Keratoconus staging: a computer-assisted ultrabiomicroscopic method compared with videokeratographic analysis

Subclinical keratoconus detection by pattern analysis of corneal and epithelial thickness maps with optical coherence tomography

PURPOSE

To screen for subclinical keratoconus by analyzing corneal, epithelial, and stromal thickness map patterns with Fourier-domain optical coherence tomography (OCT).

SETTING

Four centers in the United States.

DESIGN

Cross-sectional observational study.

METHODS

Eyes of normal subjects, subclinical keratoconus eyes, and the topographically normal eye of a unilateral keratoconus patient were studied. Corneas were scanned using a 26,000 Hz Fourier-domain OCT system (RTVue). Normal subjects were divided into training and evaluation groups. Corneal, epithelial, and stromal thickness maps and derived diagnostic indices, including pattern standard deviation (PSD) variables and pachymetric map-based keratoconus risk scores, were calculated from the OCT data. Area under the receiver operating characteristic curve (AUC) analysis was used to evaluate the diagnostic accuracy of the indices.

RESULTS

The study comprised 150 eyes of 83 normal subjects, 50 subclinical keratoconus eyes of 32 patients, and 1 topographically normal eye of a unilateral keratoconus patient. Subclinical keratoconus was characterized by inferotemporal thinning of the cornea, epithelium, and stroma. The PSD values for corneal (P < .001), epithelial (P < .001), and stromal (P = .049) thickness maps were all significantly higher in subclinical keratoconic eyes than in the normal group. The diagnostic accuracy was significantly higher for PSD variables (pachymetric PSD, AUC = 0.941; epithelial PSD, AUC = 0.985; stromal PSD, AUC = 0.924) than for the pachymetric map-based keratoconus risk score (AUC = 0.735).

CONCLUSIONS

High-resolution Fourier-domain OCT could map corneal, epithelial, and stromal thicknesses. Corneal and sublayer thickness changes in subclinical keratoconus could be detected with high accuracy using PSD variables. These new diagnostic variables might be useful in the detection of early keratoconus.

FINANCIAL DISCLOSURES

Oregon Health and Science University (OHSU) and Drs. Li, Tan, and Huang have a significant financial interest in Optovue, Inc. These potential conflicts have been reviewed and managed by OHSU. Dr. Brass receives research grants from Optovue, Inc. Drs. Chamberlain and Weiss have no financial or proprietary interest in any material or method mentioned.

Evaluation of corneal elevation, pachymetry and keratometry in keratoconic eyes with respect to the stage of Amsler-Krumeich classification

AIM

To evaluate corneal elevation, pachymetry and keratometry in keratoconic eyes according to the clinical stage of the disease.

METHODS

This prospective comparative study was performed on one hundred and twenty-six eyes of 83 patients who had keratoconus, and 42 normal eyes of 42 age-matched subjects. Corneal elevation, pachymetry and keratometry were measured using a rotating Scheimpflug camera (Pentacam HR, Oculus) in these eyes. The area under the receiver operating characteristic (AUROC) curves was used to analyse the diagnostic significance of these parameters, with respect to each stage of Amsler-Krumeich classifications. AUROC was calculated to describe the predictive accuracy of the different indices and to determine the cut-off points where sensitivity and specificity were maximised.

RESULTS

Posterior (0.980) and anterior (0.977) elevation differences showed the highest AUROCs, followed by dioptres (D) value (0.941), percentage thickness increase (PTI) 2 mm (0.931), PTI 4 mm (0.927), progression index (0.927), minimal pachymetry (0.923), average keratometry (0.914), anterior elevation (0.909), PTI 6 mm (0.906), posterior elevation (0.898), central pachymetry (0.889), PTI 8 mm (0.870), PTI 10 mm (0.864), corneal thickness spatial profile 2 mm (0.835) and cylinder (0.796). The differences in AUROC curves between anterior and posterior elevation difference measurements and other diagnostic parameters tended to be larger at the earlier stages of keratoconus.

CONCLUSIONS

Anterior and posterior corneal surface height data obtained by enhanced ectasia display, effectively discriminates keratoconus from normal corneas. Elevation difference measurements may provide useful information for improving the diagnostic accuracy of keratoconus, especially in the early stage of the disease.

Comparison of high-resolution Scheimpflug and high-frequency ultrasound biomicroscopy to anterior-segment OCT corneal thickness measurements

Background

The purpose of this study was to compare and correlate central corneal thickness in healthy, nonoperated eyes with three advanced anterior-segment imaging systems: a high-resolution Scheimpflug tomography camera (Oculyzer II), a spectral-domain anterior-segment optical coherence tomography (AS-OCT) system, and a high-frequency ultrasound biomicroscopy (HF-UBM) system.

