Allan Luz

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.

Novel Pachymetric Parameters Based on Corneal Tomography for Diagnosing Keratoconus

PURPOSE To describe pachymetric progression indices (PPI) of the Pentacam HR (Oculus Optikgeräte GmbH) and the concept of relational thickness, and to test their accuracy for differentiating keratoconic and normal corneas compared with single-point thickness values. METHODS One hundred thirteen individual eyes randomly selected from 113 normal patients and 44 eyes of 44 patients with keratoconus were studied using the Pentacam HR by acquiring central corneal thickness (CCT), thinnest point (TP), position of the TP and PPI at minimal (PPI Min) and maximal (PPI Max) meridians, and the average (PPI Ave) of all meridians. Relational thickness parameters were calculated as the ratios of TP and CCT and PPI values. Mann-Whitney U test assessed differences in groups for each variable. Receiver operating characteristic (ROC) curves were calculated for all variables and pairwise comparisons were performed. RESULTS Statistically significant differences were noted between normal and keratoconic eyes for all parameters (P<.001), except for horizontal position of TP (P=.79). The best parameters, named Ambrósios Relational Thickness (ART), were ART-Ave (TP/PPI Ave) and ART-Max (TP/PPI Max) with areas under the ROC curves of 0.987 and 0.983, respectively. The best cutoffs were 424 μm and 339 μm for ART-Ave and ART-Max, respectively. Pachymetric progression indices and ART had a greater area under the curve than TP and CCT (P<.001); TP (0.955) had a greater area under the curve than CCT (0.909; P=.002). CONCLUSIONS Tomographic-derived pachymetric parameters were better able to differentiate normal and keratoconic corneas than single-point pachymetric measurements. Further studies are needed to evaluate the role of tomography in identifying early forms of ectasia as well as ectasia risk among LASIK candidates.

Evaluation of corneal shape and biomechanics before LASIK.

The preoperative evaluation is of critical importance for success in laser in situ keratomileusis (LASIK). This examination should fulfill 3 main purposes: counseling and educating the candidates, surgery planning, and screening for cases at higher risk for complications. It is critical to interview each refractive patient to assess their individual needs and to provide realistic expectations. A thorough ophthalmologic examination is mandatory, including specific complementary examinations to characterize many aspects of the cornea and the optics of the eye. In fact, it is notable that refractive surgery has motivated tremendous development for advanced diagnostic methods, among many others advancements and innovations in Ophthalmology. One of the most important aspects of the preoperative examination of LASIK candidates is to screen cases at risk for progressive ectasia.

Dynamic ultra high speed Scheimpflug imaging for assessing corneal biomechanical properties

OBJECTIVE: To describe a novel technique for clinical characterization of corneal biomechanics using non-invasive dynamic imaging. METHODS: Corneal deformation response during non contact tonometry (NCT) is monitored by ultra-high-speed (UHS) photography. The Oculus Corvis ST (Scheimpflug Technology; Wetzlar, Germany) has a UHS Scheimpflug camera, taking over 4,300 frames per second and of a single 8mm horizontal slit, for monitoring corneal deformation response to NCT. The metered collimated air pulse or puff has a symmetrical configuration and fixed maximal internal pump pressure of 25 kPa. The bidirectional movement of the cornea in response to the air puff is monitored. RESULTS: Measurement time is 30ms, with 140 frames acquired. Advanced algorithms for edge detection of the front and back corneal contours are applied for every frame. IOP is calculated based on the first applanation moment. Deformation amplitude (DA) is determined as the highest displacement of the apex in the highest concavity (HC) moment. Applanation length (AL) and corneal velocity (CVel) are recorded during ingoing and outgoing phases. CONCLUSION: Corneal deformation can be monitored during non contact tonometry. The parameters generated provide clinical in vivo characterization of corneal biomechanical properties in two dimensions, which is relevant for different applications in Ophthalmology.

Scheimpflug imaging for laser refractive surgery.

