Comparision of central corneal thickness in pseudoexfoliation syndrome and pseudoexfoliation glaucoma

Introduction: To measure the central corneal thickness(CCT) in patients with Pseudoexfoliation Syndrome without glaucoma (PXS) and Pseudoexfoliation Syndrome with glaucoma(PXG) using ultrasonic pachymetry and compare the two. Materials and Methods: The study was conducted on 210 patients. 70 patients were with pseudoexfoliation syndrome, 70 were pseudoexfoliation glaucoma and 70 were healthy individuals. Central corneal thickness was measured using ultrasonic pachymetry. Results: Central corneal thickness is significantly thinner in patients with PXG (515±22.94μm) than in patients with PXS (525±21.22μm) and control group (528±23.35μm) with p value < 0.05. Conclusion: The study shows that corneas are thinner in patients with pseudoexfolaition glaucoma (PXG) as compared to pseudoexfoliation syndrome without glaucoma (PXS) and controls (CNT). © 2019 Published by Innovative Publication. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by/4.0/)


Introduction
Pseudoexfoliation syndrome is a microfibrillopathy, with strong genetic component. Single nucleotide polymorphism (SNP) of lysyl oxidase 1 gene (LOXL1) located on chromosome 15 is responsible for pseudoexfoliation syndrome and glaucoma. 1 It is characterized by the production and accumulation of extracellular fibrillary material in different tissues of the body.
Characteristic whitish flake material is deposited over several ocular structures including corneal endothelium, pupillary margin, anterior lens capsule, zonules, ciliary body, trabecular meshwork. It is the most common identifiable cause for secondary open angle glaucoma. 2 Other systemic conditions associated with PXF are cardiovascular disease, cerebrovascular disease, sensorineural hearing loss, Alzheimers disease. 3 The goldstandard for the measurement of intraocular pressure is Goldmann applanation tonometer . Thinner corneas underestimate the IOP and thicker corneas overestimate the IOP, thus a correction factor must be added to the measured IOP, when CCT deviates from the mean. [4][5][6][7] In case of deviatio n of CCT from the mean of 520µ, 0.7mmHg should be added for every 10µ. 7 Thus it can lead to underestimation of IOP in cases of pseudoexfoliation syndrome with thin corneas. Pseudoexfoliation glaucoma constitute about 30% cases of pseudoexfoliation syndrome and we may overlook early glaucomatous changes. PXG has a more rapid progression and worse prognosis compared to POAG.
The study aims to measure the CCT in patients with pseudoexfoliation syndrome(PXS) and pseudoexfoliation glaucoma(PXG) using ultrasonic pachymetry and compare the two.

Materials and Methods
This is a cross-sectional comparative study conducted over a period of 12  Detailed ophthalmological examination was done, including visual acuity for distant with Snellen chart and near vision with Jaeger's chart, slit lamp examination, intraocular pressure using Goldmann applanation tonometry, gonioscopy and fundus examination.
Patients were divided into 3 groups. 70 patients with pseudoexfoliation syndrome(PXS) ; diagnosed by the presence of pseudoexfoliative material over the pupil margin before pupillary dilatation, on anterior lens capsule after pupillary dilatation, on corneal endothelium and on trabecular meshwork on gonioscopy.
70 patients with pseudoexfoliation glaucoma (PXG); diagnosed by the presence of pseudoexfoliative material over pupil margin and lens capsule, IOP more than 22mmHg, open angles on gonioscopy, typical glaucomatous cupping and visual field defects. 70 were age and sex matched healthy controls(CNT) without pseudoexfoliation syndrome and pseudoexfoliation glaucoma.
Central corneal thickness was measured for all the patients with an ultrasonic pachymeter (Pachette 2 model DGH 550).
Patients were seated comfortably, local anestheticdrops were instilled in the eye. In primary gaze, the probe of the ultrasonic pachymeter was placed on the center of the cornea as such it aligns with the center of the pupil. Five consecutive readings were recorded and the average was considered as the final value.
Data has been entered into Microsoft Excel and statistical analysis was done using IBM Statistical Package for Social Sciences (SPSS Ver. 25). For continuous variables, the data values are expressed as mean ± standard deviation. To test the mean difference between three groups, ANOVA with Tukey 's post hoc test was used, P < 0.05 is considered significant.

