Comparative evaluation of Scheimpflug tomography parameters between thin non-keratoconic, subclinical keratoconic, and mild keratoconic corneas

Comparative Evaluation of Topographic Parameters Using Three Different Topographers in Keratoconic and Cross-linked Keratoconic Corneas

PURPOSE

To compare the parameters of three different topographic devices (Pentacam HR, Oculus Optikgeräte GmbH; Sirius, Costruzione Strumenti Oftalmici; and Cassini, i-Optics) in grading the severity of keratoconus in cross-linked and non-cross-linked eyes.

METHODS

This was a prospective comparative interventional study done in a tertiary eye care center, wherein 114 eyes of 68 patients with keratoconus were divided into two groups: 62 eyes that were observed and 52 eyes that were cross-linked. All eyes were evaluated on all three topographers at baseline, 3-month follow-up, and 6-month follow-up.

RESULTS

The Sirius showed significantly lower values of mean flat keratometry in comparison to the Pentacam HR (limits of agreement [LoA]: 1.75 to 3.51%) and Cassini (LoA: 1.75 to 3.51%). The mean steep keratometry values were higher for the Cassini in comparison to the Pentacam HR (LoA: 4.39 to 7.02%) and Sirius (LoA: 3.51 to 6.14%). The mean steep keratometry values of the Sirius were significantly lower than those of the Pentacam HR and in both the cross-linked and observation groups (LoA: 3.51 to 6.14%). The mean keratometry values were significantly higher in the Cassini compared to the Pentacam HR (LoA: 3.51 to 8.77%) and significantly lower in the Sirius in comparison to the Pentacam HR (LoA: 3.51 to 6.14%). The mean difference in astigmatism was also statistically significant between the three tomographers in both groups, with the Cassini showing higher values (LoA: 3.51 to 5.26%) and the Sirius lower values than the Pentacam HR (LoA: 5.26 to 6.14%).

CONCLUSIONS

The authors conclude the three devices cannot be used interchangeably. The Cassini showed better agreement and correlation with the Pentacam HR in the staging of keratoconus, whereas the Sirius tended to underdiagnose and under-stage the disease. [J Refract Surg. 2024;40(4):e260-e269.].

Corneal biomechanics in early diagnosis of keratoconus using artificial intelligence

Keratoconus is a blinding eye disease that affects activities of daily living; therefore, early diagnosis is crucial. Great efforts have been made toward an early diagnosis of keratoconus. Recent studies have shown that corneal biomechanics is associated with the occurrence and progression of keratoconus. Hence, detecting changes in corneal biomechanics may provide a novel strategy for early diagnosis. However, an early keratoconus diagnosis remains challenging due to the subtle and localized nature of its lesions. Artificial intelligence has been used to help address this problem. Herein, we reviewed the literature regarding three aspects of keratoconus (keratoconus, early keratoconus, and keratoconus grading) based on corneal biomechanical properties using artificial intelligence. Furthermore, we summarized the current research progress, limitations, and possible prospects.

Analysis of Scheimpflug Tomography Parameters for Detecting Subclinical Keratoconus in the Fellow Eyes of Patients with Unilateral Keratoconus in the Eastern Province of Saudi Arabia

Purpose

We compared the characteristics of subtle morphological changes in subclinical keratoconus (KC) and normal corneas using Scheimpflug tomography (Pentacam®) and assessed the efficacy of these parameters for distinguishing KC or subclinical KC from normal eyes.

Patients and Methods

In this multicenter comparative study at Dhahran Eye Specialist Hospital and Al Kahhal Medical Complex in the Eastern Province of Saudi Arabia, we analyzed the Scheimpflug tomography charts of patients with topographically normal eyes and those with unilateral KC. Patients were divided into the normal (NL: patients considered for refractive surgery and with normal topographic/tomographic features, 129 eyes), KC (30 patients with manifest KC in one eye based on biomicroscopy and topographical findings), and forme fruste KC (FFKC: fellow eyes of patients in the KC group that met the NL group criteria) groups. Corneal morphological parameters were analyzed using the area under the receiver operating characteristic (ROC) curves (AUCs).

Results

For distinguishing NL and KC groups, all measured corneal morphological parameters, except for flat keratometry, maximum Ambrósio relational thickness index, and minimum sagittal curvature, had AUCs >0.75. The surface variance index yielded the largest AUC (0.999). For distinguishing NL and FFKC groups, all corneal morphological parameters had AUCs <0.8. Total higher-order aberrations (RMS HOA) yielded the highest AUC, followed by Belin/Ambrỏsio Enhanced Ectasia total deviation (BAD-D), back elevation at the thinnest location, average pachymetric progression index (PPIave), and deviation of Ambrỏsio relational thickness (Da) (AUC 0.74-0.78).

Conclusion

The diagnostic performance of all tested topographic and tomographic parameters measured using Scheimpflug tomography for discriminating subclinical KC was fair at best, with the top parameters being RMS HOA, BAD-D, back elevation at the thinnest location, PPIave, and Da. Distinguishing between subclinical KC and healthy eyes remains challenging. Multimodal imaging techniques may be required for optimal early detection of subtle morphological changes.

Advanced Corneal Imaging in Keratoconus: A Report by the American Academy of Ophthalmology

PURPOSE

To review the published literature on the diagnostic capabilities of the newest generation of corneal imaging devices for the identification of keratoconus.

METHODS

Corneal imaging devices studied included tomographic platforms (Scheimpflug photography, OCT) and functional biomechanical devices (imaging an air impulse on the cornea). A literature search in the PubMed database for English language studies was last conducted in February 2023. The search yielded 469 citations, which were reviewed in abstract form. Of these, 147 were relevant to the assessment objectives and underwent full-text review. Forty-five articles met the criteria for inclusion and were assigned a level of evidence rating by the panel methodologist. Twenty-six articles were rated level II, and 19 articles were rated level III. There were no level I evidence studies of corneal imaging for the diagnosis of keratoconus found in the literature. To provide a common cross-study outcome measure, diagnostic sensitivity, specificity, and area under the receiver operating characteristic curve (AUC) were extracted. (A perfect diagnostic test that identifies all cases properly has an AUC of 1.0.) RESULTS: For the detection of keratoconus, sensitivities for all devices and parameters (e.g., anterior or posterior corneal curvature, corneal thickness) ranged from 65% to 100%. The majority of studies and parameters had sensitivities greater than 90%. The AUCs ranged from 0.82 to 1.00, with the majority greater than 0.90. Combined indices that integrated multiple parameters had an AUC in the mid-0.90 range. Keratoconus suspect detection performance was lower with AUCs ranging from 0.66 to 0.99, but most devices and parameters had sensitivities less than 90%.

CONCLUSIONS

Modern corneal imaging devices provide improved characterization of the cornea and are accurate in detecting keratoconus with high AUCs ranging from 0.82 to 1.00. The detection of keratoconus suspects is less accurate with AUCs ranging from 0.66 to 0.99. Parameters based on single anatomic locations had a wide range of AUCs. Studies with combined indices using more data and parameters consistently reported high AUCs.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Early, Forme Fruste keratoconus and normal thin cornea, evaluation of sensitive parameters by combined Placido Scheimpflug topography

PURPOSE

To determine the sensitive indices distinguishing forme-fruste keratoconus (FFKC) and early keratoconus (early KC) from thin normal corneas, and their cutoff values using Sirius topography.