Purpose of review To review the principles and clinical applications of Scheimpflug corneal and anterior segment imaging with special relevance for laser refractive surgery. Recent findings Computerized Scheimpflug imaging has been used for corneal and anterior segment tomography (CASTm) in different commercially available instruments. Such approach computes the three-dimensional image of the cornea and anterior segment, enabling the characterization of elevation and curvature of the front and back surfaces of the cornea, pachymetric mapping, calculation of the total corneal refractive power and anterior segment biometry. CASTm represents a major evolution for corneal and anterior segment analysis, beyond front surface corneal topography and single point central corneal thickness measurements. This approach enhances the diagnostic abilities for screening ectasia risk as well as for planning, evaluating the results, managing complications of refractive procedures, and selecting intraocular lens power, type, and design. In addition, dynamic Scheimpflug imaging has been recently introduced for in-vivo corneal biomechanical measurements and has also been used for anterior segment imaging of femtocataract surgery. Summary Scheimpflug imaging has an important role for laser refractive surgery with different applications, which continuously improve due to advances in technology.

Ocular Biomechanical Metrics by CorVis ST in Healthy Brazilian Patients

PURPOSE To evaluate ocular biomechanical metrics given by the CorVis ST (Oculus, Inc., Berlin, Germany) in a population of healthy Brazilian patients. METHODS An observational and cross-sectional study involving 1 eye randomly selected from 90 healthy patients. Studied parameters (including deformation amplitude, first applanation time, highest concavity time, second applanation time, first applanation length, second applanation length, curvature radius highest concavity, curvature radius normal, velocity in, and velocity out) derived from the CorVis ST were correlated to central corneal thickness from the Pentacam (Oculus, Inc.). Differences between data on the basis of gender were evaluated. RESULTS Mean patient age was 35.80 ± 12.83 years (range: 21.07 to 78.84 years). Mean central corneal thickness was 547.50 ± 32.00 μm (range: 490 to 647 μm) and mean spherical equivalent refraction was -3.29 ± 3.69 diopters (range: -9.50 to +10.37 diopters). Mean deformation amplitude was 1.05 ± 0.08 mm (range: 0.91 to 1.26 mm). Highest concavity time was 18.38 ± 0.93 ms (range: 16.95 to 21.07 ms). Intraocular pressure was 16.43 ± 2.15 mm Hg (range: 11.50 to 21.0 mm Hg). First applanation time was 8.32 ± 0.33 ms (range: 7.53 to 9.12 ms) and second applanation time was 23.80 ± 0.44 ms (range: 22.76 to 24.95 ms). First applanation length (max) was 2.07 ± 0.38 mm (range: 1.20 to 3.10 mm) and second applanation length (max) was 2.37 ± 0.47 mm (range: 1.33 to 4.12 mm). Curvature radius highest concavity was 11.09 ± 2.06 mm (range: 7.58 to 15.98 mm) and curvature radius normal was 7.59 ± 0.67 mm (range: 6.82 to 11.02 mm). Velocity in was 0.21 ± 0.05 m/s (range: 0.16 to 0.72 m/s) and velocity out was -0.33 ± 0.07 m/s (range: -0.72 to -0.20 m/s). Studied parameters were not associated with gender. CONCLUSIONS Eight of 11 ocular biomechanical metrics given by the CorVis ST were associated with central corneal thickness, but the influence of central corneal thickness on these measurements was low.

Progressão da espessura corneana do ponto mais fino em direção ao limbo: estudo de uma população normal e de portadores de ceratocone para criação de valores de referência

PURPOSE: To study the variation and progression of the pachymetric values from the thinnest point towards the limbus in normal and keratoconic corneas; to establish reference curves for this parameter. METHODS: One hundred eyes with normal corneas and twenty-five eyes with mild keratoconus (stages I and II - Krumeich) were analyzed using the Orbscan. Concentric circles were drawn on the thinnest point of the cornea with increasing radii from 1 to 7 mm. The average results of each circle were calculated and inserted in an Excel table in order to arrange a progression chart for each case starting on the thinnest point. The SPSS software was also used for statistical analysis. Students t test was then used to compare the found values. RESULTS: The average values on the thinnest point in normal patients was 511.6 µm (standard deviation 30.6). The average values of the thinnest point in patients with keratoconus was 424.4 µm (standard deviation 56.57). Statistically significant different values were reported (p<0.01) for all circles, and for 6 and 7 mm radii p=0.01. CONCLUSION: There is a greater pachymetric variability in patients with keratoconus. Keratoconic corneas also have a faster progression of pachymetric values than healthy eyes. Pachymetric progression complements traditional single point evaluation of corneal thickness and must be considered for the screening at refractive candidates. This parameter may represent an indirect index of the biomechanics of corneal tissue but this hypothesis still needs further studies.

