Anterior and posterior ratio of corneal surface areas: A novel index for detecting early stage keratoconus.

PURPOSE: To evaluate the diagnostic ability of the ratio of anterior and posterior corneal surface areas (As/Ps) comparing with other keratoconus screening indices in distinguishing forme fruste keratoconus (FFKC) from normal eyes.
METHODS: In this comparative study, 13 eyes of 13 patients with FFKC, 29 eyes of 29 patients with keratoconus (KC) and 88 eyes of 88 patients with normal subjects were involved. The As/Ps measured by the anterior segment optical coherence tomography (AS-OCT) and other indices measured by AS-OCT and rotating Scheimpflug-based corneal tomography were evaluated. The area under receiver-operating-characteristics (AU-ROC) was calculated to assess the diagnostic ability in discriminating FFKC from normal eyes.
RESULTS: The As/Ps, the Belin/Ambrosio display enhanced ectasia total derivation value (BAD-D) and posterior and anterior elevation values showed the AU-ROC 0.9 or more in differentiating FFKC from normal eyes (0.980, 0.951, 0.924 and 0.903, respectively). The sensitivity and specificity were 0.92 and 0.96 for the As/Ps, 1.00 and 0.90 for BAD-D, 0.85 and 0.86 for posterior elevation value, and 0.85 and 0.96 for anterior elevation value, respectively.
CONCLUSIONS: Among the several indices for keratoconus screening which we evaluated, the As/Ps obtained by AS-OCT had the large AU-ROC with high sensitivity and specificity in detecting FFKC, which was comparable with BAD-D obtained by rotating Scheimpflug-based corneal tomography. The As/Ps may provide information for improving the diagnostic accuracy of KC, even in the initial stage of the disease.

Introduction

Keratoconus (KC) is a progressive corneal disease characterized by local corneal thinning and protrusion of the cornea, thus resulting in loss of vision due to irregular corneal astigmatism [1,2]. It has been reported that the estimated prevalence of KC among the general population is approximately 1 in every 2000 persons, and thus, very common [3]. Following the advent of refractive surgery methods such as photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK), etc., there have been numerous reported cases of myopia progression due to rapid progression of central steepening postsurgery, classified as keratectasia and with a pathology similar to KC [4,5]. Thus, specific diagnostic criteria in regard to the development of KC is required, as it may contribute to not only the prevention of the progression into severe KC, [2,6] but also the overall safety of refractive surgery [5].

KC is basically a bilateral disease [2], and the contralateral cornea of patients with KC without any ectatic changes clinically and topographically has been designated as forme fruste keratoconus (FFKC) [7]. A recent study reported that in 50% of the subjects with unilateral KC, onset of the disease will occur in the non-affected fellow eye within 16 years [8], and that those FFKC eyes were considered to be at the earliest and mildest stage of KC [9]. In previous reports, the corneal surface area showed significant difference between KC and normal eyes [10,11] as well as several diagnostic indices obtained by corneal topography and rotating-Scheimpflug-camera-based corneal tomography that are highly successful for discriminating KC and FFKC from normal healthy eyes [9,12-15]. We previously reported that anterior and posterior corneal-surface-area imbalance derived from areas calculated via the elevation map of anterior-segment optical coherence tomography (AS-OCT) may reflect keratoconic eyes at the early stage of the disease and that using the ratio of anterior-posterior surface area (As/Ps) to distinguish normal eyes from FFKC eyes was 0.948, and quite large [16]. However, and the diagnostic accuracy of As/Ps for discriminating FFKC eyes from normal eyes has not been fully been evaluated and compared to that of other indices.

The purpose of this present study was to investigate the diagnostic ability of As/Ps to detect FFKC by comparing it with the previously-reported KC detection indices provided by AS-OCT and rotating-Scheimpflug-camera-based corneal tomography.

Methods

Patient inclusion

In this retrospective case-control study, we examined 13 eyes of 13 patients with FFKC (11 male and, 2 female), 29 eyes of 29 patients with KC (24 male and, 5 female) and 88 eyes of 88 patients with normal subjects (55 male and, 33 female) at Keratoconus and Refractive Subspecialty Clinic in the Baptist Eye Institute, Kyoto, Japan from April 2015 to June 2018. The mean ± standard deviation (SD) ages of patients with FFKC, KC and normal subjects were 28.2 ± 6.4 years (range 16–39 years), 27.1 ± 5.4 years (range 17–39 years), and 30.4 ± 7.3 years (range 16–36 years), respectively. The KC eyes were used to analyze as reference group.

The KC-group eyes were used for reference analysis. The study protocol was approved by the Kyoto Ethics Review Committee, Kyoto, Japan, an independent organization established for the approval of ethics-related issues (Approval #1604). This study was performed in accordance with the tenets set forth in the Declaration of Helsinki. Written informed consent was obtained from all subjects and their parents/guardians for patients younger than 20 years old.