Methods

Fifty eyes randomly selected from 50 patients were included in the study. Inclusion criteria were healthy, nonoperated eyes examined consecutively by the same examiner. Corneal imaging was performed by three different methods, ie, Oculyzer II, spectral-domain AS-OCT, and FH-UBM. Central corneal thickness measurements were compared using scatter diagrams, Bland-Altman plots (with bias and 95% confidence intervals), and two-paired analysis.

Results

The coefficient of determination (r²) between the Oculyzer II and AS-OCT measurements was 0.895. Likewise, the coefficient was 0.893 between the Oculyzer II and HF-UBM and 0.830 between the AS-OCT and HF-UBM. The trend line coefficients of linearity were 0.925 between the Oculyzer II and the AS-OCT, 1.006 between the Oculyzer II and HF-UBM, and 0.841 between the AS-OCT and HF-UBM. The differences in average corneal thickness between the three pairs of CCT measurements were −6.86 μm between the Oculyzer II and HF-UBM, −12.20 μm between the AS-OCT and Oculyzer II, and +19.06 μm between the HF-UBM and AS-OCT.

Conclusion

The three methods used for corneal thickness measurement are highly correlated. Compared with the Scheimplug and ultrasound devices, the AS-OCT appears to report a more accurate, but overally thinner corneal pachymetry.

Repeatability and interobserver reproducibility of Artemis-2 high-frequency ultrasound in determination of human corneal thickness

Background

The purpose of this study was to assess the repeatability and limits of agreement of corneal thickness values measured by a high-frequency ultrasound (Artemis-2), hand-held ultrasound pachymeter (DGH-500) and a specular microscope (SP-3000P).

Methods

Central corneal thickness (CCT) was analyzed in this prospective randomized study that included 32 patients (18 men and 14 women) aged 21–24 years. Measurements were obtained in two sessions, one week apart, by two examiners with three devices in a randomized order. Nine measurements were taken (three with each device) on one randomly selected eye of each patient in each measurement session. The coefficient of repeatability and interobserver reproducibility for the values of each method were calculated. The limits of agreement between techniques were also evaluated.

Results

There were no significant differences in CCT values between sessions for each of the three devices (P > 0.05). The repeatability coefficients for the Artemis-2 (±8 μm/±9 μm) were superior to those of the SP-3000P (±9 μm/±11 μm) and DGH 500 (±12 μm/±12 μm) in session 1/session 2 respectively, while the interobserver reproducibility index (differences between session 1 and session 2) was superior for the SP-3000P (±17 μm) with respect to DHG-500 (±29 μm) and the Artemis-2 (±31 μm). In session 1 and session 2, the limits of agreement between the techniques were 35 μm to −31 μm and 34 to −20 μm, respectively, for DGH-500 versus Artemis-2, 73 μm to 3 μm and 60 μm to 9 μm for Artemis-2 versus SP-3000P, and 58 μm to 22 μm and 72 μm to 10 μm for DGH-500 versus SP-3000P comparisons. The DGH-500 and Artemis-2 gave similar values (P > 0.05) in both sessions, but both (Artemis-2 and DGH-500) values were significantly greater than that of the SP-3000P (P < 0.05) in both sessions.

Conclusion

Repeatability was comparably good for the three techniques. However, interobserver reproducibility was approximately twice as good with the SP-3000P compared with the other two devices. The Artemis-2 CCT values consistently agreed with the DGH-500 and less so with the SP-3000P. The Artemis-2 provided CCT values that were, on average, 38 μm and 34 μm greater than that of the SP-3000P in session 1 and session 2, respectively.

Epithelial, stromal, and total corneal thickness in keratoconus: three-dimensional display with artemis very-high frequency digital ultrasound

PURPOSE

To characterize the epithelial, stromal, and total corneal thickness profile in a population of eyes with keratoconus.

METHODS

Epithelial, stromal, and total corneal thickness profiles were measured in vivo by Artemis very high-frequency (VHF) digital ultrasound scanning (ArcScan) across the central 6- to 10-mm diameter of the cornea on 54 keratoconic eyes. Maps of the average, standard deviation, minimum, maximum, and range of epithelial, stromal, and total corneal thickness were plotted. The average location of the thinnest epithelium, stroma, and total cornea were found. The cross-sectional semi-meridional stromal and total corneal thickness profiles were calculated using annular averaging. The absolute stromal and total corneal thickness progressions relative to the thinnest point were calculated using annular averaging as well as for 8 semi-meridians individually.

