Repeatability of a Dual-Scheimpflug Placido Disc Corneal Tomographer/Topographer in Eyes with Keratoconus

To investigate the repeatability of a combined Dual-Scheimpflug placido disc corneal tomographer/topographer (Ziemer Galilei G4) with respect to keratometric indices used to monitor progression of keratoconus (KCN).

Patients with KCN were prospectively enrolled. For each eye lacking history of corneal surgery, 5 measurements were taken in succession. Eyes in which 3 or more measurements could be obtained (defined by the device's 4 image quality metrics) were included in the analysis. The repeatability limits (RL) and interclass correlation coefficients (ICC) were calculated for various parameters.

Thirty-two eyes from 25 patients met all image quality metrics, and 54 eyes from 38 patients met at least 3/4 criteria (all except the placido image quality metric). RLs for key parameters when 4/4 or ≥3/4 image quality metrics were met included: 0.37 and 0.77 diopters (D) for steep simulated keratometry, 0.79 and 1.65 D for maximum keratometry, 13.80 and 13.88 degrees for astigmatism axis, 0.64 and 0.56 µm for vertical coma magnitude, and 3.76 and 3.84 µm for thinnest pachymetry, respectively. The ICCs for all parameters were excellent (above 0.87) except for spherical aberration (0.77), which was still considered good.

The dual-Scheimpflug placido disc corneal tomographer/topographer is highly repeatable in quantifying parameters used in monitoring KCN. Excellent placido images are difficult to capture in eyes with KCN, but when available, increase the reliability of the measurements. When clinicians find that a topographic index changes by more than the RLs defined herein, they can have confidence that this represents real change and may appropriately recommend interventions such as corneal cross-linking or intrastromal corneal ring segments.