Patients were diagnosed as having keratoconus if all the following criteria were found: 1) at least one of the following biomicroscopic signs: corneal Fleischer ring, Vogt’s striae, corneal thinning, and corneal protrusion at the apex, 2) keratoconic appearance such as focal or inferior steepening, an asymmetric bow-tie pattern with or without skewed axis [9] on placido-disc based topography (TMS-4, Tomey Corporation, Nagoya, Japan), and 3) the Keratoconus index (KCI) showed 5% or more. The KCI is video-keratographic index for detection keratoconus, calculated by Clyce/Maeda criteria on TMS-4 [13]. Subjects were classified into the FFKC group if 1) they had no clinical and topographic signs of keratoconus in one eye, 2) passed through the TMS-4 keratoconus screening program with 0% KCI, and 3) presented a diagnosis of keratoconus in the contralateral eye. The normal group subjects had no indication of KC, and enrolled as age matched control to FFKC group. Exclusion criteria were the followings: previous ocular surgery, corneal scarring, trauma, acute corneal hydrops, and a history of other ocular disease beside refractive errors. The patient who had not corneal-map with high quality and satisfactory for calculation (refer to Imaging methods) obtained by either AS-OCT or rotating Scheimpflug–based corneal topography was also excluded. Contact lens–wearing patients were asked to stop wearing contact lenses for 4 weeks, in case of rigid contact lenses, and 2 weeks for soft contact lenses, before assessment. Three representative cases are shown in Fig 1.

Fig 1

Representative data of a normal subject, a forme fruste keratoconus patient, and a keratoconus patient.

Anterior segment photo (upper left), Placido-disc based topography (upper right), anterior (bottom left) and posterior (bottom right) corneal elevation map using anterior segment optical coherence tomography (AS-OCT) images in normal subject, forme fruste keratoconus (FFKC) patient and keratoconus (KC) patient.

Imaging methods

All patients underwent topographic examination via AS-OCT imaging (SS-1000 CASIA; Tomey Corporation, Nagoya, Japan), high-resolution rotating Scheimpflug–based corneal tomography (Pentacam HR, Oculus Optikgerate GmbH, Wetzlar, Germany) and placido-disc based topography (TMS-4, Tomey Corporation, Nagoya, Japan) on the same day. We previously reported that data via AS-OCT has had high reproducibility and repeatability [16].

The AS-OCT is swept-source OCT that uses a center wavelength of 1,310 nm and measures with a speed of 30,000 A lines per second. Using the AS-OCT, 16 cross-sectional images, which consisted of 512 A-scans were obtained for 0.34 second during 1 measurement to assess the corneal topography. The built-in program identified and digitized the anterior and posterior corneal surfaces for further analysis.

The indices from the AS-OCT included the anterior and posterior mean Ks (steep keratometry), and mean Kf (flat keratometry), the different sector index (DSI), the opposite sector index (OSI), central corneal thickness (CCT) at the apex, corneal thickness at the thinnest point (CTmin), anterior or posterior best-fit sphere (BFS), elevation values, and corneal surface areas, the As/Ps and the ectasia screening index (ESI). The DSI is the greatest difference in the average power between any two sectors and the OSI is the greatest difference in the average power of two opposite sectors from a divided eight pie-shaped image. The BFS was calculated by AS-OCT built-in software with float option within central 9.0 mm cornea and used as reference surface. Anterior or posterior elevations were measured as the maximum value above the BFS in the central 5 mm of the anterior or posterior cornea. The anterior or posterior corneal surfaces were calculated within a central 5.0 mm (not radius) area of cornea based on the anterior or posterior elevation maps, as we previously reported [17]. Briefly, we assumed the cornea was a spherical cap and used the formula , in which S = surface area, PI = ratio of a circle’s circumference (3.14), R = curvature, and D = corneal diameter, at each measurement point based on the elevation map which was calculated by AS-OCT. The calculation scheme is shown in Fig 2.

Fig 2

Schema used to compute the corneal surface area.

To calculate the corneal surface areas, we assumed the corneal surface as a fan-shaped surface that lacked the central area (ΔS). Each fan-shaped surface was based-on an adjacent point, which was merged into each calculation. The calculation formulas are shown in the top part of the figure, as we previously reported [17]. i = measuring point in the direction of rotation (range: 1–32), j = ring number [1 to RingNum (※)], ※ = outermost ring number (determined by the measurement diameter), r = distance from the center to the measuring point in the XY direction, dr = diameter of the unit segment joining 2 adjacent points [(i, j) and (i, j + 1)], dh = height of the unit segment joining 2 adjacent points [(i, j) and (i, j + 1)], DS = fan-shaped surface lacking the central area.

The As/Ps was calculated as follows: As/Ps = anterior corneal surface area / posterior corneal surface area. The ESI was measured by AS-OCT built-in software which estimates the ectasia similarity of the scan. The ESI consisted of anterior and posterior score derived from Fourier analysis [18]. In each rotating Scheimpflug–based corneal tomography scans measurement, a total of 25 single Scheimpflug images captured within 1 second to measure the anterior and posterior corneal configurations. KC index from the rotating Scheimpflug-based corneal tomography included the index of surface variance (ISV), the index of vertical asymmetry (IVA), the maximum (PPImax), minimum (PPImin) and the average (PPIave) pachymetric progression index, the maximum Ambrosio relational thickness (ARTmax), the back difference elevation (BDE) and the Belin/Ambrosio display enhanced ectasia total derivation value (BAD-D). The PPI was calculated as the progression value at the different rings referenced to the mean curve. The ARTmax was calculated as follows: ARTmax = the corneal thickness at the thinnest point / PPImax. The BDE and BAD-D were extrapolated from the difference map of the Belin/Ambrosio-enhanced ectasia display.

Placido-disc based topography takes 4 measurements capturing images of 25 mire ring during 3.0 seconds, and evaluates 256 points on each 25 mire rings. We evaluated Keratoconus Index which is a video-keratographic index calculated by Klyce/Maeda criteria on TMS-4, which is based on linear discriminant analysis and a binary decision tree [14].