RESULTS

The mean corneal vertex epithelial, stromal, and total corneal thicknesses were 45.7+/-5.9 microm, 426.4+/-38.5 microm, and 472.2+/-41.4 microm, respectively. The average epithelial thickness profile showed an epithelial doughnut pattern characterized by localized central thinning surrounded by an annulus of thick epithelium. The thinnest epithelium, stroma, and total cornea were displaced on average by 0.48+/-0.66 mm temporally and 0.32+/-0.67 mm inferiorly, 0.31+/-0.45 mm temporally and 0.54+/-0.37 mm inferiorly, and 0.31+/-0.43 mm temporally and 0.50+/-0.35 mm inferiorly, respectively, with reference to the corneal vertex. The increase in semi-meridional absolute stromal and total corneal thickness progressions was greatest inferiorly and lowest temporally.

CONCLUSIONS

Three-dimensional thickness mapping of the epithelial, stromal, and total corneal thickness profiles characterized thickness changes associated with keratoconus and may help in early diagnosis of keratoconus.

Stromal thickness in the normal cornea: three-dimensional display with artemis very high-frequency digital ultrasound

PURPOSE

To characterize the stromal thickness profile in a population of normal eyes.

METHODS

Stromal thickness profile was measured in vivo by Artemis very high-frequency digital ultrasound scanning (ArcScan, Morrison, Colo) across the central 10-mm corneal diameter on 110 normal eyes. Maps of the average, standard deviation, minimum, maximum, and range of stromal thickness were plotted. The average location of the thinnest stroma was found. The cross-sectional hemi-meridional stromal thickness profile was calculated using annular averaging. The absolute stromal thickness progression relative to the thinnest point was calculated using annular averaging as well as for 8 hemi-meridians individually.

RESULTS

The mean stromal thickness at the corneal vertex and at the thinnest point were 465.4+/-36.9 mum and 461.8+/-37.3 mum, respectively. The thinnest stroma was displaced on average 0.17+/-0.31 mm inferiorly and 0.33+/-0.40 mm temporally from the corneal vertex. The average absolute stromal thickness progression from the thinnest point could be described by the quadratic equation: stromal thickness = 6.411 x radius(2) + 2.444 x radius (R(2) = 0.999). Absolute stromal thickness progression was independent of stromal thickness at the thinnest point. The increase in hemi-meridional absolute stromal thickness progression was greatest superiorly and lowest temporally.

CONCLUSIONS

Three-dimensional thickness mapping of the corneal stroma and stromal thickness progression in a population of normal eyes represent a normative data set, which may help in early diagnosis of corneal abnormalities such as keratoconus and pellucid marginal degeneration. Absolute stromal thickness progression was found to be independent of stromal thickness.

Fluorescein pattern interpretation in keratoconus

PURPOSE

The purpose of this study is to assess the effect of disease severity on how accurately contact lens fluorescein patterns can be interpreted in keratoconus by clinician assessment.

METHODS

Two clinicians evaluated fluorescein patterns on 111 eyes of 60 patients with mild (<45 D, 14 eyes), moderate (45 D to 52 D, 61 eyes,) and severe (>52 D, 36 eyes) keratoconus. The masked clinicians were given six contact lenses in random order, the lens that just cleared the corneal apex (the first definite apical clearance lens), three lenses flatter (in 0.1 mm increments), and two lenses steeper (in 0.1 mm increments) than the first definite apical clearance lens. They ranked the lenses from flattest to steepest, based on the fluorescein patterns. The percentage of lenses correctly ranked was determined using (1) exact match with actual; (2) within 0.1 mm of actual; and (3) within 0.2 mm of actual. Accuracy was assessed as the sum of the squared differences between the actual base curve value and each clinician's ranking. Comparison of the mean percentage correctly ranked and accuracy for each keratoconus severity groups was performed using a mixed linear model.

RESULTS

Neither percentage correctly ranked (using any of the three protocols) nor accuracy was found to be related to severity of keratoconus (p > 0.15 for all comparisons).

CONCLUSIONS

Accuracy of ranking contact lenses in order of base curve radius based on fluorescein pattern assessment by clinicians does not seem to be related to severity of keratoconus. Many factors influence interpretation of fluorescein patterns including all components of the system, fluorescein, tears, cornea, contact lens, external forces, and technique.

