Corneal Astigmatism and Intraocular Lens Power Calculation before and after Pterygium Surgery

 INTRODUCTION

Pterygium is an elastotic degeneration that occurring firstly in the bulbar conjunctiva and spread to the surface of the cornea. It may impair patients' quality of life and vision by resulting in discomfort, corneal abnormalities, and aesthetic problems.¹

Pterygium prevalence was estimated to range from 10.2% to 12% globally, with greater rates in tropical areas. Pterygia has been linked to a number of risk factors, including geographic latitude, living in rural areas, aging, race, gender, sun exposure, chronic irritation, and inflammation.²

After pterygium surgery, corneal irregularity is a crucial factor in preserving good visual acuity and function, and it takes time to restore the corneal surface^[3]. According to Nejima et al. ³, a larger sized pterygium requires 6 to 12 months to stabilise corneal irregularity. They have also demonstrated that the major pterygium's size affects how well post-operative corneal topographical anomalies are restored.

One of the clinical issues with pterygium surgery is the recurrence, and it occurs at high incidence rate in the patients who undergo surgery⁴. Degradation of the Bowman's layer and invasiveness are two difficulties in pterygium surgery that make it challenging to distinguish proliferative tissue from the corneal stroma, with the challenge being more obvious in recurrent-pterygium surgery.⁵

Pterygium is a widespread condition affecting the ocular surface. It might happen simultaneously with cataract because both diseases are very common in elderly persons ⁶. The precision of the calculated intraocular lens power can be impacted by a large pterygium, which can cause substantial corneal astigmatism^[7]. Numerous studies have found a connection between pterygium size and pterygium-induced astigmatism, and they also projected that IOL power would raise in pterygium-affected eyes.⁷

The current study was designed to study astigmatism of the cornea and to calculate IOL power pre and post-operative pterygium excision in patient with coexistence cataract and primary pterygium.

PATIENTS AND METHODS

Patients

This is a clinical trial study involved adult patients (48-62 years) of both genders. All patients exhibited the following inclusion criteria: simultaneous cataract and primary pterygium, limbus conjunctival autograft and pterygium excision surgery.

Patients with recurring pterygium, previous trauma or operation, corneal disorders that can compromise the regularity of the cornea, keratoconus, keratitis, and uveitis were excluded from the study.

All patients gave a written consent after explaining the purpose and the procedure of the study.

Methods

All patients had ptergyium surgery excision with limbus conjunctival autograft that conducted as follow: The head of Pterygium was removed from the cornea and its body was trimmed to be nearly 4 mm away from the corneal limbus under subconjunctival anaesthetic. The corneal damaged area was carefully polished after the subconjunctival tissue was removed. Conjunctival autograft tissue was then applied over the damaged area, limbus-to-limbus overlapped, and 8-0 absorbable vicryl sutures were used for the stitching.

Prior to and following pterygium surgery, the corneal astigmatism was assessed by the Pentacam HR (Oculus Inc., Wetzlar, Germany).

Prior to and following pterygium surgery, the IOL power was determined using the IOL master 500 (Carl Zeiss Meditec, Germany). Many patients could not utilise the IOL master preoperatively to achieve an appropriate corneal curvature because the pterygium had infiltrated the cornea. Therefore, we computed the IOL power using the corneal curvature data from Pentacam.

Data analysis

Statistical analysis was done by SPSS software statistical computer package version 21.

Mean, standard deviation, range, and IQR are used to describe quantitative variables. Qualitative variables analyzed by Independent t-test and Wilcoxon signed ranks test. Independent t-test was used in comparing parametric data (when SD<50% mean). Wilcoxon signed ranks test was used in comparing non parametric data (when SD>50% mean). Cut off point of significant p value <0.05.

RESULTS

Demographic data of the studied patients are presented in Table 1. The study showed that 9 (45.0%) of the studied cases were males and 11 (55.0%) were females. there mean age was 54.95 ± 4.74 SD with range (48.0 – 62.0). Table 2 showed that 17 (85.0%) of the studied cases were OS (left eye) and 3 (15.0%) were OD (right eye).

IQR: Inter quartile range                         SD: Standard deviation

Table 1: Distribution of the studied cases according to demographic data (n = 20)

Table 2: Distribution of the studied cases according to OS/OD (n = 20)

As regarding K1 measurement, mean and SD of pre-operative measurement was 42.55 ± 2.61 D while, it was 42.51 ± 2.19 D in post-operative assessment. So there was no statistically significant difference between pre and post-operative assessment as regard K1 by Independent sample t-test as shown in table 3.

IQR: Inter quartile range, SD: Standard deviation, t: Independent sample t-test. Cut off point of significance p value<0.05

Table 3: Comparison between pre and post-operative assessment according to K1 (n = 20)

Another important finding was the significant change of K2 in post-operative assessment as Mean and SD was 44.84 ± 1.20 D in pre-operative assessment, while it changed to 44.06 ± 1.21 D in post-operative assessment as shown in table 4.   

IQR: Inter quartile range,        SD: Standard deviation,         t: Independent sample t-test. Cut off point of significance p value<0.05

Table 4: Comparison between the pre and post-operative assessment according to K2 (n = 20)

Table 5 shows that IOL power was 18.50 ± 8.24 D in pre-operative assessment while in post-operative assessment it was 18.98 ± 8.15 D. So there was highly statistically significant difference between pre and post-operative assessment as regard IOL power.

IQR: Inter quartile range,        SD: Standard deviation,         Z: Wilcoxon signed ranks test. Cut off point of significance p value <0.05

Table 5: Comparison between the pre and post-operative assessment according to IOL (n = 20)