METHODS

156 eyes with normal thin corneas < 500 um (group 1), 99 eyes with early KC (group 2) and 41 eyes with FFKC (group 3), were assessed retrospectively for: corneal keratometric indices, pachymetry indices, corneal aberrations, elevation indices; thinnest corneal point elevation, Q value, root mean square (RMS) withRMS/ area (RMS/A); and KC summary indices of front and back surfaces; surface asymmetry index (SIf, SIb), elevation at KC vertex (KVf, KVb), Baiocchi-Calossi-Versaci index (BCVf, BCVb) and summation of its vector (BCV). Cutoff values were calculated.

RESULTS

Keratometry indices were significantly different between early KC and thin normal cases (apex front curvature had the highest area-under-the-ROC-curve (AUROC) (0.926) in early KC, while only apex curvature and coordinates were significant in FFKC cases. Pachymetry indices did not show any significance in differentiating either early KC or FFKC from normal thin corneas. KC summary indices were highly significant among the 3 groups. The highest AUROC was observed with KVb in early KC (0.987) and with KVf in FFKC (0.831). Vertical coma and vertical trefoil showed the highest significance of all aberration parameters differentiating the 3 groups. Thinnest point elevation, RMS and RMS/A showed the highest AUROC in differentiating early KC and FFKC cases from thin normal corneas.

CONCLUSION

Comparing early KC and FFKC to thin normal corneas, Sirius provided high precision in prediction.

Scheimpflug-Derived Keratometric, Pachymetric and Pachymetric Progression Indices in the Diagnosis of Keratoconus: A Systematic Review and Meta-Analysis

Scheimpflug Pentacam Tomography is becoming crucial in the diagnosis and monitoring of keratoconus, as well as in pre- and post-corneal refractive care, but there are still some inconsistencies surrounding its evidence base diagnostic outcome. Therefore, this study aimed at employing meta-analysis to systematically evaluate the keratometric, pachymetric, and pachymetric progression indices used in the diagnosis of Keratoconus. The review protocol was registered with PROSPERO (Identifier: CRD4202310058) and followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. PubMed, MEDLINE, Web of Science, and EMBASE were used for data search, followed by a quality appraisal of the included studies using the revised tool for the quality assessment of diagnostic accuracy studies (QUADAS-2). Meta-analysis was conducted using the meta (6.5.0) and metafor (4.2.0) packages in R version 4.3.0, as well as Stata. A total of 32 studies were included in the analysis. All keratometry (K) readings (flattest meridian, K1; steepest meridian, K2, maximum, Kmax) were significantly steeper in keratoconic compared to normal eyes: [MD (95% CI)], K1 [2.67 (1.81; 3.52)], K1-back [-0.71 (-1.03; -0.39)], K1-front [4.06 (2.48; 5.63)], K2 [4.32 (2.89; 5.75)], K2-back [-1.25 (-1.68; -0.82)], K2-front [4.82 (1.88; 7.76)], Kmax [7.57 (4.80; 10.34)], and Kmean [2.80 (1.13; 4.47)]. Additionally, corneal thickness at the center, CCT [-61.19 (-73.79; -48.60)] and apex, pachy-apex [-41.86 (-72.64; -11.08)] were significantly thinner in keratoconic eyes compared to normal eyes. The pooled estimates for pachymetric progression index (PPI): PPImin [0.66 (0.43; 0.90)], PPImax [1.26 (0.87; 1.64)], PPIavg [0.90 (0.68; 1.12)], and Ambrosio relational thickness (ART): ARTmax [-242.77 (-288.86; -196.69)], and ARTavg [-251.08 (-308.76; -195.39)] revealed significantly more rapid pachymetric progression in keratoconic eyes than in normal eyes. The Pentacam Scheimpflug-derived keratometric, pachymetric, and pachymetric progression indices are good predictors in discriminating KC from normal eyes.

Parental Corneal Tomographic and Biomechanical Characteristics of Patients With Keratoconus

PURPOSE

To investigate the hereditability of corneal tomographic and biomechanical parameters in keratoconus (KC).

DESIGN

Prospective cohort study.

METHODS

This study was conducted at Qingdao Eye Hospital of Shandong First Medical University in Qingdao, China. Forty-four patients with KC and their biological parents (n = 88) were recruited as the study group. The control group consisted of 84 healthy adults with matched age and gender. Both eyes of each participant underwent clinical examinations, and 1 eye was selected for statistical analysis. Exclusion criteria were as follows: individuals with glaucoma, ocular surgery, systemic diseases known to affect the eyes, or poor cooperation during examination. Subjects were asked to discontinue soft contact lens (CL) wear for 2 weeks and rigid gas permeable CL wear for 4 weeks before ocular examination. All participants underwent a comprehensive assessment including Pentacam Scheimpflug tomography, Corvis ST, visual acuity, refraction examination, axial length, and slitlamp examination for both eyes. Individuals presenting with KC manifestations in at least 1 eye were classified as having KC. A total of 9 Pentacam indices including keratometry in the flat/steep meridian (K1/K2), maximal keratometry (Kmax), thinnest point pachymetry (TP), and maximum/average Ambrósio relational thickness (ARTmax/ARTave), anterior and posterior surfaces elevation of the cornea (Ef/Eb) and total deviation value (Final D), and 21 biomechanical indices were collected. Associations of these factors with KC were evaluated using multiple comparison and binary logistics regression analyses.

RESULTS

Two parents (2.27%) from 2 different families were diagnosed with KC. Parents of patients with KC had thinner corneas with altered corneal biomechanical parameters compared with healthy controls (P < .05). The combined tomographic and biomechanical index demonstrated the highest discriminatory power (area under the receiver operating characteristic curve 0.785) and strong specificity (84.5%). Parental corneal tomographic and biomechanical index, Corvis biomechanical index, and TP were identified as the major influential factors for KC in their offspring by logistic regression analysis, with a 73.3% accuracy in identifying offspring with KC.

CONCLUSIONS

Parental corneal tomographic and biomechanical properties of patients with KC suggest a possible predisposition to KC. A combination of tomography and corneal biomechanics can be helpful in predicting the incidence rate of KC in the offspring of patients with subclinical KC.