In each examined eye, a corneal map with at least 9.0 mm of corneal coverage and no extrapolated data by AS-OCT and rotating Scheimpflug corneal tomography respectively, was required.

Statistical analyses

Statistical analyses were accomplished using R version 3.1.0 (The R Foundation) statistical software, with the data being presented as mean ± SD where applicable. The data were not normally distributed; therefore, Kruskal-Wallis test followed by paired Wilcoxon test was used to compare each parameter between normal eyes and FFKC or KC eyes. A Bonferroni correction was used to control type I error. A P-value less than 0.05 was considered to be statistically significant. Receiver- operating-characteristic (ROC) curves were used to determine the overall predictive accuracy of the test parameters as described by the area under the curve (AUC) and to calculate the sensitivity and specificity in distinguishing FFKC from normal eyes. Optimal cutoff points for each index were obtained from the ROC curves as those closest to the particular point, where sensitivity equals specificity. This point of the curve is where the product of these two indices (ie. sensitivity and specificity) is maximum. [19]

Results

Intergroup differences

The median of all parameters used for statistical analysis in all groups was summarized in Table 1. Statistically significant differences were noted between normal and FFKC eyes for all parameters with the exception of anterior and posterior keratometric reading (i.e., Ks and Kf), BFS and anterior surface areas.

Table 1

All parameters of Normal, FFKC and KC eyes acquired from AS-OCT and rotating Scheimpflug tomography.

Indices, Mean ± SD		Normal (n = 88)	FFKC (n = 13)	KC (n = 29)	N vs FFKC	N vs KC
AS-OCT	aKs	48.62 ± 1.52	48.84 ± 2.74	60.73 ± 8.09	0.33	p < 0.01
	aKf	47.31 ± 1.36	46.69 ± 2.32	56.27 ± 7.25	0.52	p < 0.01
	pKs	-6.30 ± 0.23	-6.40 ± 0.36	-7.78 ± 1.87	0.23	p < 0.01
	pKf	-5.98 ± 0.18	-6.04 ± 0.20	-7.33 ± 1.51	0.48	p < 0.01
	DSI	1.61 ± 0.60	2.87 ± 1.13	11.27 ± 4.84	p < 0.01	p < 0.01
	OSI	0.77 ± 0.46	1.98 ± 1.07	10.69 ± 4.80	p < 0.01	p < 0.01
	CCT	541.15 ± 26.23	501.46 ± 34.83	438.07 ± 60.87	p < 0.01	p < 0.01
	CTmin	536.86 ± 25.96	493.46 ± 36.88	411.86 ± 59.03	p < 0.01	p < 0.01
	aBFS	7.88 ± 0.20	7.88 ± 0.45	6.45 ± 0.89	0.31	p < 0.01
	pBFS	6.57 ± 0.23	6.48 ± 0.32	5.10 ± 0.93	0.18	p < 0.01
	As	20.15 ± 0.03	20.14 ± 0.06	20.40 ± 0.20	0.63	p < 0.01
	Ps	20.38 ± 0.05	20.44 ± 0.07	20.90 ± 0.36	p < 0.01	p < 0.01
	As/Ps	0.989 ± 0.001	0.986 ± 0.001	0.976 ± 0.008	p < 0.01	p < 0.01
	AE	5.43 ± 1.38	13.85 ± 7.23	44.00 ± 22.15	p < 0.01	p < 0.01
	PE	10.00 ± 2.82	22.69 ± 12.07	89.93 ± 41.93	p < 0.01	p < 0.01
	ESI	0.00 ± 0.00	5.77 ± 6.98	76.00 ± 24.88	p < 0.01	p < 0.01
Rotating Scheimpflug tomography	ISV	16.19 ± 5.67	27.85 ± 13.99	109.69 ± 48.68	p < 0.01	p < 0.01
	IVA	0.10 ± 0.04	0.26 ± 0.19	1.05 ± 0.40	p < 0.01	p < 0.01
	PPImin	0.71 ± 0.14	0.89 ± 0.26	2.29± 1.24	p < 0.01	p < 0.01
	PPImax	1.28 ± 0.18	1.67 ± 0.43	4.42± 1.85	p < 0.01	p < 0.01
	PPIave	1.00 ± 0.16	1.31 ± 0.24	2.98 ± 1.33	p < 0.01	p < 0.01
	BDE	5.31 ± 3.40	8.92 ± 4.29	55.83 ± 36.95	p < 0.01	p < 0.01
	ARTmax	436.30 ± 73.69	318.08 ± 84.24	118.07± 57.10	p < 0.01	p < 0.01
	BAD-D	0.94 ± 0.48	2.11 ± 0.77	12.34 ± 6.28	p < 0.01	p < 0.01

FFKC: forme fruste keratoconus, KC: keratoconus, N: Normal, aKs: Anterior steep keratometry, aKf: Anterior flat keratometry, pKs: Posterior steep keratometry, pKf: Posterior flat keratometry, DSI: differential sector index, OSI: Opposite sector index, CCT: central corneal thickness, CTmin: corneal thickness at the thinnest point, aBFS: anterior best-fit sphere, pBFS: posterior best-fit sphere, As: anterior corneal surface area, Ps: posterior corneal surface area, As/Ps: ratio of anterior and posterior corneal surface area, AE: anterior elevation values, PE: posterior elevation values ESI: ectasia screening index, ISV: index of surface variance, IVA: index of vertical asymmetry, PPImin: minimum pachymetric progression index, PPImax: maximum pachymetric progression index, PPIave: average pachymetric progression index,BDE: back difference elevation, ARTmax: maximum Ambrosio relation thickness, BAD-D: Belin/Ambrosio display enhanced ectasia total derivation value, SD: standard deviation.

