Year : 2003 | Volume
: 51 | Issue : 2 | Page : 187--188
Effect of pterygium excision on pterygium induced astigmatism.
Pterygium is known to affect refractive astigmatism, which can have a significant impact on vision. This study was undertaken to evaluate the effect of pterygium excision on refractive astigmatism. Thirty-six eyes with primary pterygium with astigmatism of 2D or more were analysed before and after pterygium excision. Astigmatism increased with the increase in the grade of pterygium (P = 0.000001). The preoperative refractive cylinder decreased from 4.60±2D to 2.20±2.04D (P = 0.00001) after pterygium excision.
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Maheshwari S. Effect of pterygium excision on pterygium induced astigmatism. Indian J Ophthalmol 2003;51:187-188
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Maheshwari S. Effect of pterygium excision on pterygium induced astigmatism. Indian J Ophthalmol [serial online] 2003 [cited 2022 Nov 29 ];51:187-188
Available from: https://www.ijo.in/text.asp?2003/51/2/187/14703
Pterygium is a very common degenerative condition seen in the Indian subcontinent. The indications for pterygium surgery are (a) pterygium either invading or threatening visual axis; (b) visual impairment due to astigmatism; (c) irritative symptoms and inflammation; (d) restricted movements; and (e) cosmesis. Vision may be reduced due to direct invasion of the visual axis or astigmatism induced by the pterygium.
Several mechanisms have been suggested to explain the induced astigmatism. These include (a) pooling of the tear film at the leading edge of the pterygium, and (b) mechanical traction exerted by the pterygium on cornea. This abnormality has been measured by keratometry, corneal topography,,, and refraction., In the present study an attempt was made to assess the effect of pterygium excision on the induced astigmatism, and the relationship between the size of pterygium and the amount of astigmatism.
Materials and Methods
Thirty-six eyes with primary pterygia were studied before and after surgery. The inclusion criteria was >2D with-the-rule astigmatism as measured by refraction. Patients with history of trauma or surgery were excluded.
All surgeries were performed by the author under peribulbar anaesthesia. A small conjunctival incision was made medial to the head of pterygium and subconjunctival dissection carried out up to the caruncle, superior and inferior fornix. The corneal epithelium 2mm anterior of the head was scraped off with no.15 Bard-Parker blade and the pterygium head avulsed using a combination of blunt dissection and traction. The body of the pterygium was then excised. Haemostasis was achieved by applying light cautery to bleeders. The exposed scleral bed was covered with free conjunctival graft from the superotemporal quadrant.
Examination included Snellen's visual acuity, manifest refraction and slitlamp examination pre-operatively and one month postoperatively. Pterygium was graded depending on the extent of corneal involvement: Grade I -crossing limbus, Grade II -midway between limbus and pupil, Grade III -reaching up to pupillary margin, Grade IV -crossing pupillary margin.
Visual acuity was expressed as a decimal. The result was expressed as an arithmetic mean.
Preoperative and postoperative values were compared using the paired t-test. Preoperative astigmatism was compared against the grade of pterygium using one-way analysis of variance.
Nasal pterygium was present in all eyes except one which had double-headed pterygium [Table 1]. None of the eyes had grade I pterygium; 16 eyes (44.45%) had grade II pterygia, 12 eyes (33.33%) had grade III pterygia, 7 eyes (19.45%) had grade IV pterygia and 1 eye (2.77%) had a double headed pterygium. [Table 1].
The amount of astigmatism varied with the grade of pterygium. Mean astigmatism in eyes with grade II pterygium was 2.92D ± 0.65D. In eyes with grade III pterygium the mean astigmatism of 3.83 ±1.75D. 9.42 ± 2.64D of mean astigmatism was noted in eyes with grade IV pterygium. The amount of astigmatism was seen to increase with the grade of pterygium (P = 0.000001). One eye with double monocular pterygium had 7D of astigmatism. [Table 1].
The preoperative refractive cylinder was 4.60 ± 2 D, which improved to 2.20 ± 2.04 D (P = 0.00001) postoperatively [Table 2]. Visual acuity remained the same in 21 (58.33%) eyes. 15 eyes (41.67%) showed 1 or 2-line improvement in vision. The mean visual acuity preoperatively was 0.53 ± 0.35 D which improved to 0.68 ± 0.34 D (P = 0.001) postoperatively [Table 2].
Pterygium-induced astigmatism can lead to visual complaints. Previous studies have shown pterygium induces with-the-rule astigmatism., The astigmatism appears to be due to an alteration in the tear film caused by the lesion. As the head of the pterygium approaches the apex of cornea, a tear meniscus develops between the corneal apex and the elevated pterygium, causing an apparent flattening of normal corneal curvature.
Lin et al have reported that the pterygium begins to induce significant degrees of hemiastigmatism once it reaches up to 45% of the distance from the limbus to the visual axis or within 3.2mm of visual axis. Such an observation was made in the present series . With increase in the degree of corneal involvement, the induced astigmatism increased. The maximum degree of astigmatism was noted in eyes with grade IV pterygium (9.42±2.64D) and least was noted in eyes with grade II pterygium (2.92±0.65D) (P = 0.000001). One eye with double-headed pterygium had 7D of astigmatism; due to the increase in corneal involvement in double-headed pterygium, the induced astigmatism is higher.
The astigmatism decreased significantly following pterygium excision. The mean preoperative refractive cylinder decreased from 4.60±2D to 2.20±2.04D postoperatively (P=0.00001). Visual improvement was noted in 15 eyes(41.67%). The improvement in vision may be due to two causes - a) reduction in astigmatism, and b) removal of pterygium from visual axis as in grade IV pterygium.
The astigmatism seen in the patients represents both naturally occuring astigmatism and induced astigmatism. It may be incorrect to label the entire astigmatism as "induced". I would like to believe that majority of the astigmatism seen in the study was caused by the pterygium itself since it was always "with-the-rule" whereas naturally occuring astigmatism can occur at any of the axes.
The present study verifies that as the size of pterygium increases, the amount of induced astigmatism increases in direct proportion. Successful pterygium surgery reduces the pterygium-induced refractive astigmatism and improves the visual acuity.
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