Accuracy of tomographic and biomechanical parameters in detecting unilateral post-LASIK keratoectasia and fellow eyes

Background: Patients with unilateral post-LASIK keratectasia (KE) have clinical ectasia in one eye but not in the fellow eye. As serious complications, these cases are rarely reported but are worth investigating. This study aimed to explore the characteristics of unilateral KE and the accuracy of corneal tomographic and biomechanical parameters to detect KE and distinguish fellow eyes from control eyes. Methods: The study analyzed 23 KE eyes, 23 KE fellow eyes, and 48 normal eyes from age- and sex-matched patients who had undergone LASIK. The Kruskal-Wallis test and further paired comparisons were performed to compare the clinical measurements of the three groups. The receiver operating characteristic curve was used to evaluate the ability to distinguish KE and fellow eyes from the control eyes. Binary logistic regression with the forward stepwise method was performed to produce a combined index, and the DeLong test was used to compare the discriminability difference of the parameters. Results: Males accounted for 69.6% of patients with unilateral KE. The duration between corneal surgery and the onset of ectasia ranged from 4 months to 18 years, with a median time of 10 years. The KE fellow eye had a higher posterior evaluation (PE) value than the control eyes (5 vs. 2, p = 0.035). Diagnostic tests showed that PE, posterior radius of curvature (3 mm), anterior evaluation (FE), and Corvis biomechanical index-laser vision correction (CBI-LVC) were sensitive indicators for distinguishing KE in the control eyes. The ability of PE to detect the KE fellow eye from the control eye was 0.745 (0.628 and 0.841), with 73.91% sensitivity and 68.75% specificity at a cut-off value of 3. The ability of a combined index, constructed using PE and FE, to distinguish fellow eyes of KE from controls was 0.831 (0.723 and 0.909), which was higher than that of PE and FE individually (p < 0.05). Conclusion: The fellow eyes of patients with unilateral KE had significantly higher PE values than control eyes, and a combination of PE and FE enhanced this differentiation in a Chinese population. More attention should be paid to the long-term follow-up of patients after LASIK and to be wary of the occurrence of early KE.

Comparison of corneal biomechanical properties in healthy thin corneas with matched keratoconus eyes

PURPOSE

To compare corneal biomechanical parameters of normal thin corneas with matched keratoconus eyes.

SETTING

Eye Research Center, Mashhad University of Medical Sciences, Mashhad, Iran.

DESIGN

Cross-sectional comparative study.

METHODS

Dynamic corneal response parameters of Corvis ST were compared in 61 eyes with keratoconus with 61 matched healthy thin corneas (corneal thinnest point <500 μm), while corneal thickness, biomechanically corrected intraocular pressure, and age were considered covariates. The receiving operator sensitivity curve analysis was used to determine the cutoff point with the highest sensitivity and specificity, and the area under the curve (AUC) for each parameter.

RESULTS

All biomechanical parameters were statistically significant between the 2 groups except for the first ( P = .947) and second ( P = .582) applanation length, first ( P = .783) and second ( P = .301) applanation velocity, and deformation amplitude in the highest concavity phase ( P = .106). The highest mean difference between groups (12.89 ± 2.03 mm Hg/mm) was related to the stiffness parameter at the first applanation (SPA1). Although the Corvis biomechanical index and tomographic biomechanical index had the highest detection ability based on their AUC (0.912 and 0.959, respectively), among the standard and combined biomechanical parameters except for keratoconus screening parameters, the highest discriminative ability was related to SPA1 with AUC, sensitivity, and specificity of 0.793, 60.66%, and 90.16%, respectively.

CONCLUSIONS

Keratoconus corneas were significantly softer compared with healthy thin corneas of matched thickness. Optimal cutoff points close to the maximum value defined for screening parameters limit their clinical use for differentiation purposes in these particular types of cases.

Corneal densitometry in bilateral keratoconus patients with unilateral corneal Vogt’s striae: a contralateral eye study

PURPOSE

To investigate corneal densitometry and correlations with corneal morphological parameters in patients with bilateral keratoconus (KC) with unilateral Vogt's striae.

METHODS

This prospective contralateral study enrolled 112 patients (224 eyes) with evident KC characteristics (corneal topography with asymmetric bow-tie pattern, inferior steepening), and at least one KC sign (conical protrusion of the cornea at the apex, corneal stromal thinning, Fleischer ring, Vogt's striae) on slit-lamp examination. Corneal densitometry and morphological parameters were measured using Pentacam HR.

RESULTS

The mean age was 23.93 ± 6.81 years. Fifty-two (23.22%), 111 (49.55%), and 61 (27.23%) eyes were in mild, moderate, and severe groups, respectively. Corneal densitometry values of the anterior 0-2 mm and 2-6 mm, intermediate 0-2 mm and 2-6 mm, posterior 2-6 mm, and total cornea 2-6 mm were significantly higher in eyes with Vogt's striae (P < 0.05), whereas those of the anterior 6-10 mm, posterior 0-2 mm, and total cornea 6-10 mm were significantly lower in eyes with Vogt's striae (P < 0.05). Anterior 0-2 mm and total cornea 2-6 mm corneal densitometry values were positively correlated with anterior K1 (A-K1), K2 (A-K2), Km (A-Km), Kmax (A-Kmax), anterior corneal elevation, and posterior corneal elevation (P < 0.05), and negatively correlated with central corneal thickness and thinnest corneal thickness in eyes with Vogt's striae (P < 0.05). A-K2, A-Km, and A-Kmax were significantly correlated with the densitometry values of the anterior 0-2 mm and intermediate 0-2 mm in eyes without Vogt's striae (P < 0.05).

CONCLUSION

Vogt's striae mainly occur on the anterior and intermediate layers during KC progression.

Evaluation of artificial intelligence models for the detection of asymmetric keratoconus eyes using Scheimpflug tomography

BACKGROUND

To evaluate artificial intelligence (AI) models based on objective indices and raw corneal data from the Scheimpflug Pentacam HR system (OCULUS Optikgeräte GmbH, Wetzlar, Germany) for the detection of clinically unaffected eyes in patients with asymmetric keratoconus (AKC) eyes.

METHODS

A total of 1108 eyes of 1108 patients were enrolled, including 430 eyes from normal control subjects, 231 clinically unaffected eyes from patients with AKC, and 447 eyes from keratoconus (KC) patients. Eyes were divided into a training set (664 eyes), a test set (222 eyes) and a validation set (222 eyes). AI models were built based on objective indices (XGBoost, LGBM, LR and RF) and entire corneal raw data (KerNet). The discriminating performances of the AI models were evaluated by accuracy and the area under the ROC curve (AUC).

RESULTS

The KerNet model showed great overall discriminating power in the test (accuracy = 94.67%, AUC = 0.985) and validation (accuracy = 94.12%, AUC = 0.990) sets, which were higher than the index-derived AI models (accuracy = 84.02%-86.98%, AUC = 0.944-0.968). In the test set, the KerNet model demonstrated good diagnostic power for the AKC group (accuracy = 95.24%, AUC = 0.984). The validation set also proved that the KerNet model was useful for AKC group diagnosis (accuracy = 94.12%, AUC = 0.983).

CONCLUSIONS

KerNet outperformed all the index-derived AI models. Based on the raw data of the entire cornea, KerNet was helpful for distinguishing clinically unaffected eyes in patients with AKC from normal eyes.

An exploratory analysis of forme fruste keratoconus sensitivity diagnostic parameters

PURPOSE

To secondary statistical analysis of the Pentacam or Corvis ST parameters from literatures, and to obtain more sensitive diagnostic parameters for clinical keratoconus (CKC) and forme fruste keratoconus (FFKC), respectively.

METHODS

The parameters and the corresponding area of ROC curve (AUC) in previous studies were extracted and screened to obtain the database of CKC (Data-CKC) and FFKC (Data-FFKC), respectively. Two different importance evaluation methods (%IncMSE and IncNodePurity) of random forest were used to preliminary select the important parameters. Then, based on the partial dependency analysis, the sensitive diagnostic parameters that had promotion to the diagnostic performance were obtained. Data-FFKC was analyzed in the same way. Finally, a diagnostic test meta-analysis on the sensitive parameter of interest was conducted to verify the reliability of the above analysis methods.