To evaluate the ability of indices to discriminate between normal eyes and FFKC eyes, the results of the ROC analysis including AU-ROC and 95% confidence intervals (CI) for all parameters were plotted (Table 2). The AU-ROC of the As/Ps, the Belin/Ambrosio display enhanced ectasia total derivation value (BAD-D) and posterior and anterior elevation were 0.9 or more in differentiating FFKC from normal eyes (0.980, 0.951, 0.924 and 0.903, respectively). The cut-off points with sensitivity and specificity of As/Ps were 0.98 with 0.92 and 0.96, respectively. The sensitivity and specificity of other indices were 1.00 and 0.90 for BAD-D, 0.85 and 0.86 for posterior elevation value, and 0.85 and 0.96 for anterior elevation value, respectively.

Table 2

Receiver-operating-characteristic curve analysis for the Normal versus FFKC eyes.

	Indices	AU-ROC	95% CI	Cut-off	Sensitivity	Specificity
AS-OCT	aKs	0.584	0.374–0.795	50.26	0.46	0.84
	aKf	0.556	0.363–0.750	47.00	0.54	0.59
	pKs	0.603	0.396–0.810	-6.42	0.62	0.66
	pKf	0.566	0.367–0.765	-6.15	0.46	0.76
	DSI	0.826	0.667–0.984	1.95	0.85	0.76
	OSI	0.868	0.737–0.999	1.24	0.85	0.86
	CCT	0.809	0.648–0.971	507.00	0.69	0.89
	CTmin	0.822	0.665–0.980	500.00	0.69	0.91
	aBFS	0.587	0.353–0.821	7.72	0.54	0.78
	pBFS	0.615	0.413–0.818	6.48	0.62	0.59
	As	0.542	0.333–0.751	20.20	0.46	0.71
	Ps	0.751	0.548–0.955	20.42	0.69	0.80
	As/Ps	0.98	0.954–1.00	0.99	0.92	0.96
	AE	0.903	0.770–1.000	8.00	0.85	0.96
	PE	0.924	0.853–0.997	13.00	0.85	0.86
	ESI	0.731	0.59–0.872	7.00	1.00	0.46
Rotating Scheimpflug tomography	ISV	0.801	0.659–0.943	22.00	0.69	0.83
	IVA	0.884	0.75–1.000	0.16	0.85	0.85
	PPImin	0.782	0.606–0.958	0.85	0.69	0.84
	PPImax	0.832	0.692–0.973	1.37	0.85	0.78
	PPIave	0.866	0.74–0.993	1.15	0.77	0.85
	BDE	0.743	0.583–0.904	7.00	0.69	0.67
	ARTmax	0.871	0.749–0.992	388.00	0.92	0.78
	BAD-D	0.951	0.910–0.991	1.33	1.00	0.90

AU-ROC: the area under the receiver operation characteristics curve, CI: cofidence interval, aKs: Anterior steep keratometry, aKf: Anterior flat keratometry, pKs: Posterior steep keratometry, pKf: Posterior flat keratometry, DSI: differential sector index, OSI: Opposite sector index, CCT: central corneal thickness, CTmin: corneal thickness at the thinnest point, aBFS: anterior best-fit sphere, pBFS: posterior best-fit sphere, As: anterior corneal surface area, Ps: posterior corneal surface area, As/Ps: ratio of anterior and posterior corneal surface area, AE: anterior elevation values, PE: posterior elevation values ESI: ectasia screening index, ISV: index of surface variance, IVA: index of vertical asymmetry, PPImin: minimum pachymetric progression index, PPImax: maximum pachymetric progression index, PPIave: average pachymetric progression index, BDE: back difference elevation, ARTmax: maximum Ambrosio relation thickness, BAD-D: Belin/Ambrosio display enhanced ectasia total derivation value.

The ROC curves of these parameters to differentiate FFKC from normal eyes are shown in Fig 3.

Fig 3

Receiver operating characteristic curves of each index.

Combined receiver operating characteristic curves for the ratio of anterior and posterior corneal surface areas (As/Ps), Belin/Ambrosio display enhanced ectasia total derivation value (BAD-D), Posterior elevation values (PE) to differentiate forme fruste keratoconus (FFKC) from normal controls. Note that the As/Ps has the largest AUC.

Discussion

In this present study, we found that the As/Ps calculated by elevation map of the AS-OCT, which we previously reported as a novel comprehensive index reflecting the both of anterior and posterior cornea surfaces, revealed the large AU-ROC (0.980). Although previous geometric reports demonstrated anterior surface and posterior surface areas as new indices [10,11], to the best our knowledge, the present study is a first report showing the discriminating ability of the As/Ps obtained by AS-OCT to detect FFKC via comparison with several KC screening parameters obtained by AS-OCT and rotating Scheimpflug–based corneal tomography.

