Comparison of Central Corneal Thickness using Anterior Segment Optical Coherence Tomography Versus Ultrasound Pachymetry

Abstract

Corneal thickness is an important and sensitive indicator of corneal health. It is useful in monitoring corneal diseases such as corneal oedema and keratoconus, and selecting patients for refractive surgery. Central Corneal Thickness (CCT) is a risk factor for Primary Open-Angle Glaucoma (POAG). To compare CCT using ultrasound pachymetry and Anterior Segment Optical Coherence Tomography (AS-OCT), and also to find out the reproducibility of AS-OCT readings for both central and pericentral corneal areas. This prospective cross-sectional study was conducted on 120 patients above 40 years of age, and with clinically normal corneas, who underwent CCT measurements by both ultrasound and AS-OCT. Both the eyes were analysed. Two measurements by AS-OCT and 25 measurements by ultrasound pachymetry were taken. The readings were averaged and compared by paired t-test. Repeatability of the OCT pachymetry map sector averages, was assessed by pooled standard deviation, obtained from the two measurements taken from each eye. The CCT in right eye by OCT and ultrasound was 516.28±29.76 µm and 532.42±29.71 µm, respectively. The CCT in left eye by OCT and ultrasound was 515.82±29.88 µm and 532.36±29.83 µm, respectively. The difference in CCT measurement by AS-OCT and ultrasound was statistically significant (p<0.001); mean ultrasound CCT being 16.14 µm and 16.54 µm greater than the mean AS-OCT, CCT in right eye and left eye respectively. For AS-OCT, intra-session repeatability was measured. Repeatability of the OCT mapping was 0.01 µm to 1.6 µm and 0.01 µm to 1.9 µm in the right eye and left eye respectively. The CCT measurement by ultrasound pachymetry gives higher values compared to AS-OCT measurement. Hence, they cannot be interchangeably used in clinical practice. The AS-OCT provides highly repeatable pachymetry map measurements both centrally and pericentrally.