Table 1.

Modalities and indices used for diagnosing established and early keratoconus and the corresponding anatomical location

Imaging modality	Indices for KC diagnosis	Indices for early KC detection	Corneal anatomy
Reflection-based topography	K-value, I-S value, KPI, KISA%, KSI	-	Anterior
Elevation-based tomography	Anterior and posterior elevation of the corneal apex, keratometric values, pachymetric data based on comparing reconstruction of the anterior and posterior corneal surface to a best-fit sphere	ISV, IHA, BAD-D, PRFI, I-S value (Pentacam), CSI (Galilei), SIb, KVf (Sirius) Corneal densitometry (The anterior layer in the 0 to 2 mm area) PE from BFTE	Anterior/stromal/posterior
Anterior segment optical coherence tomography	Min, Min-Max, SN-IT, and Std Dev	Pachymetric indices C1–C2 Epithelial donut pattern Superonasal epithelial thickening and inferotemporal epithelial thinning The vertical maximum ectasia index of epithelium and horizontal maximum ectasia index of Bowman’s layer Epithelial thickness in the thinnest corneal zone Increased central epithelial/stromal (E/S) ratio CCT As/Ps Epithelial PSD	Anterior/epithelial/stromal/posterior
Noncontact tonometry	CH and CRF with ORA Radius value of central concave curvature at highest concavity, central corneal thickness, CBI, and TBI with the Corvis ST	TBI, CBI, HCR, IR, and DAR	Stromal
Brillouin light-scattering microscopy	Brillouin frequency shift	The mean and minimum values in the anterior plateau region and anterior 150 microns	Stromal
Wavefront aberrations	Vertical coma, total coma, total HOAs, front/back Zernike coefficient Z3–1	Vertical coma, BCVf, ocular total HOA, total coma Minimal corneal thickness, anterior coma to 90°, and posterior coma to 90°	-
In vivo confocal microscopy	Anterior and posterior stromal keratocyte density, epithelial cell shape, corneal nerve fiber length, subbasal nerve density, and stromal nerve diameter	-	Epithelial/stromal/endothelial
Optical coherence elastography	Ka/Kp	-	Stromal

K-value = keratometry value; I-S value = inferior-superior value; KPI = keratoconus prediction index; KISA% = keratoconus percentage index; KSI = keratoconus severity index; ISV = index of surface variance; IHA = index of height asymmetry; BAD-D = Belin-Ambrósio enhanced ectasia display deviation; PRFI = Pentacam random forest index; CSI = center/surround index; SIb = symmetry index back; KVf = keratoconus vertex front; PE = posterior elevation; BFTE = best-fit toric ellipsoid; Min = minimum corneal epithelial thickness; Min-Max = minimum minus maximum corneal epithelial thickness; SN-IT = superonasal-inferotemporal; Std Dev = standard deviation; C1 = average corneal thickness at the points placed within a 1 mm diameter circle around the point with the lowest thickness, C2 = the average corneal thickness at the points located diametrically opposite to the first one; C1-C2 = the difference between C1 and C2; CCT = central corneal thickness; As/Ps = anterior/posterior corneal surface areas ratio; PSD = pattern standard deviation; CH = corneal hysteresis; CRF = corneal resistance factor; ORA = ocular response analyzer; CBI = Corvis biomechanical index; TBI = tomographic/biomechanical index; HCR = highest concavity radius; IR = integrated radius; DAR = deformation amplitude ratio; HOAs = higher-order aberrations; BCVf = Baiocchi Calossi Versaci front; Ka/Kp = slope of the force/displacement function for anterior (a) and posterior (p) corneal stromal regions