RESULTS

There were 88 parameters with 766 records in Data-CKC, 57 parameters with 346 records in Data-FFKC. Based on two importance evaluation methods, 60 important parameters were obtained, of which 20 were further screened as sensitive parameters of keratoconus, and most of these parameters were related to the thinnest point of cornea. The stiffness parameter at first applanation (SPA1) was the only Corvis ST output parameter sensitive to FFKC except the Tomographic and Biomechanical Index and the Corvis Biomechanical Parameter (CBI). A total of 4 records were included in the meta-analysis of diagnostic tests on SPA1. The results showed that there was threshold effect, but no significant heterogeneity (I² = 33%), and the area under the SROC curve was 0.87 (95% CI, 0.84-0.90).

CONCLUSIONS

For the diagnosis of FFKC, the sensitivity of SPA1 is not inferior to the well-known CBI, and may be the earliest Corvis ST output parameter to reflect the changes of corneal biomechanics during keratoconus progression. The elevation parameters based on the typical position of the thinnest point of corneal thickness are of great significance for the diagnosis of keratoconus.

Effect of white-to-white corneal diameter on biomechanical indices assessed by Pentacam Scheimpflug corneal tomography and corneal visualization Scheimpflug technology

PURPOSE

To provide evidence for more accurate evaluation of refractive surgery candidates in clinics, this retrospective study investigated the effect of corneal diameter on the biomechanical indices assessed by Pentacam Scheimpflug cornea tomography (Pentacam) and corneal visualization Scheimpflug technology (Corvis ST).

METHODS

The relevant data were collected of 132 eyes from 132 participants with moderate myopia who were candidates for refractive surgery. Eligible participants were apportioned to 2 groups based on the white-to-white (WTW) corneal diameter: Group A, ≤ 11.5 mm, and Group B, ≥ 11.6 mm. A single clinician performed Pentacam and Corvis ST imaging on each subject for 3 consecutive measurements, and the means were used for statistical analyses.

RESULTS

Each group comprised 66 eyes. As measured by Pentacam, the 2 groups were comparable regarding Df and Da. For other measurements, Group A had significantly higher K1, K2, Db, Dp, Dt, Do, PPI_min, PPI_max, PPI_avg, while Group B had significantly higher CCT, BFSf, BFSb, and ART_max. Corvis ST data included DA ratio, SPA1, CBI, TBI, and ARTh. Only the latter showed a significant difference, with ARTh of group A (437.04 ± 76.60) larger than group B (470.46 ± 103.36, p = 0.04).

CONCLUSION

In a Chinese population, WTW corneal diameter showed effect on biomechanical indices assessed by Pentacam and Corvis ST. Personalized evaluation of these measurements based on corneal diameter should improve the sensitivity and specificity for screening of keratoconus by these devices.

Distribution of Corneal Geometric Landmarks and Relationship Between Their Distances and Biomechanical Parameters in the Development of Keratoconus

Purpose: To analyze the changes in coordinates and distances among three typical geometric landmarks of the cornea, namely, the thinnest point (TP), maximum curvature (Kmax), and corneal apex (AP) during the development of keratoconus, and explore the potential relationship between these changes and the abnormalities of corneal biomechanics. Methods: Normal eyes (n = 127), clinical keratoconic eyes (CKC, n = 290), and the eyes of forme fruste keratoconus (FFKC, n = 85) were included; among them, the CKC group was classified into four grades based on the Topographic Keratoconus Classification (TKC) provided by Pentacam. A total of 38 Corvis ST output parameters and three distance parameters of three typical landmarks (D_Kmax-AP, D_TP-AP, and D_Kmax-TP) based on Pentacam were included. The differences of parameters among the abovementioned six groups (Normal, FFKC, and CKC stage I to CKC stage IV) were analyzed. Spearman's rank correlation test was performed to choose several dynamic corneal response (DCR) parameters that could best reflect the changes of corneal biomechanical characteristics during the progression of the disease, and the Pearson's or Spearman's correlation test was conducted to determine the association between the three distances and the selected DCR parameters in each grade. In addition, by flipping the X coordinate of the left eye on the vertical axis to reflect the direction of the right eye, the coordinates of TP and Kmax in different developmental grades were highlighted. Results: From CKC stage II, the three geometric landmark distances commenced to correlate with the corneal DCR parameters (CBI, SPA1, IR, DA Ratio 2, ARTh, MIR, Radius, Pachy, and DA Ratio 1), which could better represent the changes of biomechanical properties from normal cornea to keratoconus. From normal cornea to CKC stage IV, the coordinates of Kmax were gradually tended to the inferior temporal region from dispersion, while TP was always concentrated in the inferior temporal region. Although D_Kmax-AP, D_Kmax-TP, and D_TP-AP all showed a gradual decreasing trend with the progress of the disease, the first two did not change significantly, and only D_TP-AP significantly approached AP in the later stage of disease development. In addition, from the FFKC group, the corresponding values of D_Kmax-TP in each disease development group were smaller than D_Kmax-AP. Conclusions: In the later stage of keratoconus, the relationship between the three typical landmark distance parameters and DCR parameters is stronger, and even the weakening of corneal biomechanical properties may be accompanied by the merger of typical landmark positions.

Changes in Anterior and Posterior Corneal Elevation in Patients With Allergic Conjunctivitis

Purpose: To evaluate corneal elevation changes in patients with allergic conjunctivitis (AC) and to analyze their correlations with ocular allergy signs and corneal biomechanical alterations. Methods: Thirty patients (30 eyes) with AC and twenty normal subjects (20 eyes) were included in this prospective study. All participants underwent a complete ocular examination, including corneal tomography by Pentacam and corneal biomechanics evaluation by Corvis ST. AC patients were evaluated for their eye rubbing frequency and ocular allergic signs. Results: The elevation at the thinnest location (TE) and the central location (CE), the elevation difference at the thinnest location (TED) and the central location (CED), and the mean value of elevation difference in the central 4 mm zoom (MED) of both the anterior and posterior corneal surface were significantly higher in the AC group than in the normal group (p < 0.05 for all). In AC patients, only anterior corneal elevation parameters were positively correlated with eye rubbing frequency and ocular allergy sign severity (p < 0.05 for all), while the tomography and biomechanical index (TBI) was positively correlated with the elevation parameters of both the anterior and posterior corneal surfaces (p < 0.05 for all). Conclusion: AC patients carry an increased risk of corneal ectasia. Posterior corneal elevation parameters are sensitive and reliable predictors of keratoconus (KC) risk in AC patients. Clinical Trial Registration: https://clinicaltrials.gov/ct2/show/NCT04299399, identifier [NCT04299399].

Comparisons of corneal biomechanical and tomographic parameters among thin normal cornea, forme fruste keratoconus, and mild keratoconus

Background

To compare the dynamic corneal response (DCR) and tomographic parameters of thin normal cornea (TNC) with thinnest corneal thickness (TCT) (≤ 500 µm), forme fruste keratoconus (FFKC) and mild keratoconus (MKC) had their central corneal thickness (CCT) matched by Scheimpflug imaging (Pentacam) and corneal visualization Scheimpflug technology (Corvis ST).

