Central Corneal Thickness: Important Considerate in Ophthalmic Clinic

Abstract

Purpose: To evaluate the relationship of central corneal thickness with age, sex, refractive status and keratometry.

Method: In this cross sectional study, 1000 eyes from 500 patients from outpatient department were randomly selected between July 2014 and December 2015. Central corneal thickness was measured with Humphrey Ultrasonic Pachymeter. Horizontal and vertical curvature of cornea was measured with Bausch &Laumb Keratometer and mean was calculated. Refractive state was measured with Priestely Smith retinoscope and converted to SE. The patients were divided into three groups as per their age [Group A (16 to 30 yrs); Group B (31 to 45 yrs) and Group C (46 to 60 yrs)]. Subsequently data was analyzed statistically.

Results: The mean SE, KM and CCT of the patients under the study were (-) 0.47 ± 2.26,43.79 ± 1.18D and 528.41 ± 19.1μm. Mean CCT was higher in age group C (46 to 60 yrs) than other groups (p = 0.008) but no impact was found on increasing age after regression analysis. CCT was not affected by gender (p = 0.168). Mean CCT for Myopic was found to be 522.87±18.03μm which was lower compared to in Hypermetropic (536.39 ±17.753μm) (p = <0.001). There was a positive correlation between CCT and SE (r = 0.520, p = <0.001). However KM showed negative correlation with CCT (r = -0.288, p = <0.001).

Conclusion: From above study we concluded that CCT was related to age, refractive status and keratometry but not to sex.

Keywords: Central corneal thickness; Mean Keratometry; Refractive state; Age; Sex

Abbreviations

CCT: Central Corneal Thickness; KM: Mean Keratometry; SE: Spherical Equivalent

Introduction

The Cornea occupy one third of ocular tunic and forms the anterior meniscus-shaped transparent portion of the ocular globe. It serves as the principal refractive element in the eye, while maintaining a highly impermeable barrier between an eye and environment.

It serves as transparent window of an eye that allows the entry of light. Maintenance of corneal shape and transparency is critical for light refraction, considering the cornea accounts for more than two third of total refractive power of an eye.

Central Corneal Thickness (CCT), one of the most important corneal parameter, plays an important role in planning various intraocular procedure, refractive surgeries, corneal transplantation etc. Recently there have been increased interests in corneal thickness [1]. Different aspects such as correlation between CCT & intraocular pressure and progressive thinning in cornea have been well established in many studies [2-6].

CCT is an important diagnostic and prognostic factor in determining whether patient is suitable for refractive surgery and also to determine the required procedure. It also helps in classification as well as diagnosis of glaucoma.

Pachymeter measures corneal thickness, which is now in routinely used and its use is increasingly important in ophthalmic practice to avoid postoperative complications. Other uses of corneal pachymetry include determining the ‘health’ of a corneal transplant, evaluating a patient with keratoconus, and monitoring the degree of stromal edema. In contact lens wear, corneal edema and hypoxia can be assessed in daily wear easily.

Non corrected refractive error is increasingly recognized as a significant cause of avoidable visual disability worldwide and has been included as one of the priority areas of Vision 2020 [7]. In-depth knowledge of ocular biometric parameters is therefore required in understanding the risk factors and determinants of ammetropia [8-11] and in formulating appropriate preventative and treatment strategies. The most important factor in the refractive errors is relationship to the ocular components [9].

Stephan J. et al [2] evaluated 4600 eyes to study relationship between thinnest point in corneal thickness and the refractive state, keratometry, age, sex and the ocular side. They concluded that refractive state, Mean Keratometry (KM) and age were statistically significant, although marginal impact, on the thinnest point in corneal thickness. Sex and the ocular side had no effect. Altinoket al [12] and Archana Prasad et al [13] found no correlation for age, sex, refractive status with CCT.

Recently phakic intraocular lenses are being used increasingly to correct refractive error [14] which require different ocular parameter to consider. Effective visual rehabilitation after cataract surgery also depends on accurate intraocular lens power calculations which are primarily derived from normative ocular biometric data [15-19]. Therefore, it is important to understand the factors that have an influence on the CCT.

In our study, we had analyzed different biometric ocular factors and investigated the possible influences of age, sex, refractive state, keratometry on CCT.

Methods

This cross sectional study was conducted at Dr. Mohal Lal Memorial Gandhi Eye Hospital, Aligarh. Permission for this study was obtained from the ethical committee of the hospital. Informed consent was taken from the patients in their own language in a prescribed bilingual format. 500 patients (1000 eyes) who fulfilled the inclusion criteria were selected from all those attending outpatient department.

Inclusion criteria

Patients between the age of 16 years and 60 years of both the genders having Visual Acuity 6/6 using Snellen’s Chart and Landolt’s C Chart were selected.

Exclusion criteria

Patient below 16 years and above 60 years, patient with any ocular surgery or ocular trauma, patient with pre-existing eyelid disease or anterior segment pathology like conjunctivitis, keratitis, uveitis, glaucoma, corneal dystrophy, corneal degeneration, Keratoconus, congenital corneal disease (Keratoglobus), patient with systemic disease like diabetes mellitus, hypertension etc. and patient with contact lens wear.

Detailed histories of all patients were elicited and thorough ophthalmic examination was done. Patients were divided into three groups according to their age {Group A (16 to 30 yrs); Group B (31 to 45 yrs) and Group C (46 to 60 yrs)}.

Visual acuity

Visual acuity of both eyes were recorded by Snellen’s Chart or Landolt’s C Chart and refractive state was determined by Priestely Smith retinoscope. Spherical Equivalent (SE) was calculated as sphere plus half cylindrical power for left and right eye. Refraction subgroups were again subdivided into Myopic & Hypermetropic as per their SE (myopic SE < 0.00 D and hypermetropic SE > 0.00 D).

Slit lamp examination

A Haag Streit 900 slit lamp was used for examination. Both the eyes were examined under diffuse, focal and retro illumination on the day of examination. The cornea was examined carefully for any abnormality such as corneal opacity, corneal dystrophy, corneal degeneration, keratitis, corneal ulcer, stromal thinning as in Keratoconus etc. The anterior chamber, lens and pupillary reactions were examined to rule out any abnormality. Slit lamp biomicroscopy 90 D was done to rule out fundus abnormality. Specific examination like Pachymeter and keratometer were included as a part of examination.

Pachymetry

CCT was measured with Humphrey ultrasonic pachymeter for left and right eye. Three consecutive readings were taken and mean was calculated.

Keratometry

Curvature of cornea was measured with Bausch and Laumb Keratometer. Vertical and horizontal curvature of cornea was measured. Three consecutive readings were taken and mean KM was calculated for each eye separately.

Results

41.2% of the patients were under 16-30 years of age, 29.8% were under 31-45 years and 29% were under the age group of 45-60 years. The mean age of group was 36.09 ± 13.05 years with median age of 35 years. 53.2% of the patients were males while 46.8% were females.

Table 1 shows the mean SE was (-) 0.47 ± 2.26 with median of 0.50, while mean KM was 43.79 ± 1.18 D with median of 43.88 D. Mean CCT was 528.41 ± 19.1μm with median of 530μm.