In order to detect early stage of KC, there were already a great variety of indices, derived from the posterior elevation parameters, corneal thickness and corneal aberration [9,12,13,20]. In the studies of index measured by Scheimpflug imaging system, the ART max [12], the MAX PE5 [13], which measured the maximum posterior elevation from the reference BFS at the central 5.0 mm of cornea, and the back difference elevation (BDE) [9] were reported to show the large AU-ROC in FFKC compared to normal eyes as 0.88 (0.82 sensitivity and 0.70 specificity), 0.93 (0.68 sensitivity and 0.70 specificity)and 0.76 (0.74 sensitivity and 0.65 specificity), respectively. Naderan and associates [20] found that the ocular total higher aberration had the highest diagnostic ability in discriminating FFKC from normal eyes, as 0.946 (0.94 sensitivity and 0.94 specificity). However, those previous studies were comprised of a different patient population with different measurement techniques. Thus, those indices cannot be directly compared with the diagnostic accuracy of the As/Ps in this present study.

The corneal surface area ratio in this preset study used data at the central 5.0-mm, because our previous study revealed that AU-ROC of As/Ps at the central 5.0-mm areas was larger than that at the central 6.0-mm and 7.0-mm.15 This might possibly be explained by that the apex of the both anterior and posterior surfaces was located within the central 5.0-mm, and the difference between anterior surface are and posterior surface at larger area was underestimated.

In this present study, AU-ROC of the As/Ps, BAD-D and anterior and posterior elevation values showed 0.9 or more, and furthermore, the As/Ps and BAD-D showed high sensitivity and specificity (>0.9). The test with AU-ROC greater than 0.9 can be interpreted as having high accuracy [21], thus our results suggest that the As/Ps had sufficient strength in differentiate FFKC from normal eyes. Although this finding does not reduce the importance of known indices which showed AU-ROC of less than 0.9, it highlights the importance of the As/Ps derived from AS-OCT in detecting KC at the early stage of disease.

In this present study, the As/Ps had the large AU-ROC (0.980), in distinguishing the FFKC from the normal eyes. This large AU-ROC in As/Ps was explained by the characteristic of the index, which included not only the components of anterior and posterior cornea surfaces, but also imbalance of anterior-posterior ratio, and corneal thickness, as we previously reported [16]. However, there is ongoing debates as to whether changes in the anterior surface [22], posterior surface [2,9], or both surface of the cornea [23] are the first arise in the early stage of KC. Our findings, which demonstrated the smaller As/Ps in FFKC and KC eyes, suggest that protrusion of the posterior corneal surface prior to changes being observed on the anterior corneal surface, leading to be imbalance of anterior and posterior corneal surface area at the initial stage of the disease. The BAD-D, also provided the large AU-ROC (0.951) in our study, and it has been reported as multi-metric parameter, with calculation based on the front and back enhanced elevations, thinnest value, pachymetric distribution, and vertical displacement of the thinnest in relation to the apex [24]. In previous reports, the combined index generated from the curvature, elevation, and corneal thickness has showed high accuracy in detecting subclinical keratoconus or forme fruste keratoconus [7,25]. The findings in those studies may support our finding in which several components index, namely the As/Ps and BAD-D, had higher diagnostic ability than single component index in early-stage KC and indicate the importance of comprehensive approach to characterize the initial change in KC. Consequently, the character of the As/Ps as multi-metric index which reflects the imbalance of anterior-posterior ratio, leads to large AU-ROC in distinguishing FFKC from normal eyes.

It should be noted that this current study did have several limitations. First, in order to be widely applied the As/Ps for the screening of the early stage of KC, the optimal cut-off value should be determined. This was a retrospective study and it involved a small number of cases because most of subjects were not able to stop wearing the lenses for 2 to 4 weeks prior to assessment, and did not have the images with high quality from both AS-OCT and Scheimpflug imaging system. Hence, we are now conducting the extensive research to determine the optimal cut-off value using another patient population in a larger sample size. Second, we did not evaluate the corneal biomechanics or the total corneal wavefront aberrations, which are reportedly useful in the diagnosis of ectatic disease [19, 26–28]. Thus, in order to identify the significant independent predictors of the presence of early-stage KC, additional investigation involving logistic regression analysis with a large number of cases is required. We hope that the present findings lead to the early detection of KC, and then the early therapeutic intervention including cross-linking will be administered for preventing progress to severe KC.

In conclusion, it was found that the As/Ps, which was calculated by using the surface area obtained from AS-OCT, was a reliable index among the several previously reported indices, demonstrating the large AU-ROC (0.980) in detecting FFKC.

Data analyzed.

(XLSX)

Click here for additional data file.

6 Jan 2020

PONE-D-19-34557

Anterior and Posterior Ratio of Corneal Surface Areas: A Novel Index for Detecting Early Stage Keratoconus

PLOS ONE

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We will update your Data Availability statement to reflect the information you provide in your cover letter.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

Reviewer #3: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: No

Reviewer #3: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: No

Reviewer #3: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: No

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: it is a novel work and should be studied on larger population and other ethnic groups to be applicable and dependable anywhere. minimal writing errors need correction. I recommend accepting this manuscript.

Reviewer #2: There are some important points I would like to address:

Methods: Lines 126- 133

1. Explain KCN and FFKCN classification methods more clearly. For example, how much skewed-axis was included in the KCN group? Same for other group classifications.

Analysis:

2. The important point in this paper is this study does not evaluate the sensitivity and specificity of the As/Ps index compared to other indices. If you want to say that this index is better than the others, a specific analysis should be performed to compare the area under the curves (AUCs). The area under the curve alone cannot confirm As/Ps ratio has a high diagnostic ability for early KCN detection. It is recommended to use the “DeLong” method to compare the area under the curves and report sensitivity, specificity and optimal cut points. In retrospective data reporting these diagnostic indices also possible.

