Review Article

Corneal Epithelial Thickness Mapping: A Major Review

Table 3

Studies evaluating specific indices or methods in the diagnosis of FFK, S-KCN, and KCN vs. normal eyes.

Author, yearDeviceSignificant indices or algorithmsComparison eyesResults

Li et al. 2012 [7]SD-OCT
6 mm D
RMSV: root mean square of variation
RMSPD: root mean square pattern deviation
76 NL
35 KCN
All indices were positive in KCN
RMSPD has the greatest AUC

Silverman et al. 2014 [104]VHF-US
10 mm
6 variables, including 4 ETM variables analyzed by linear discriminant analysis (LDA)
Neural network (NN) analysis
130 NL
74 KCN
LDA: 100% AUC
NN: 100% AUC

Temstet et al. 2015 [8]SD-OCT
6 mm
Location and CET of thinnest42 NL
36 FFK
32 severe
KCN
Inferior thinnest CET in 91.3%
Thinnest central CET was thinner in FFK compared to NL

Catalan et al. 2016 [105]SD-OCT
6 mm
Combination of 7 ETM and other pachymetry variables104 NL
22 FFK
22 KCN
No good discrimination of individual ETM variables
A combination of ETM and pachymetry had good discrimination power

Tang et al. 2016 [106]SD-OCT
6 mm D
Epi-PSD (pattern standard deviation)
Ant. ectasia index
Warpage index
22 NL
31 KCN
11 CLW
8 FFK
Epi-PSD: NL <4.1% vs. KCN, CLW, FFK >4.1%
Ant. ectasia index: KCN > FFK > CLW > NL
Warpage index: positive in CLW vs. negative in KCN, FFK

Li et al. 2016 [107]SD-OCT
6 mm D
Epi-PSD
Corneal PSD
Stromal PSD
83 NL
50 S-KCN
1 FFK
All 3 parameters were successful
Epi-PSD had the greatest AUC

Xu et al. 2016 [55]UHR-OCTEEI: epithelium ectasia index
BEI: Bowman’s layer ectasia index
EEI-max: maximum epithelium ectasia index
BEI-max: maximum BL ectasia index
81 NL
37 KCN
32 FFK
EEI-max and BEI-max had the highest power of discrimination

Silverman et al. 2017 [108]VHF-US and Scheimpflug device3 variables of VHF-US including 2 ETM variables and 4 variables of Scheimpflug imaging111 NL
30 KCN
97.3% specificity
100% sensitivity

Hwang et al. 2018 [109]SD-OCT 6 mm D and Scheimpflug imagingTwo ETM parameters (ETM SD and greater min-max) and 11 other OCT pachymetry and Scheimpflug indices60 NL
30 FFK
None of the individual ETM parameters showed a good discrimination power
Combination of parameters: 100% sensitivity and 100% specificity

Vega-Strada et al. 2019 [46]SD-OCT+
Placido disc
Thinner 3 mm central
Greater S-I (8 mm)ratio
Greater S-I (6 mm)ratio
60 NL
107 KCN
Combined 3 parameters: AUC: 0.92

Pircher et al. 2019 [54]UHR-OCTBLTM
Minimum BL thickness
R1ET (thinnest to thickest ET)
20 NL
47 KCN
Moth-like change in BLTM
Min BLT: AUC: 0.983
R1ET: AUC: 0.926

Pavlatos et al. 2020 [110]SD-OCTCoincident thinning index82 NL
133 KCN, S-KCN, FFK
CTN: KCN > S-KCN > FFK > NL

Yang et al. 2020 [111]SD-OCT2-step decision tree54 NL
19 FFK
11 S-KCN
91 KCN
NL: 100% specificity
KCN: 97.8% sensitivity
S-KCN: 100% sensitivity
FFK: 73.3% sensitivity

Pavlatos et al. 2021 [112]SD-OCT
6 mm
Epi-MI (modulation index)
Epi-PSD
32 NL
20 CLW
89 KCN
16 S-KCN
26 FFK
Epi-PSD: NL<4.1%, KCN, CLW, FFK> 4.1%
Epi-MI: NL=CLW << FFK < S-KCN < KCN

Toprak et al. 2021 [113]SD-OCT+
Placido disc
E/S (epithelium to stroma) ratio66 NL
27 FFK
55 S-KCN
ETM parameters failed to show a good discrimination power between FFK or S-KCN and normal

Yücekul et al. 2022 [44]SD-OCT2-step decision tree172 NL
20 S-KCN
172 KCN
100% sensitivity, 100% specificity in KCN
90.4% sensitivity in S-KCN

Shi et al. 2022 [114]UHR-OCT and Scheimpflug imagingEpithelial pattern variation (EPV)
Analysis: neural network
Logistic regression
50 NL
33 S-KCN
38 KCN
EPV: best discrimination power
Combination OCT and Scheimpflug: AUC about 0.9

AUC: area under the curve, BL: Bowman’s layer, BLTM: Bowman’s layer thickness map, CET: corneal epithelial thickness, CLW: contact lens wearer, ETM: epithelial thickness mapping, FFK: forme fruste KCN, KCN: keratoconus, KCN: keratoconus, NL: normal eye, RMSPD: root mean square pattern deviation, RMSV: root mean square of variation, SD: standard deviation, SD-OCT: spectral-domain OCT, S-KCN: subclinical KCN, UHR-OCT: ultra-high resolution OCT, and VHF-US: very high-frequency ultrasound.