Keratoconus staging: a computer-assisted ultrabiomicroscopic method compared with videokeratographic analysis.

PURPOSE

The aim of this study was to introduce a new paradigm for keratoconus assessment, the keratoconus index (KI), generated from the ratio of peripheral corneal thickness (PCT) to the thinnest corneal thickness (TCT), and calculated by a computer-assisted procedure after ultrabiomicroscope (UBM) examination. Then we compared KI and the keratoconus severity index (KSI), obtained by videokeratography in patients with different stages of keratoconus.

METHODS

We studied 60 eyes with different forms of keratoconus using the TMS-3 autotopographer, provided with a keratoconus screening program (using Smolek-Klyce methods) and the commercial version of the ultrasound biomicroscope (Paradigm UBM Plus Model P45) equipped with a 50-MHz probe, which was provided with our computer-assisted program. The proportion test Z and the correlation coefficient R were applied to the outcomes.

RESULTS

The keratoconus severity index, KSI, obtained by color-coded videokeratographic maps, was in the range 95% to 32% (mean 52.22%). By means of UBM examination, we obtained 60 images and found values of TCT 0.278-0.592 mm and PCT 0.475-0.704 mm. Applying the computer-assisted method, we obtained values for KI of 1.112-2.159 (mean 1.428).

CONCLUSIONS

KI is correlated as well as KSI with the severity of the keratoconus (R = 0.76, P < 0.0001). It can be used as a similar parameter to measure the evolution of the disease, on the basis of corneal thickness rather than the curvature.