High-resolution ultrasound imaging of the eye - a review

This report summarizes the physics, technology and clinical application of ultrasound biomicroscopy (UBM) of the eye, in which frequencies of 35 MHz and above provide over a threefold improvement in resolution compared with conventional ophthalmic ultrasound systems. UBM allows imaging of anatomy and pathology involving the anterior segment, including regions obscured by overlying optically opaque anatomic or pathologic structures. UBM provides diagnostically significant information in conditions such as glaucoma, cysts and neoplasms, trauma and foreign bodies. UBM also can provide crucial biometric information regarding anterior segment structures, including the cornea and its constituent layers and the anterior and posterior chambers. Although UBM has now been in use for over 15 years, new technologies, including transducer arrays, pulse encoding and combination of ultrasound with light, offer the potential for significant advances in high-resolution diagnostic imaging of the eye.

Keratoconus diagnosis with optical coherence tomography pachymetry mapping

OBJECTIVE

To detect abnormal corneal thinning in keratoconus using pachymetry maps measured by high-speed anterior segment optical coherence tomography (OCT).

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Thirty-seven keratoconic eyes from 21 subjects and 36 eyes from 18 normal subjects.

METHODS

The OCT system operated at a 1.3 microm wavelength with a scan rate of 2000 axial scans per second. A pachymetry scan pattern (8 radials, 128 axial scans each; 10 mm diameter) centered at the corneal vertex was used to map the corneal thickness. The pachymetry map was divided into zones by octants and annular rings. Five pachymetric parameters were calculated from the region inside the 5 mm diameter: minimum, minimum-median, inferior-superior (I-S), inferotemporal-superonasal (IT-SN), and the vertical location of the thinnest cornea. The 1-percentile value of the normal group was used to define the diagnostic cutoff. Placido-ring-based corneal topography was obtained for comparison.

MAIN OUTCOME MEASURES

The OCT pachymetric parameters and a quantitative topographic keratoconus index (keratometry, I-S, astigmatism, and skew percentage [KISA%]) were used for keratoconus diagnosis. Diagnostic performance was assessed by the area under the receiver operating characteristic (AROC) curve.

RESULTS

Keratoconic corneas were thinner. The pachymetric minimum averaged 452.6+/-60.9 microm in keratoconic eyes versus 546+/-23.7 microm in normal eyes. The 1-percentile cutoff was 491.6 microm. The thinnest location was inferiorly displaced in keratoconus (-805+/-749 microm vs -118+/-260 microm; cutoff, -716 microm). The thinning was focal (minimum-median: -95.2+/-41.1 microm vs -45+/-7.7 microm; cutoff, -62.6 microm). Keratoconic maps were more asymmetric (I-S, -44.8+/-28.7 microm vs -9.9+/-9.3 microm; cutoff, -31.3 microm; and IT-SN, -63+/-35.7 microm vs -22+/-11.4 microm; cutoff, -48.2 microm). Keratoconic eyes had a higher KISA% index (2641+/-5024 vs 21+/-19). All differences were statistically significant (t test, P<0.0001). Applying the diagnostic criteria of any 1 OCT pachymetric parameter below the keratoconus cutoff yielded an AROC of 0.99, which was marginally better (P = .09) than the KISA% topographic index (AROC, 0.91).

CONCLUSIONS

Optical coherence tomography pachymetry maps accurately detected the characteristic abnormal corneal thinning in keratoconic eyes. This method was at least as sensitive and specific as the topographic KISA.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

Incidence and prevalence of keratoconus in Denmark

PURPOSE

To estimate the prevalence and incidence of hospitalized keratoconus (KC) in Denmark.

METHODS

Data extracts from the National Patient Registry under the National Board of Health (which covers the entire Danish population) were analysed.

RESULTS

The prevalence of KC was estimated at 86 patients per 100,000 residents and the incidence at 1.3 per 100 000 per year.

CONCLUSION

KC is rather widespread in Denmark, with more than 4600 affected individuals.

Corneal-thickness spatial profile and corneal-volume distribution: Tomographic indices to detect keratoconus

PURPOSE

To evaluate whether the corneal-thickness spatial profile and corneal-volume distribution differentiate keratoconic corneas from normal corneas using new tomography parameters.

SETTING

Subspecialty cornea and refractive practice, Fluminense Federal University, Rio de Janeiro, Brazil.

