Several methods have been described in the literature to both evaluate and document possible changes in keratoconus, but keratoconus progression per se has not been consistently or clearly defined.
When corneal deformation progresses, steepening of the anterior corneal surface with corneal thinning would be expected.
Progression (or stabilization) of the corneal deformation must be assessed by reliable examinations.
The direct inspection of a series of corneal topography maps performed at different time points can be instructive, but it is sometimes difficult to confirm an impression.
The repeatability (test–retest reliability) of topography instruments is lower in keratoconic eyes than in normal eyes. For example, we found that the « critical difference » for simulated keratometry (or maximum keratometry) is about 1.50 D for advanced keratoconic eyes i.e. differences in monitored values that are smaller than this critical difference should not be considered as statistically significant (see here for more).
In topography, the difference map allows us to compare two examinations carried out at different times. All the cases reported on this website are monitored with several corneal topography examinations at each visit. We perform a difference map whenever possible, as they provide a direct quantified, and comprehensive representation of the presence (or absence) of significant variations between two examinations.