Results:

3. Line 117, you said keratosis was a reference for the analysis. Why did you compare FFKCN with the control group in Table 1?

4. You should remove the KCN column from the table or report that the p-value related to the comparison of KCN with the normal group.

Discussion:

5. Explain in the discussion why the AUC of As had weak value however, the anterior (As) and posterior (Ps) ratio had high AUC value. (based on Table 2)

6. Curvature indices (e.g ISValue) and new combined indices (e.g BAD-D) appear to had better diagnostic accuracy for early KCN detection. Why this new elevation based index (As/Ps ) had better diagnostic power than curvature indices OR new combined indices to detect early stages of the disease?

Reviewer #3: The authors have retrospectively collected 13 forme fruste keratoconus (FFKC) eyes, 29 keratoconus eyes, and 88 normal eyes and analyze their keratoconus related indices by three devices: anterior segment OCT (CASIA), Scheimpflug imaging tomography (Pentacam), and Placido disc-based topography (TMS-4). In their cohort, they reported the ratio of anterior/posterior corneal surfaces (As/Ps)(CASIA) has the largest AUROC in differentiating FFKC from normal eyes, which may improve the diagnosis of early keratoconus. This study is an extended study of the authors' previous work (Ref 15) that focused on the imbalance of anterior/posterior corneal surface as a pathological change of keratoconus. The experiments, statistics and data analyses are performed in an adequate technical standard.

Few specific suggestions are listed below:

Material:

1. Line 124-125: (2) keratoconic appearance such as focal or inferior steepening, a “bow-tie pattern, or a skewed axis.” Should the author clarify the criteria as “an asymmetric bow-tie pattern with or without skewed axis?” (Ref: 8, Muftuoglu O, et al. J Cataract Refract Surg 2013)

2. Line 172: formula for calculation the posterior corneal surface is a bit confusing. Please explain the assumption and derivation of your formula and present it in a clear way with references.

3. Line 176: please explain the derivation of ESI.

General recommendation:

1. Use Scheimpflug tomography for Pentacam instead of topography since the rotating Scheimpflug imaging system also provides posterior corneal parameters and pachymetry data.

2. Please check all the references in your manuscript, some references are missing.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Ismail Ahmed Nagib Omar

Reviewer #2: No

Reviewer #3: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

Submitted filename: revise.pdf

Click here for additional data file.

25 Jan 2020

Response letter

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at http://www.journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and http://www.journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

Response to Comment 1: We greatly appreciate the Editor’s very helpful comment. We have revised the manuscripts to meet PLOS ONE’s style requirements throughout the paper. (i.e. Heading with Bold type, Figure citation, Inline Equation, Reference Citation)

2. Thank you for your financial disclosure: "Koji Kitazawa received Grant-in-Aid for Young Scientists B for this work." Please provide, in the cover letter accompanying your revised manuscript a reworded financial disclosure which states specifically whether the funders played any role in the study.

Response to Comment 2: We greatly appreciate the Editor’s very insightful and helpful comment. To clarify, we have added the text regarding the financial disclosure in the cover letter.

3. Please carefully proofread your manuscript for typographical errors. For example, in the Patient Inclusion section there is a missing space “… 88 eyes of 88patients ….”.

Response to Comment 3: We greatly appreciate the Editor’s very insightful and helpful comment. We have revised the manuscripts in regard to the typographical errors throughout the paper.

4. Please state the participant recruitment date in your methods section.

Response to Comment 4: We greatly appreciate the Editor’s very insightful and helpful comment. As suggested, please note that we have now added the text regarding the participant recruitment date in method section. (p. 6, lines 2-3)

5. You indicated that you had ethical approval for your study. In your Methods section, please ensure you have also stated whether you obtained consent from parents or guardians of the minors included in the study or whether the research ethics committee or IRB specifically waived the need for their consent.

Response to Comment 5: We greatly appreciate the Reviewer’s very insightful and helpful comment. We have now added the text regarding informed consent in the Method sections. (p. 6, lines 11-12)

6. In your Data Availability statement, you have not specified where the minimal data set underlying the results described in your manuscript can be found. PLOS defines a study's minimal data set as the underlying data used to reach the conclusions drawn in the manuscript and any additional data required to replicate the reported study findings in their entirety. All PLOS journals require that the minimal data set be made fully available. For more information about our data policy, please see http://journals.plos.org/plosone/s/data-availability.

Response to Comment 6: We greatly appreciate the Editor’s very insightful and helpful comment. We have now added the standard deviations of all indices which were reported in our study in table 1. Since the values behind the means, sample size, statistical method, P value, the values used for ROC analysis in figure 3 were included in our manuscripts. We believe that this new information adequately addresses the editor's comment.

Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: it is a novel work and should be studied on larger population and other ethnic groups to be applicable and dependable anywhere. minimal writing errors need correction. I recommend accepting this manuscript.

Response to Comment1: We greatly appreciate the Reviewer’s very helpful comment. As suggested, please note that we have now added the standard deviation of each indices to correct minimal writing errors in table 1.

Reviewer #2: There are some important points I would like to address:

Methods: Lines 126- 133

1. Explain KCN and FFKCN classification methods more clearly. For example, how much skewed-axis was included in the KCN group? Same for other group classifications.