METHODS

Forty-six eyes diagnosed with mild to moderate keratoconus and 364 normal eyes were studied by the Pentacam Comprehensive Eye Scanner. Corneal thickness at the thinnest point and the averages of the points on 22 imaginary circles centered on the thinnest point with increased diameters at 0.4 mm steps were calculated to create a corneal-thickness spatial profile. Corneal volume was calculated within diameters from 1.0 to 7.0 mm with 0.5 mm steps centered on the thinnest point to create the corneal-volume distribution. The percentage increase in thickness and the percentage increase in volume were calculated for each position of the corneal-thickness spatial profile and corneal-volume distribution from their first value. Statistical analysis was done using the Wilcoxon 2-independent-sample test to compare mean levels using S-Plus-4.0 software (MathSoft) and a normal linear model under a Bayesian frame for estimating the mean variation in thickness and volume using the BUGS 0.6 package.

RESULTS

Statistically significant differences were observed between the groups (P<.05) in all positions of corneal-thickness spatial profile and corneal-volume distribution and in the percentage increase in thickness and percentage increase in volume between 3.5 mm and 7.0 mm diameters.

CONCLUSIONS

Corneal-thickness spatial profile, corneal-volume distribution, percentage increase in thickness, and percentage increase in volume were different between keratoconic corneas and normal corneas and could serve as indices to diagnose keratoconus and screen refractive candidates. Further studies are necessary to evaluate whether these tomographic indices are more sensitive and specific than the classic Placido-based topography.

Automated Keratoconus Detection Using Height Data of Anterior and Posterior Corneal Surfaces

PURPOSE

To develop a keratoconus detection algorithm using the corneal topographic data of the anterior and posterior corneal surfaces.

METHODS

Topographic measurements of the cornea were made with a slit-scanning corneal topographer. We examined 120 subjects (165 eyes); keratoconus patients and keratoconus suspect patients comprised the keratoconus group, and post-photorefractive keratectomy patients, with-the-rule astigmatism patients, and controls without disease comprised the nonkeratoconus group. Two variables of the anterior corneal surface, two variables of the posterior corneal surface, and one corneal thickness variable were obtained by applying the Fourier harmonic decomposition formula. By performing a logistic regression analysis with a training set to differentiate the keratoconus group from the nonkeratoconus group, the Fourier-incorporated keratoconus detection Index (FKI) was created. The validity of the FKI was determined by using independent validation sets.

RESULTS

The FKI distinguished the keratoconus group from the nonkeratoconus group with 96.9% sensitivity and 95.4% specificity in the validation set.

CONCLUSIONS

A newly developed automated keratoconus classifier can be used to screen keratoconic patients. The index is based on information obtained by Fourier analysis from not only the anterior corneal surface but also from the posterior corneal surface and corneal thickness.

A New Method for Grading the Severity of Keratoconus

PURPOSE

To define a new method for grading severity of keratoconus, the Keratoconus Severity Score (KSS).

METHODS

A rationale for grading keratoconus severity was developed using common clinical markers plus 2 corneal topographic indices, creating a 0 to 5 severity score. An initial test set of 1012 eyes, including normal eyes, eyes with abnormal corneal and topographic findings but not keratoconus, and eyes with keratoconus having a wide range of severity, was used to determine cutpoints for the KSS. Validation set 1, comprising data from 128 eyes, was assigned a KSS and compared with a clinician's ranking of severity termed the "gold standard" to determine if the scale fairly represented how a clinician would grade disease severity. kappa statistics, sensitivity, and specificity were calculated. A program was developed to automate the determination of the score. This was tested against a manual assignment of KSS in 2121 (validation set 2) eyes from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study, as well as normal eyes and abnormal eyes without keratoconus. Ten percent of eyes underwent repeat manual assignment of KSS to determine the variability of manual assignment of a score.

RESULTS

From initial assessments, the KSS used 2 corneal topography indices: average corneal power and root mean square (RMS) error for higher-order Zernike terms derived from the first corneal surface wavefront. Clinical signs including Vogt striae, Fleischer rings, and corneal scarring were also included. Last, a manual interpretation of the map pattern was included. Validation set 1 yielded a kappa statistic of 0.904, with sensitivities ranging from 0.64 to 1.00 and specificities ranging from 0.93 to 0.98. The sensitivity and specificity for determining nonkeratoconus from keratoconus were both 1.00. Validation set 2 showed kappa statistics of 0.94 and 0.95 for right and left eyes, respectively. Test-retest analysis yielded kappa statistics of 0.84 and 0.83 for right and left eyes, respectively.

CONCLUSION

A simple and reliable grading system for keratoconus was developed that can be largely automated. Such a grading scheme could be useful in genetic studies for a complex trait such as keratoconus requiring a quantitative measure of disease presence and severity.