriction; (d) cosmetic disfigurement.

Pterygium leads to a considerable effect on corneal refractive status which has been previously measured in various studies by refraction,2,3 keratometry4 and corneal topography.5,6,7,8,9,10 In the present study pre and postoperative corneal topography was evaluated for each case.

Materials and Methods

A retrospective analysis of 151 eyes with primary pterygia was done. Patients with history of trauma, previous surgery, patients having corneal scars were not included in the study. On slit-lamp examination with slit beam focused on the nasal limbus, pterygium was graded depending on the extent of corneal involvement:

Grade I - between limbus and a point midway between limbus and pupillary margin

Grade II - head of the pterygium present between a point midway between limbus and pupillary margin and pupillary margin (nasal papillary margin in case of nasal pterygium and temporal margin in case of temporal pterygium)

Grade III - crossing pupillary margin

Corneal topography analysis was obtained with computerized videokeratography (TMS -2, Computed Anatomy Inc, New York, NY), which was performed by an experienced optometrist one day prior to surgery and one month after it [Figs. 1-2].

Figure 1

Corneal topography of a patient with grade II Pterygium showing flattening in the horizontal meridian

Figure 2

Corneal topography of the same patient 1 month after surgery

All eyes underwent pterygium excision with conjunctival autograft transplantation.

Results were expressed as an arithmetic mean ± standard deviation. Values were compared against the grades of pterygium using one-way analysis of variance (ANOVA). Statistical analysis of average corneal power (ACP), corneal astigmatism was calculated from the Sim K values, surface regulatory index (SRI), surface asymmetry (SAI), min K and refractive cylinder was done before and one month after pterygium surgery using paired t -test. Corneal cylinder and refractive cylinder were compared using Mann-Whitney test; corneal astigmatism amongst various grades was compared using one-way analysis of variance (ANOVA with Bonferroni′s correction of P values for multiple comparison).

Results

Preoperative topographic indices in various grades of pterygium have been shown in Table 1. Corneal astigmatism, SRI and SAI were seen to increase with the increase in the grade of pterygium. Comparing the corneal astigmatism amongst the various grades of pterygium [Table 2] a statistically significant increase in astigmatism was noted with the increase in the grade from I to III.

Table 1

Grade of pterygium and topographic indices

Table 2

Comparison between corneal astigmatism among various grades

The comparison and analysis of pre and postoperative values are presented in Table 3. The mean ACP increased from 42.91±2.20 diopter (D) to 44.25±1.77D ( P value < 0.001) after pterygium excision.

Table 3

Pre and postoperative statistics

The mean preoperative topographic astigmatism was 4.40±3.64D and reduced postoperatively to 1.55±1.63D ( P value < 0.001).

Refraction was done in 140 eyes; in 11 eyes it was not possible due to cataract. The refractive cylinder before surgery was 1.94±2.24D, which was less than the topographic cylinder of 4.25±3.63D ( P value < 0.001). The refractive cylinder reduced from 1.94±2.24D to 0.78±1.07D.

Discussion

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.