Results
The study was conducted on 210 patients. Patients were divided into 3 groups. Group

Discussion
Acccording to our study, the corneas are significantly thinner in patients with pseudoexfoliative glaucoma compared to pseudoexfoliation syndrome and controls. Pseudoexfoliation syndrome patients have thinner corneas than that of controls with no statistical significance.
Similar results are shown by several studies. Kitsos 8 and colleagues conducted a study to evaluate the CCT in patients with pseudoexfoliation syndrome, pseudoexfoliation glaucoma using ultrasound pachymetry. They concluded that the corneas are significantly thinner in patients with pseudoexfoliation glaucoma (526±34.30 µm) compared to individuals with pseudoexfolaition syndrome (550.64±39 µm) and controls (547.36±33.1 µm) p<0.05.
In another study conducted by Inoue and colleagues, 9 patients with pseudoexfoliation syndrome, pseudoexfoliation glaucoma and controls were included and central corneal thickness was measured in all the cases. The study concluded that the corneas are thinner in PXS (529±31µ m )compared to controls(547±28µ m) with p = 0.03. No significant difference was found between the cases of PXS with and without glaucoma.
In the study conducted by Shah 10 and colleagues, the central corneal thickness was measured in normal individuals and pseudoexfoliation glaucoma. The study concluded that the cornea is thinner in pseudoexfoliation glaucoma patients (530.7µm) compared to the normal individuals(553.9µm), with P<0.001.
Bechmann 11 and colleagues conducted a stud y to evaluate the CCT in different types of glaucoma using OCT. The study concluded that patients with pseudoexfolia tion glaucoma havethinner corneas(493±33 µm) compared to healthy individuals(530± 32µm) with p< 0.0001. Another study conducted by Sobothka 12 and colleagues where CCT was measured using OCT in different types of glaucoma, concluded that the CCT was less in pseudoexfoliation glaucoma (507±25 µm) compared to normal individuals (524±25 µm), but was not statistically significant. Yagci 13 and colleagues conducted a study to evaluate the relation between CCT and IOP among glaucomatous eyes and normals, where CCT was measured using ultrasound pachymetry.
CCT was lower in pseudoexfoliation glaucoma cases( 526.28±31.73 µm) compared to normals (533.96±29.25 µm), the difference being statistically in significant. In another study conducted by Aghaian 13 and colleagues comparing CCT using ultrasonic pachymetry    15 As per this study, the central corneal thickness in patients with pseudoexfoliation syndrome is 533±32µm and in controls is 527±42µ m. Thus CCT is greater in PXS patients compared to controls, the difference being statistically in significant, p=0.232. As per study conducted by Acar 16 and colleagues, central corneal thickness is lower in pseudoexfoliation syndrome (540.8±30.2 µm) than in controls (551.5±28.3µ m). Statistical significance was not achieved p=0.315 The study conducted by Zheng 17 and colleagues to measure cell density in different layers of cornea in eyes with pseudoexfolaition syndrome, observed the presence ofpseudo exfoliative deposits in the corneal stroma and reduced number of stromal keratocytes in eyes with pseudoexfoliation syndrome compared to those without pseudoexfoliation syndrome. They concluded that the presence of pseudoexfoliative material is responsible for inducing a poptosis of stromal keratocytes, leading to weakening of extracellular matrix; thus resulting in corneal thinning.

Conclusion
The study concludes that the patients with pseudoexfolaition glaucoma have thinner corneas compared to those with pseudoexfoliation syndrome and healthy individuals. Thus the intraocular pressure measurement in pseudoexfoliation syndrome patients should be correlated with central corneal thickness, as underestimation of intraocular pressure in such patients may lead to overlooking glaucoma, that has rapid progression and poor prognosis.

Source of funding
None.

Conflicts of interest
None.