Methods

CCT were matched in 50 eyes with FFKC, 50 eyes with MKC, and 53 TNC eyes with TCT ≤ 500 µm. The differences in DCR and tomographic parameters among the three groups were compared. The receiver operating characteristic (ROC) curve was used to analyze the diagnostic significance of these parameters. Back propagation (BP) neural network was used to establish the keratoconus diagnosis model.

Results

Fifty CCT-matched FFKC eyes, 50 MKC eyes and 50 TNC eyes were included. The age and biomechanically corrected intraocular pressure (bIOP) did not differ significantly among the three groups (all P > 0.05). The index of height asymmetry (IHA) and height decentration (IHD) differed significantly among the three groups (all P < 0.05). IHD also had sufficient strength (area under the ROC curves (AUC) > 0.80) to differentiate FFKC and MKC from TNC eyes. Partial DCR parameters showed significant differences between the MKC and TNC groups, and the deflection amplitude of the first applanation (A1DA) showed a good potential to differentiate (AUC > 0.70) FFKC and MKC from TNC eyes. Diagnosis model by BP neural network showed an accurate diagnostic efficiency of about 91%.

Conclusions

The majority of the tomographic and DCR parameters differed among the three groups. The IHD and partial DCR parameters assessed by Corvis ST distinguished FFKC and MKC from TNC when controlled for CCT.

Anterior pituitary, sex hormones, and keratoconus: Beyond traditional targets

"The Diseases of the Horny-coat of The Eye", known today as keratoconus, is a progressive, multifactorial, non-inflammatory ectatic corneal disorder that is characterized by steepening (bulging) and thinning of the cornea, irregular astigmatism, myopia, and scarring that can cause devastating vision loss. The significant socioeconomic impact of the disease is immeasurable, as patients with keratoconus can have difficulties securing certain jobs or even joining the military. Despite the introduction of corneal crosslinking and improvements in scleral contact lens designs, corneal transplants remain the main surgical intervention for treating keratoconus refractory to medical therapy and visual rehabilitation. To-date, the etiology and pathogenesis of keratoconus remains unclear. Research studies have increased exponentially over the years, highlighting the clinical significance and international interest in this disease. Hormonal imbalances have been linked to keratoconus, both clinically and experimentally, with both sexes affected. However, it is unclear how (molecular/cellular signaling) or when (age/disease stage(s)) those hormones affect the keratoconic cornea. Previous studies have categorized the human cornea as an extragonadal tissue, showing modulation of the gonadotropins, specifically luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Studies herein provide new data (both in vitro and in vivo) to further delineate the role of hormones/gonadotropins in the keratoconus pathobiology, and propose the existence of a new axis named the Hypothalamic-Pituitary-Adrenal-Corneal (HPAC) axis.

Application of a scheimpflug-based biomechanical analyser and tomography in the early detection of subclinical keratoconus in chinese patients

Background

In vivo corneal biomechanics evaluation has been used to help screen early keratoconus in recent years. This study is to evaluate the value of a Scheimpflug-based biomechanical analyser combined with tomography in detecting subclinical keratoconus by distinguishing normal eyes from frank keratoconus (KC) and forme frusta keratoconus (FFKC) eyes in Chinese patients.

Methods

Study design: diagnostic test. This study included 31 bilateral frank keratoconus patients, 27 unilateral clinically manifesting keratoconus patients with very asymmetric eyes, and 79 control subjects with normal corneas. Corneal morphological and biomechanical parameters were measured using a Pentacam HR and a Corvis ST (OCULUS, Wetzlar, Germany). The diagnostic ability of computed parameters reflecting corneal biomechanical and morphological traits [including the Belin-Ambrósio deviation index (BAD_D), the Corvis biomechanical index (CBI) and the tomographic and biomechanical index (TBI)] was determined using receiver operating characteristic (ROC) curve analysis and compared by the DeLong test. Additionally, the area under the curve (AUC), the best cut-off values, and the Youden index for each parameter were reported. A novel corneal stiffness parameter, the stress-strain index (SSI), was also compared between KC, FFKC and normal eyes.

Results

Every morphological and biomechanical index analysed in this study was significantly different among KC, FFKC and normal eyes (P = 0.000). The TBI was most valuable in detecting subclinical keratoconus (FFKC eyes), with an AUC of 0.928 (P = 0.000), and both forms of corneal ectasia (FFKC and frank KC eyes), with an AUC of 0.966 (P = 0.000). The sensitivity and specificity of the TBI was 97.5 and 77.8 % in detecting FFKC and 97.5 and 89.7 % in detecting any KC, respectively, with a cut-off value of 0.375. The morphological index BAD_D and the biomechanical index CBI were also very useful in distinguishing eyes with any KC from normal eyes, with AUCs of 0.965 and 0.934, respectively. The SSI was significantly different between KC, FFKC and normal eyes (P = 0.000), indicating an independent decrease in corneal stiffness in KC eyes.

Conclusions

The combination of a Scheimpflug-based biomechanical analyser and tomography could increase the accuracy in detecting subclinical keratoconus in Chinese patients. The TBI was the most valuable index for detecting subclinical keratoconus, with a high sensitivity and specificity. Evaluation of corneal biomechanical properties in refractive surgery candidates could be helpful for recognizing potential keratoconic eyes and increasing surgical safety.

KerNet: A Novel Deep Learning Approach for Keratoconus and Sub-clinical Keratoconus Detection Based on Raw Data of the Pentacam System

Keratoconus is one of the most severe corneal diseases, which is difficult to detect at the early stage (i.e., sub-clinical keratoconus) and possibly results in vision loss. In this paper, we propose a novel end-to-end deep learning approach, called KerNet, which processes the raw data of the Pentacam system to detect keratoconus and sub-clinical keratoconus. First, we collect raw data from the Pentacam system. The raw data is of a specific format, that is, each sample consists of five numerical matrices, corresponding to the front and back surface curvature, the front and back surface elevation, and the pachymetry of an eye. Then, we propose a novel convolutional neural network, called KerNet, containing five branches as the backbone with a multi-level fusion architecture. The five branches receive five slices separately and capture effectively the features of different slices by several cascaded residual blocks. The multi-level fusion architecture (i.e., low-level fusion and high-level fusion) moderately takes into account the correlation among five slices and fuses the extracted features for better prediction. Specifically, five spatial attention modules are utilized, each in a branch, to guide the operation of the low-level fusion. The high-level fusion is implemented by simply concatenating the output feature maps of the last residual block in each branch. Experimental results show that: (1) our novel approach outperforms state-of-the-art methods on an in-house dataset, by  ∼ 1\% for keratoconus detection accuracy and  ∼ 4\% for sub-clinical keratoconus detection accuracy; (2) the attention maps visualized by Grad-CAM show that our KerNet places more attention on the inferior temporal part for sub-clinical keratoconus, which has been proved as the identifying regions for ophthalmologists to detect sub-clinical keratoconus in previous clinical studies. To our best knowledge, we are the first to propose an end-to-end deep learning approach utilizing raw data obtained by the Pentacam system for keratoconus and subclinical keratoconus detection. Further, the prediction performance and the clinical significance of our KerNet are well evaluated and proved by two clinical experts. Our code is available at \url{https://github.com/upzheng/Keratoconus}.

