Table 1.

Variability of the criteria used to define keratoconus progression in different studies.

Study	Visual acuity	Refraction	Keratometry	Pachymetry	Topography	Considerations
Caporossi et al. [30]	UCVA/BSCVA decrease ≥ 1 Snellen line	ΔSph or ΔCyl > 0.5 D	ΔK mean > 0.5 D	Decrease in thinnest pachymetry ≥ 10 μm	ΔSAI/SI > 0.5 D	At least 2 parameters in 3 months
Caporossi et al. [31]	UCVA/BSCVA decrease ≥ 1 Snellen line	ΔSph or ΔCyl > 0.5 D	ΔK max⁡ > 1 D	Decrease in thinnest pachymetry ≥ 10 μm	ΔSAI/SI > 0.5 D	At least 3 parameters (1 clinical and 2 instrumental) in 3 months
Vinciguerra et al. [32]		Change in Sph or Cyl ≥ 3 D	ΔK mean ≥ 1.5 D on 2 consecutive topographies	Decrease in CCT ≥ 5% on 3 consecutive tomographies		Any parameter in 3 months
Zotta et al. [33]		ΔSE > 0.75 D	ΔK max⁡ of cone apex > 0.75 D			Any parameter in 6 months
Chatzis and Hafezi [34]			ΔK max⁡ > 1 D			Follow-up period of maximum 12 months
Bakshi et al. [35]		ΔCyl ≥ 1.5 D	ΔK max⁡ ≥ 1.5 D			Any parameter at 3 time points in 12 months
Magli et al. [36]		ΔCyl > 1 D	ΔK max⁡ of cone apex > 1 D			Any parameter in 6 months
Shetty et al. [32, 37]			ΔK max⁡ > 1–1.5 D with corresponding change in refraction	Decrease in thinnest pachymetry ≥ 5%		Any parameter in 6 months
Zotta et al. [33]		ΔCyl ≥ 1 D and ΔSE ≥ 0.5 D	ΔK max⁡ ≥ 1 D			All 3 parameters on consecutive examinations

UCVA: uncorrected visual acuity, BSCVA: best spectacle corrected visual acuity, Sph: sphere, Cyl: cylinder, SAI: surface asymmetry index, SI: symmetry index, CCT: central corneal thickness, SE: spherical equivalent, and Δ: increase in.