EVALUATION OF CENTRAL AND PERIPHERAL CORNEAL THICKNESS WITH ULTRASOUND BIOMICROSCOPY IN NORMAL AND KERATOCONIC EYES

Purpose. Our study was designed to calculate central and pe ripheral c orneal thickness in patients affected with early stages of keratoconus and in normal subjects using ultrasound biomicroscopy (UBM). To obtai n an objective and reliable assessment of the corneal thinning in affe cted eyes, we developed a keratoconus index (KI) by means of the UBM m easurements. Methods, By means of the commercial version of the ultras ound biomicroscope (system model 840; Zeiss-Humphrey Instruments, San Leandro, CA, U.S.A.) using a 50-MHz probe, we studied 30 normal and af fected eyes. In keratoconic eyes, we measured the thinnest corneal thi ckness (TCT) at the apex of the conus and at four peripheral sites at a distance of 2.5 mm from the central site (peripheral corneal thickne ss: PCT). The same procedure was performed in the normal eyes. To obta in an objective and reliable assessment of the corneal thinning, we ca lculated the ratio between the mean PCT and the mean TCT (Keratoconus Index: KI = PCT/TCT), in keratoconic eyes. In normal eyes, the KI was calculated on the basis of the ratio between the mean PCT and the mean central corneal thickness (CCT). Results, In keratoconic eyes, the me an corneal thickness at the thinnest part of the conus was significant ly different from the CCT in normal patients (Student's t test, p < 0. 001). The peripheral measurements were not significantly different fro m keratoconic and normal eyes. The mean KI was 1.482 (SD, 0.095) in th e keratoconic eyes, whereas it was 1.189 (SD, 0.086) in the normal sub jects. The statistical analysis of the KI calculated on the basis of t he UBM measurements showed that the KI values are significantly differ ent from healthy subjects and from keratoconic patients (Student's t t est, p < 0.001). Conclusions, UBM can be considered a useful tool in t he study of keratoconus. We believe that calculation of the KI by mean s of UBM gives the possibility of obtaining an objective assessment of corneal thinning. Therefore this parameter can be useful in the stagi ng and in the follow-up of these patients.