Contribution of reference bodies in diagnosis of keratoconus

To evaluate the contribution of reference bodies for elevation maps in diagnosis of keratoconus (KC) and forme fruste keratoconus (FFK).

Study groups included (1) 80 eyes with KC, (2) 55 eyes with FFK, and (3) 95 eyes as control eyes. The maximum elevation value at the central 5 mm of the posterior cornea (MEL) and the elevation value at thinnest corneal point (TEL) were evaluated for their diagnostic capacity using receiver operating characteristic curves under the following reference bodies: sphere, ellipsoid, toric ellipsoid, and torus, with autodiameter and fixed 8-mm-diameter settings. Moreover, characteristic parameters for each reference body and the root mean square (RMS) were evaluated for their diagnostic capacity.

Significant differences were detected in TEL, MEL, and RMS parameters among groups, for almost all reference bodies. The highest predictive accuracy (KC group, >99.97%; FFK group, >90.9%) was estimated for the TEL and MEL parameters using the toric ellipsoid (8 mm) reference body with a fixed eccentricity of 0.4. Moreover, the RMS parameter demonstrated sufficient diagnostic capacity (KC group, 99.95%; FFK group, 85.1%). The cutoff points for the TEL parameter were found to be 7 and 3 μm in the KC group and the FFK group, respectively; for the MEL parameter, they were 11 and 6 μm, respectively; and for the RMS parameter, they were 6.87 and 7.19 μm, respectively.

The posterior corneal elevation values and the RMS parameter can effectively discriminate KC from normal corneas, although measured values and cutoff points depend on the selection of reference body. Moreover, the use of a toric ellipsoid reference body, with a fixed eccentricity of 0.4, seems to be an effective tool even in diagnosis of FFK.