Evaluation of topographic, tomographic, topometric, densitometric, and aberrometric features of cornea with pentacam HR system in subclinical keratoconus

PURPOSE

To investigate topographic, tomographic, topometric, densitometric, and aberrometric parameters in subclinical keratoconus with the Pentacam HR imaging system.

METHODS

Data of 3128 patients were evaluated, finding in 108 patients clinical keratoconus in one eye and subclinical keratoconus in the other. Corneal topographic, tomographic, topometric, densitometric, and aberrometric values obtained using the Pentacam HR imaging system were compared between clinical keratoconus, subclinical keratoconus, and normal eyes.

RESULTS

Comparing eyes with subclinical keratoconus and the control group, while flat K, horizontal coma, horizontal trefoil, and vertical trefoil values were similar (p > 0.05 for each), all other parameters were significantly different (p < 0.05 for each). Densitometry values of eyes with subclinical keratoconus were significantly higher in all layers of the 0-2 mm annular area and in the anterior and central layers of the 2-6 mm annular area compared to the control group (p < 0.05 for each). According to the receiver operating characteristic curve analysis, the densitometry region with the largest area under the curve was the anterior layer of the 0-2 mm annular area. The sensitivity in this region was 79.4% and the specificity 73.2% in distinguishing eyes with subclinical keratoconus from normal eyes when 19.3 GSU was considered the threshold.

CONCLUSION

Corneal densitometry values in the 0-2 and 2-6 mm annular areas, especially in the anterior layers, are parameters that can be used to predict and distinguish subclinical keratoconus from normal eyes.

Revisiting the Robustness of Pentacam HR Indices Against Soft Contact Lens Wear

Purpose

To evaluate various selected Pentacam HR indices in a cohort of soft contact lens (SCL) wearers before and after SCL discontinuation.

Methods

Ninety right eyes of 90 SCL wearers (group 1) were compared to healthy controls (group 2). For group 1 eyes, Pentacam HR images were captured within one hour of SCL removal and after one month of discontinuation. The evaluated indices included curvature, elevation, and pachymetric parameters.

Results

Elevation indices and two pachymetric indices (central corneal thickness and thinnest corneal thickness) did not show significant differences, either between the two groups or in group 1 after SCL discontinuation. Some pachymetric indices (average and maximum corneal pachymetry progression indices, and maximum Ambrosio's relational thickness index) showed statistically significant (P-value <0.001), yet clinically irrelevant differences. Contrarily, changes in curvature indices were both statistically significant (P-value <0.001) and clinically relevant.

Conclusion

From a clinical point of view, many elevation and pachymetric indices are robust against SCL wear, and can be relied upon for corneal ectasia screening of SCL wearers, even without abstaining from wearing SCLs.

A hospital-based study on clinical data, demographic data and visual function of keratoconus patients in Central China

China is a populous country but lacks epidemiological data on keratoconus (KC). The present study aimed to investigate the clinical data, demographic data, and visual function (VF) data of KC patients in Central China. A total of 524 KC eyes in 307 KC patients (217 bilateral and 90 unilateral) from Henan Eye Hospital were included in the current study. Demographic and VF data were assessed with questionnaires administered by well-trained staff during face-to-face interviews. Visual acuity value was examined by a qualified optometrist, and the clinical data were measured by professional clinicians. The distributions of sex, residence and education level of KC patients were compared by Chi-square tests, and the ratios of people wearing glasses and rigid gas permeable (RGP) lenses were compared by McNemar tests. General linear models/Chi-squared tests were used to compare the clinical and demographic data according to KC severity. Spearman’s correlation analysis was used to test the associations between the data and KC severity. The mean age at diagnosis was 20.98 ± 6.06 years, and males had a higher ratio of KC than females (P < 0.001). Patients in rural areas had a higher rate of KC than those in urban areas (P = 0.039), and the proportion of KC patients with a higher education level (above high school) was high (P < 0.001). A total of 68.40% of the patients reported eye rubbing and 3.52% had a positive family history. The percentage of people wearing glasses was higher than that of patients wearing RGP lenses (P < 0.001). The total VF score of KC patients was 69.35 ± 15.25. The thinnest corneal thickness (TCT) and stiffness parameter at the first applanation (SP-A1) values were inversely correlated with KC severity (P < 0.05). The mean, steep, and max keratometry (Km, Ks and Kmax) values, the RGP lens use and keratoplasty were positively correlated with KC severity (all P < 0.05). The total VF score of the eye with better VA decreased as the severity increased (r = − 0.21, P = 0.002). The present study comprehensively describes various associated features of KC patients from a tertiary hospital in Central China, providing a reference for understanding the characteristics of KC patients in China.

Changes in Anterior and Posterior Corneal Elevation in Patients With Allergic Conjunctivitis

Purpose: To evaluate corneal elevation changes in patients with allergic conjunctivitis (AC) and to analyze their correlations with ocular allergy signs and corneal biomechanical alterations. Methods: Thirty patients (30 eyes) with AC and twenty normal subjects (20 eyes) were included in this prospective study. All participants underwent a complete ocular examination, including corneal tomography by Pentacam and corneal biomechanics evaluation by Corvis ST. AC patients were evaluated for their eye rubbing frequency and ocular allergic signs. Results: The elevation at the thinnest location (TE) and the central location (CE), the elevation difference at the thinnest location (TED) and the central location (CED), and the mean value of elevation difference in the central 4 mm zoom (MED) of both the anterior and posterior corneal surface were significantly higher in the AC group than in the normal group (p < 0.05 for all). In AC patients, only anterior corneal elevation parameters were positively correlated with eye rubbing frequency and ocular allergy sign severity (p < 0.05 for all), while the tomography and biomechanical index (TBI) was positively correlated with the elevation parameters of both the anterior and posterior corneal surfaces (p < 0.05 for all). Conclusion: AC patients carry an increased risk of corneal ectasia. Posterior corneal elevation parameters are sensitive and reliable predictors of keratoconus (KC) risk in AC patients. Clinical Trial Registration: https://clinicaltrials.gov/ct2/show/NCT04299399, identifier [NCT04299399].

Accuracy of the posterior corneal elevation values of Pentacam HR from different reference surfaces in early ectasia diagnosis

PURPOSE

Detecting the accuracy of various posterior elevation (PE) indices of Pentacam HR, and correlating them with some possibly related factors or parameters, in a cohort with early keratoconus (KC).

METHODS

A cross sectional study that was conducted at Eye World Hospital, Egypt. One hundred and two corneas were enrolled, including two groups; group 1 (50 corneas) having forme fruste or early KC, and group 2 (52 corneas) for healthy controls. Corneas were scanned using Pentacam HR (Oculus, Wetzlar, Germany). The investigated PE parameters were: PE from best fit sphere (BFS), PE from best fit toric ellipsoid (BFTE), PE from exclusion map of Belin Ambrosio's display (BAD), and PE from difference map of BAD. The four PE values were correlated to age, thinnest corneal thickness "TCT," posterior aberrations, and posterior Q value.

RESULTS

All the investigated indices were significantly different in group 1 compared to group 2 (p < 0.001). Accuracy of PE parameters revealed the highest AUROC for PE from BFTE (AUROC = 0.989, and best cutoff > 4 um with sensitivity 96.00% and specificity 96.15%). PE from difference map was the least accurate. Correlation coefficients showed a significant correlation between all the studied PE parameters and some of the posterior aberrations (root mean square of higher order aberrations, vertical coma, and spherical aberrations), besides a significant correlation with posterior Q value.