Response to Comment 1: We greatly appreciate the Reviewer’s very insightful and helpful comment. We used the Keratoconus index (KCI), calculated by Clyce/Maeda criteria on TMS-4, as objective indication for diagnosis of KC and FFKC. Patients were diagnosed as having keratoconus if they have clinical and topographic signs and showed KCI 5 % or more. All patients enrolled in the FFKC group had to pass through the KCI with 0%. We have now added some text in the Method sections to describe the classification methods for KC and FFKC. (p. 6, lines 12-18 and p. 7, lines 1-4)

Analysis:

2. The important point in this paper is this study does not evaluate the sensitivity and specificity of the As/Ps index compared to other indices. If you want to say that this index is better than the others, a specific analysis should be performed to compare the area under the curves (AUCs). The area under the curve alone cannot confirm As/Ps ratio has a high diagnostic ability for early KCN detection. It is recommended to use the “DeLong” method to compare the area under the curves and report sensitivity, specificity and optimal cut points. In retrospective data reporting these diagnostic indices also possible.

Response to Comment 2: We greatly appreciate the Reviewer’s very insightful and helpful comment.　We consulted another biostatistics expert to clarify the method to evaluate diagnostic ability of the As/Ps. We received his opinion as below ”It is difficult to compare and judge the diagnostic accuracy of each indices using sensitivity and specificity in this study. Because the indices which were used in this paper, were not defined universal cut-off value to distinct FFKC from normal subjects. As sensitivity and specificity of each indices changed depending on cut-off value, it is difficult to use sensitivity and specificity in judging diagnostic accuracy of indices which were not defined universal cut-off value. In addition, it is also difficult to compare the AU-ROC via “Delong method” in this study, because it is not easy to find the difference in AUROC using multiple comparison analysis among the numerous indices.” In the other word, it is difficult to　conclude superiority of the As/Ps in diagnostic accuracy. Thus, we tone down our statements and added the text regarding to this matter in Discussion section. We revised “largest” AU-ROC to “large” AU-ROC in order to avoid misleading. We believe that this new statement adequately addresses the reviewer's comment. (p. 2, line 15, p. 3, line 3, p. 18, lines 15-17, p. 20, line 2 and p. 21, line 2)

Results:

3. Line 117, you said keratosis was a reference for the analysis. Why did you compare FFKCN with the control group in Table 1?

Response to Comment 3: We greatly appreciate the Reviewer’s very insightful and helpful comment. The aim of this study was to evaluate the diagnostic ability of the ratio of anterior and posterior corneal surface areas (As/Ps) comparing with other keratoconus screening indices in distinguishing forme fruste keratoconus (FFKC) from normal eyes. We removed the text and add the text (p. 12, line 4). We hope these revisions clarify the aim of our study.

4. You should remove the KCN column from the table or report that the p-value related to the comparison of KCN with the normal group.

Response to Comment 4: We greatly appreciate the Reviewer’s very insightful and helpful comment.　In accordance with the Reviewer’s request, please note that we have now added the p-value related to the comparison of KC with the normal group in table 1, as well as a description of appropriate statistical analysis in the main text. (p. 11, lines 16-18 and p. 12, line 1)

Discussion:

5. Explain in the discussion why the AUC of As had weak value however, the anterior (As) and posterior (Ps) ratio had high AUC value. (based on Table 2)

Response to Comment 5: We greatly appreciate the Reviewer’s very insightful and helpful comment. We think that protrusion of the posterior corneal surface prior to changes being observed on the anterior corneal surface lead to small AU-ROC in As, while As/Ps showing large AU-ROC. Although, there have been numerous previous reports in regard to either anterior or posterior curvature in formation of KC eyes, the findings in our study suggest that the changes in the posterior cornea occurred earlier than the anterior cornea. We have now added some text in the Discussion sections to describe the reason why the AUC of As had weak value however, the anterior (As) and posterior (Ps) ratio had high AUC value. (p. 19, lines 4-10)

6. Curvature indices (e.g ISValue) and new combined indices (e.g BAD-D) appear to had better diagnostic accuracy for early KCN detection. Why this new elevation based index (As/Ps ) had better diagnostic power than curvature indices OR new combined indices to detect early stages of the disease?

Response to Comment 6: We greatly appreciate the Reviewer’s very insightful and helpful comment. As suggested, there has been numerous previous reports in regard to the diagnostic ability of curvature indices and new combined indices in detecting KC. We do not intend to reduce the importance of these known indices. We have now removed the text to avoid misleading and added the text to clarify the points we attempted to make in Discussion section. (p. 2, line 15, p. 3, line 3, p. 18, lines 15-17, p. 20, line 2 and p. 21, line 2)

Reviewer #3: The authors have retrospectively collected 13 forme fruste keratoconus (FFKC) eyes, 29 keratoconus eyes, and 88 normal eyes and analyze their keratoconus related indices by three devices: anterior segment OCT (CASIA), Scheimpflug imaging tomography (Pentacam), and Placido disc-based topography (TMS-4). In their cohort, they reported the ratio of anterior/posterior corneal surfaces (As/Ps)(CASIA) has the largest AUROC in differentiating FFKC from normal eyes, which may improve the diagnosis of early keratoconus. This study is an extended study of the authors' previous work (Ref 15) that focused on the imbalance of anterior/posterior corneal surface as a pathological change of keratoconus. The experiments, statistics and data analyses are performed in an adequate technical standard.

