Corneal Thinning Reduces Tomographic Repeatability in Keratoconus

Corneal thinning in patients with keratoconus may significantly limit the repeatability of corneal tomography measurements, according to research published in Eye. Measurements obtained from patients with keratoconus with moderate thinning demonstrated greater repeatability compared with those with more advanced thinning, the report shows.

Researchers enrolled 228 patients with keratoconus in the prospective, single-center study and included individuals with advanced corneal thinning (thinnest corneal thickness [TCT], ≤400 µm; n=114; mean age, 25.76 years; 59.65% men) and age- and sex-matched control group participants with moderate corneal thinning (TCT, 450-500 µm; n=114; mean age, 25.76 years; 59.65% men). An investigator performed 3 sets of corneal tomography measurements, which included maximal corneal power (Kmax), simulated flat and steep keratometry of the anterior and posterior corneal surfaces, and corneal pachymetry at the thinnest point.

The repeatability of all parameters evaluated was lower among patients with advanced thinning (interclass correlation coefficient [ICC] range, 0.84-0.98) compared with those with moderate corneal thinning (ICC range, 0.92-0.99). In both cohorts, the most repeatable measures were flat keratometry of the anterior surface (ICC, 0.97 vs 0.98) and steep keratometry of anterior surface (ICC, 0.98 vs 0.99) for advanced and moderately thin corneas, respectively. The least repeatable parameter was astigmatism for individuals with advanced thinning (ICC, 0.84) and TCT for participants with moderate corneal thinning (ICC, 0.92).

The standard deviation of the 3 tomography measurements correlated with the maximal corneal power among the advanced thinning (r, 0.27; P <.01) and moderate thinning (r, 0.42; P <.01) groups and with TCT among the advanced corneal thinning group (r, -0.37; P <.01).

In a subgroup comparison of patients with very severe corneal thinning (TCT, ≤330 mm; n=27) and individuals with severe thinning (TCT, 331-400 mm; n=87), the ICC values ranged between 0.82-0.99 and 0.95-0.99, respectively. As with the main analysis, flat and steep keratometry of anterior surface were the most repeatable parameters.

“The repeatability limits of different tomographic parameters quantified in this study have important implications for both the application of CXL in patients with advanced keratoconus as well as monitoring progression of disease,” according to the researchers. “Parameters such as TCT are pivotal when deciding whether to offer CXL to patients with keratoconus. Knowing the repeatability limits reported in our study can help practitioners make informed decisions about the type of CXL protocol…”

A failure to assess reproducibility between investigators and differences in contact lens wear among the 2 cohorts are acknowledged limitations to the research.