CONCLUSION

PE indices are sensitive detectors of early ectasia. PE from BFTE had the highest deduced AUROC. Alterations in PE values can significantly alter many posterior corneal aberrations and the posterior Q.

Effect of Down syndrome and keratoconus on corneal density and volume: a triple comparative study

Keratoconus (KCN) and Down syndrome affect the corneal density and volume. In this study included Down syndrome patients with and without KCN (24 Down-KCN and 204 Down-nonKCN eyes) and normal age- and gender-matched individuals (184 eyes). Studied parameters were the corneal density measured with Pentacam HR in 5 concentric zones and annuli (0-2 mm, 2-6 mm, 6-10 mm, 10-12 mm, and 0-12 mm) in 4 different depth layers (anterior 120 µm, posterior 60 µm, middle layer, and the full thickness of the cornea), and the 10 mm zone corneal volume. In Down-KCN, Down-nonKCN, and control groups, respectively, mean full thickness density in the 0-12 mm zone was 19.35 ± 2.92, 17.85 ± 2.55, and 15.78 ± 2.67 GSU, and mean corneal volume was 57.45 ± 4.37, 56.99 ± 3.46, and 61.43 ± 3.42mm3. All density readings were significantly different between the three studied groups (all P < 0.01) except full thickness density in 0-2 mm and 2-6 mm (P > 0.05) and corneal volume (P = 0.519) between Down-KCN and Down-nonKCN groups; these inter-group densitometry differences within the 6 mm zone were only in the middle layer, and not the anterior or posterior thickness layers (all P > 0.05). Corneal density increased with age and corneal thickness, but there was no significant relationship with gender. Overall, Down syndrome is associated with increased density and light scatter in all corneal layers up to the 12 mm diameter. In Down patients with KCN, the increased light scatter and density in the 6 mm zone is only in the middle thickness layer. Corneal volume is reduced in Down syndrome irrespective of the presence or absence of KCN.

EMKLAS: A New Automatic Scoring System for Early and Mild Keratoconus Detection

Purpose

Create a unique predictive model based on a set of demographic, optical, and geometric variables with two objectives: classifying keratoconus (KC) in its first clinical manifestation stages and establishing the probability of having correctly classified each case.

Methods

We selected 178 eyes of 178 subjects (115 males; 64.6%; 63 females, 35.4%). Of these, 74 were healthy control subjects, and 104 suffered from KC according to the RETICS grading system (61 early KC, 43 mild KC). Only one eye from each patient was selected, and 27 different parameters were studied (demographic, clinical, pachymetric, and geometric). The data obtained were used in an ordinal logistic regression model programmed as a web application capable of using new patient data for real-time predictions.

Results

EMKLAS, an early and mild KC classifier, showed good training performance figures, with 73% global accuracy and a 95% confidence interval of 65% to 79%. This classifier is particularly accurate when validated by an independent sample for the control (79%) and mild KC (80%) groups. The accuracy of the early KC group was remarkably lower (69%). The variables included in the model were age, gender, corrected distance visual acuity, 8-mm corneal diameter, and posterior minimum thickness point deviation.

Conclusions

Our web application allows fast, objective, and quantitative assessment of early and mild KC in detection and classification terms and assists ophthalmology professionals in diagnosing this disease.

Translational Relevance

No single gold standard exists for detecting and classifying preclinical KC, but the use of our web application and EMKLAS score may aid the decision-making process of doctors.

Topometric and Tomographic Evaluation of Subclinical Keratoconus

PURPOSE

To investigate the corneal topometric and tomographic findings that can be used in the diagnosis of subclinical keratoconus.

METHODS

A retrospective cohort study. The study group was selected from patients with clinically evident keratoconus in one eye and subclinical keratoconus without evident topographic findings in fellow eye. The age-matched control group was selected from patients who were candidates for laser in situ keratomileusis (LASIK) and did not develop ectasia after LASIK surgery at least 1-year follow-up. All subjects underwent topographic, topometric and tomographic (Belin-Ambrósio Enhanced Ectasia Display III) analyses via a Pentacam HR rotating Scheimpflug camera (Oculus, Germany, version 1.20r.98) before LASIK surgery.

RESULTS

The study group consisted of 151 patients (69 male and 82 female, mean age of 24.8 ± 7.2 years) and the control group also consisted of 150 patients (70 male and 80 female, mean age of 26.0 ± 6.3 years). There were statistically significant differences in all measured topometric (p˂.05) and tomographic (p˂.001) parameters between the eyes with subclinical keratoconus and those of the control group. In discriminating eyes with subclinical keratoconus from normal eyes, final D showed the highest area under curve value (0.858, sensitivity 85.2%, specificity 66.7%), followed by maximum pachymetric progression index (0.809, sensitivity 81.9%, specificity 69.4%) and average pachymetric progression index (0.796, sensitivity 81.9%, specificity 68.1%) in receiver operating characteristic analysis.

CONCLUSION

Topometric and tomographic parameters might be useful for early detection of keratoconus, but the sensitivity and specificity of any parameter are not high enough to be used alone.

A Machine-Learning Model Based on Morphogeometric Parameters for RETICS Disease Classification and GUI Development

This work pursues two objectives: defining a new concept of risk probability associated with suffering early-stage keratoconus, classifying disease severity according to the RETICS (Thematic Network for Co-Operative Research in Health) scale. It recruited 169 individuals, 62 healthy and 107 keratoconus diseased, grouped according to the RETICS classification: 44 grade I; 18 grade II; 15 grade III; 15 grade IV; 15 grade V. Different demographic, optical, pachymetric and eometrical parameters were measured. The collected data were used for training two machine-learning models: a multivariate logistic regression model for early keratoconus detection and an ordinal logistic regression model for RETICS grade assessments. The early keratoconus detection model showed very good sensitivity, specificity and area under ROC curve, with around 95% for training and 85% for validation. The variables that made the most significant contributions were gender, coma-like, central thickness, high-order aberrations and temporal thickness. The RETICS grade assessment also showed high-performance figures, albeit lower, with a global accuracy of 0.698 and a 95% confidence interval of 0.623–0.766. The most significant variables were CDVA, central thickness and temporal thickness. The developed web application allows the fast, objective and quantitative assessment of keratoconus in early diagnosis and RETICS grading terms.

Robust keratoconus detection with Bayesian network classifier for Placido-based corneal indices

PURPOSE

To evaluate in a sample of normal and keratoconic eyes a simple Bayesian network classifier for keratoconus identification that uses previously developed topographic indices, calculated directly from the digital analysis of the Placido ring images.

METHODS

A comparative study was performed on a total of 60 eyes from 60 patients (age 20-60 years) from the Department of keratoconus of INVISION Ophthalmology clinic (Almería, Spain). Patients were divided into two groups depending on their preliminary diagnosis based on the classical topographic criteria: a control group without topographic alteration (30 eyes) and a keratoconus group (30 eyes). The keratoconus group included all grades except grade IV with excessively distorted corneal topography. All cases were examined using the CSO topography system (CSO, Firenze, Italy), and primary corneal Placido-indices were computed, as described in literature. Finally, a classifier was built by fitting a conditional linear Gaussian Bayesian network to the data, using the 5- and 10-fold cross-validation. For comparison, the original data were perturbed with random white noise of different magnitude.

