Correlation between Central Corneal Thickness and Intraocular Pressure among Normal IOP, Ocular Hypertensive and Primary Open Angle Glaucoma Patients

Abstract

Background: Glaucoma is an optic neuropathy characterized by a typical appearance of optic nerve head and characteristic visual field loss. The diagnosis of primary glaucoma, normal tension glaucoma and ocular hypertension is made on the basis of an IOP cut-off point of 21 mmHg. Thus, any factor that alters the value of the IOP can lead to a misclassification of the patient. There is a growing consensus that assessment of Central Corneal Thickness (CCT) has become an important element of the clinical evaluation of the glaucoma patient. This study intends to analyse the CCT trends amongst three study groups: Normal subjects; Primary Open Angle Glaucoma subjects; and Ocular Hypertension subjects. It also intends to explore the relationship between CCT and GAT IOP amongst the three study groups. Lastly, this study aims to evaluate if CCT can be used as a useful tool for prognostication of disease progression in ocular hypertensives, and for glaucoma diagnosis. Aims and Objective: 1. To study and compare the CCT amongst patients with Normal IOP, ocular hypertension and glaucoma. 2. To study and compare the correlation between CCT and IOP amongst patients with Normal IOP, ocular hypertension and glaucoma. Material and Methods: IOP was measured using Goldmann's Applanation Tonometer and CCT was measured using the ultrasound pachymeter in 60 eyes of 30 normal subjects, 60 eyes of 30 glaucoma patients and 60 eyes of 30 ocular hypertensive patients. A correction factor based on the CCT was applied and corrected IOP values were calculated for the three cohorts. Analysis of Variance and Pearson Correlation Test was used to determine any significant difference in CCT, the correction factors and the CCT adjusted IOPs between the three groups. The study was conducted between November 2014 to November 2016. Results: There was a statistically significant difference in the mean CCT of the normals (554.38 + 17.67μm) as compared to the glaucomas (554.15 + 16.39μm) and ocular hypertensives (568.18 + 30.52 μm). Applying the described correction factor for corneal thickness, 40% of eyes with ocular hypertension were found to have a corrected IOP of 21mmHg or less. Conclusion: Increased corneal thickness in ocular hypertension may lead to an overestimation of IOP in 39% of cases. Measurement of central corneal thickness is advisable when the clinical findings do not correlate with the applanation IOP.