Few specific suggestions are listed below:

Material:

1. Line 124-125: (2) keratoconic appearance such as focal or inferior steepening, a “bow-tie pattern, or a skewed axis.” Should the author clarify the criteria as “an asymmetric bow-tie pattern with or without skewed axis?” (Ref: 8, Muftuoglu O, et al. J Cataract Refract Surg 2013)

Response to Comment 1: We greatly appreciate the Reviewer’s very insightful and helpful comment. We revised the text in the Method section to clarify the criteria, which we used for diagnosis of KC. (p. 6, lines 14-16)

2. Line 172: formula for calculation the posterior corneal surface is a bit confusing. Please explain the assumption and derivation of your formula and present it in a clear way with references.

Response to Comment 2: We greatly appreciate the Reviewer’s very insightful and helpful comment. In accordance with the Reviewer’s request, please note that we have now added the new figure and text regarding the assumption and derivation of formula with reference citation in Method section. (p. 9, lines 9-10 and 13 and Figure 2)

3. Line 176: please explain the derivation of ESI.

Response to Comment 3: We greatly appreciate the Reviewer’s very insightful and helpful comment. As suggested, please note that that we have now added appropriate reference citation in regards to the derivation of ESI in the Method section. (p. 10, lines 9-12)

General recommendation:

1. Use Scheimpflug tomography for Pentacam instead of topography since the rotating Scheimpflug imaging system also provides posterior corneal parameters and pachymetry data.

Response to Comment 4: We greatly appreciate the Reviewer’s very insightful and helpful comment. We have replaced the term Scheimpflug topography for Pentacam with Scheimpflug tomography throughout the paper to use more precise terms.

2. Please check all the references in your manuscript, some references are missing.

Response to Comment 5: We greatly appreciate the Reviewer’s very insightful and helpful comment. We have reflected this comment and now revised the text in Reference section.

Submitted filename: PLoSone_Response letter.docx

Click here for additional data file.

19 Feb 2020

PONE-D-19-34557R1

Anterior and posterior ratio of corneal surface areas: A novel index for detecting early stage keratoconus

PLOS ONE

Dear Dr. Kitazawa,

Thank you for submitting your manuscript to PLOS ONE. The authors have addressed the majority of comments, however, few minor points still need to be revised.  We invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

We would appreciate receiving your revised manuscript by Apr 04 2020 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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Please include the following items when submitting your revised manuscript:

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Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Yu-Chi Liu, M.D

Academic Editor

PLOS ONE

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: (No Response)

Reviewer #3: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Partly

Reviewer #3: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: No

Reviewer #3: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

Reviewer #3: No

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: Comment 2 (analysis section) not been addressed correctly. You can select the optimal cut-off points based on lowest difference between specificity and sensitivity for your cases. (with SPSS or STATA software)

The "larger AUC" is not correct for comparing the variables (in the abstract and text).

Table 2 should be revised accordingly.

Reviewer #3: The authors have revised the majority of the manuscript according to the reviewers' suggestions. However, the mini-data set is still not available for further review.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: No

Reviewer #3: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

Submitted filename: Comment.docx

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26 Feb 2020

RESPONSE TO REVIEWER2

1. Comment 2 (analysis section) not been addressed correctly. You can select the optimal cut-off points based on lowest difference between specificity and sensitivity for your cases. (with SPSS or STATA software) The "larger AUC" is not correct for comparing the variables (in the abstract and text). Table 2 should be revised accordingly.

Response to Comment 1: We greatly appreciate the Reviewer’s very insightful and helpful comment. In accordance with the Reviewer’s suggestion, please note that we have now revised the text in the statistical analysis and table 2 regarding the optimal cut-off points with sensitivity and specificity of each indices. In addition, we revised “larger” AU-ROC to “large” AU-ROC in order to avoid misleading. We believe that this new statement adequately addresses the reviewer's comment. (p. 2, lines 15-18, p. 3, lines 3, p 12, lines 5-9, p 15, lines 3-7, p 17, lines 9, p 18, lines 1-2,4-5, p18 lines14-18)

RESPONSE TO REVIEWER3

1. The authors have revised the majority of the manuscript according to the reviewers' suggestions. However, the mini-data set is still not available for further review

Response to Comment 1: We greatly appreciate the Reviewer’s very insightful and helpful comment. We have now added the data set which include the values to use the ROC analysis as supporting information (S1 dataset). We believe that this new information adequately addresses the reviewer's comment.

Submitted filename: RESPONSE TO REVIWER3.docx

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17 Mar 2020

Anterior and posterior ratio of corneal surface areas: A novel index for detecting early stage keratoconus

PONE-D-19-34557R2

Dear Dr. Kitazawa,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

Within one week, you will receive an e-mail containing information on the amendments required prior to publication. When all required modifications have been addressed, you will receive a formal acceptance letter and your manuscript will proceed to our production department and be scheduled for publication.

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If your institution or institutions have a press office, please notify them about your upcoming paper to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, you must inform our press team as soon as possible and no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

With kind regards,

Yu-Chi Liu, M.D

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: (No Response)

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: No

20 Mar 2020

PONE-D-19-34557R2

Anterior and posterior ratio of corneal surface areas: A novel index for detecting early stage keratoconus

Dear Dr. Kitazawa:

I am pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please notify them about your upcoming paper at this point, to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

For any other questions or concerns, please email plosone@plos.org.

Thank you for submitting your work to PLOS ONE.

With kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Yu-Chi Liu

Academic Editor

PLOS ONE