RESULTS

The naïve Bayes classifier showed perfect discrimination ability among normal and keratoconic corneas, with 100% of sensibility and specificity, even in the presence of a very significant noise.

CONCLUSIONS

The Bayesian network classifiers are highly accurate and proved a stable screening method to assist ophthalmologists with the detection of keratoconus, even in the presence of noise or incomplete data. This algorithm is easily implemented for any Placido topographic system.

Assessment of Corneal Pachymetry Distribution and Morphologic Changes in Subclinical Keratoconus with Normal Biomechanics

Purpose

To investigate the pachymetry distribution of central cornea and morphologic changes in subclinical keratoconus with normal biomechanics and determine their potential benefit for the screening of very early keratoconus.

Methods

This retrospective comparative study was performed in 33 clinically unaffected eyes with normal topography and biomechanics from 33 keratoconus patients with very asymmetric ectasia (VAE-NTB; Corvis Biomechanical Index defined) and 70 truly normal eyes from 70 age-matched subjects. Corneal topographic, tomographic, and biomechanical metrics were measured using Pentacam and Corvis ST. The distance and pachymetry difference between the corneal thinnest point and the apex were defined as D_TCP-Apex and DP_TCP-Apex, respectively, to evaluate the pachymetry distribution within the central cornea. The discriminatory power of metrics was analysed via the receiver operating characteristic curve. A logistic regression analysis was used to establish predictive models.

Results

The parameters, D_TCP-Apex and DP_TCP-Apex, were significantly higher in VAE-NTB than those in normal eyes. For differentiating normal and VAE-NTB eyes, the Belin-Ambrósio deviation (BAD-D) showed the largest area under the curve (AUC; 0.799), followed by ARTmax (0.798), D_TCP-Apex (0.771), tomography and biomechanical index (0.760), maximum pachymetry progression index (PPImax, 0.756), DP_TCP-Apex (0.753), and back eccentricity (B_Ecc, 0.707) with no statistically significant differences among these AUCs. In the VAE-NTB group, the parameter B_Ecc was significantly and positively correlated with D_TCP-Apex (P=0.011) and DP_TCP-Apex (P=0.035), whereas the posterior elevation difference had a significant positive association with DP_TCP-Apex (P=0.042). A model using the indices D_TCP-Apex, B_Ecc, PPImax, and index of height asymmetry demonstrated the highest AUC of 0.846 with 91.43% specificity.

Conclusions

Abnormal pachymetry distribution within the central cornea and subtle morphologic changes are detectable in subclinical keratoconus with normal biomechanics. This may improve VAE-NTB eyes detection.

Assessment of the Association between In Vivo Corneal Morphogeometrical Changes and Keratoconus Eyes with Severe Visual Limitation

Assessing changes suffered by the cornea as keratoconus progresses has proven to be vital for this disease diagnosis and treatment. This study determines the corneal biometric profile in eyes considered as affected by keratoconus (KC) showing severe visual limitation, by means of in vivo 3D modelling techniques. This observational case series study evaluated new objective indices in 50 healthy and 30 KC corneas, following a validated protocol created by our research group, which has been previously used for diagnosis and characterization of KC in asymptomatic (preclinical) and mild visually impaired eyes. Results show a statistically significant reduction of corneal volume and an increase of total corneal area in the severe KC group, being anterior and posterior corneal surfaces minimum thickness points the best correlated parameters, although with no discrimination between groups. Receiving operator curves were used to determine sensitivity and specificity of selected indices, being anterior and posterior apex deviations the ones which reached the highest area under the curve, both with very high sensitivity (96.7% and 90%, respectively) and specificity (94.0% and 99.9%, respectively). The results suggest that once severe visual loss appears, anterior corneal topography should be considered for a more accurate diagnosis of clinical KC, being anterior apex deviation the key metric discriminant. This study can be a useful tool for KC classification, helping doctors in diagnosing severe cases of the disease, and can help to characterize corneal changes that appear when severe KC is developed and how they relate with vision deterioration.

Diagnostic Sensitivity of Different Reference Bodies When Using Scheimpflug Tomography in a Myopic Population with Keratoconus

Purpose

To establish which reference body offers the greatest sensitivity in keratoconus (KC) diagnosis, obtain normative data for the myopic population with toric ellipsoid reference bodies, and determine the cutoff points for a population with KC.

Methods

A retrospective, observational study of the entire Scheimpflug tomographer database of the Fundación Jiménez Díaz in Madrid was conducted to identify a normal myopic and a KC myopic population. Three different reference bodies were tested on all patients: best fit sphere (BFS), best fit toric ellipsoid with fixed eccentricity (BFTEFE), and best fit toric ellipsoid (BFTE). Anterior and posterior elevation measurements at the apex and thinnest point were recorded, as well as the root mean square of posterior elevations (RMS-P). Normative data were extracted, and receiver operating characteristic (ROC) curves were generated to obtain cutoff points between the normal and KC population.

Results

A total of 301 eyes were included, comprising 219 normal myopic and 82 myopic KC eyes. BFS and BFTEFE produced the best results when measuring posterior elevation at the thinnest point. BFTE had better sensitivity with the RMS-P. From all measurements, best sensitivity (100%) was achieved with a cutoff point of 8 μm of posterior elevation at the thinnest point using the BFTEFE. BFTE was found to hide the cone in certain patients.

Conclusions

Posterior elevation measured at the thinnest point with a BFTEFE is the best-performing parameter and, therefore, is recommended to discriminate between normal and KC patients within a myopic population.

Trends in Research Related to Keratoconus From 2009 to 2018: A Bibliometric and Knowledge Mapping Analysis

Supplemental Digital Content is Available in the Text.

Purpose:

To map the publication trends in and explore hotspots of keratoconus research.

Methods:

A bibliometric analysis based on the Web of Science Core Collection was conducted to investigate the publication trends in research related to keratoconus. The records extracted were analyzed, and a knowledge map was constructed using VOSviewer v.1.6.10 to visualize the annual publication number, distribution of countries, international collaborations, author productivity, source journals, intellectual base, and research hotspots in the field of keratoconus.

Results:

In total, 3194 peer-reviewed publications on keratoconus published between 2009 and 2018 were retrieved, and the annual research output increased with time. The United States ranked the highest among the countries with the most publications, and Tehran University of Medical Sciences was the most active institution. JL Alio contributed to the most number of publications on keratoconus, and Cornea was the most prolific journal publishing keratoconus research. The top cited references mainly focused on corneal collagen cross-linking. The keywords formed 6 clusters: 1) pathogenesis of keratoconus, 2) corneal collagen cross-linking, 3) management for early-stage keratoconus, 4) corneal parameter measurement, 5) surgical treatment of keratoconus, and 6) corneal biomechanics-related research.

Conclusions:

On the basis of the data extracted from the Web of Science Core Collection, the quantity and quality of publications on keratoconus were assessed using bibliometric techniques. The cited references and research hotspots could provide insights into keratoconus research as well as valuable information to cornea specialists for performing research in this field and discovering potential collaborators.