Scheimpflug-Based Tomography and Biomechanical Assessment in Pressure-Induced Stromal Keratopathy

PURPOSE To report the tomographic and biomechanical findings before and after treatment of a case of pressure-induced stromal keratopathy (PISK), which was misdiagnosed as diffuse lamellar keratitis (DLK). METHODS A case report of a referred patient with supposed diagnosis of DLK after LASIK in the right eye. Scheimpflug-based corneal tomography and biomechanical assessment were provided by the Pentacam HR and CorVis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany). RESULTS A layer of corneal opacity beneath the flap with a presumably fluid-filled interface area was observed on slit-lamp biomicroscopy. Scheimpflug image from Pentacam revealed a hyperreflective area underneath the flap interface. Goldmann applanation tonometry was 12 mm Hg, whereas CorVis intraocular pressure was 53.5 mm Hg with deformation amplitude of 0.42 mm. Two days after starting oral and topical ocular hypotensive therapy, CorVis intraocular pressure was 14 mm Hg and deformation amplitude was 1.02 mm. CONCLUSIONS Ocular hypertension in PISK was associated with lower deformation response, along with steepening and thickening of the cornea.

Assessing ectasia susceptibility prior to LASIK: the role of age and residual stromal bed (RSB) in conjunction to Belin-Ambrósio deviation index (BAD-D)

Purpose: To compare the ability to detect preoperative ectasia risk among LASIK candidates using classic ERSS (Ectasia Risk Score System) and Pentacam Belin-Ambrosio deviation index (BAD-D), and to test the benefit of a combined approach including BAD-D and clinical data. Methods: A retrospective nonrandomized study involved preoperative LASIK data from 23 post-LASIK ectasia cases and 266 stable-LASIK (follow up > 12 months). Preoperative clinical and Pentacam (Oculus; Wetzlar, Germany) data were obtained from all cases. Mann-Whitneys test was performed to assess differences between groups. Stepwise logistic regression was used for combining parameters.The areas under the Receiver Operating Characteristic (ROC) curves (AUC) were calculated for all parameters and combinations, with pairwise comparisons of AUC (DeLongs method). Results: Statistically significant differences were found for age, residual stromal bed (RSB), central corneal thickness and BAD-D (p 0.05). ERSS was 3 or more on 12/23 eyes from the ectasia group (sensitivity = 52.17%) and 48/266 eyes from the stable LASIK group (18% false positive). BAD-D had AUC of 0.931 (95% CI: 0.895 to 0.957), with cut-off of 1.29 (sensitivity = 87%; specificity = 92.1%). Formula combining BAD-D, age and RSB provided 100% sensitivity and 94% specificity, with better AUC (0.989; 95% CI: 0.969 to 0.998) than all individual parameters (p>0.001). Conclusion: BAD-D is more accurate than ERSS. Combining clinical data and BAD-D improved ectasia susceptibility screening. Further validation is necessary. Novel combined functions using other topometric and tomographic parameters should be tested to further enhance accuracy.

Enhanced ectasia screening: the need for advanced and objective data.

To the Editor: We read with interest the report from Drs. Abdulmassih-Gonçalves and Gonçalves.1 The authors should be commended for publishing a case of ectasia after LASIK, which was detected and addressed by collagen cross-linking. We also appreciate their willingness in sharing the preoperative raw data from rotatingScheimpflug tomography, which generated interesting information that we address in this letter. The tomography examination was taken by the Oculyzer (Wavelight, Inc., Erlangen, Germany), which has the same hardware characteristics as the Oculus Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany). Although the Oculyzer software is designed for topography-guided ablations with the Wavelight excimer lasers, there are significant differences on diagnostic parameters when compared to Pentacam. Objective tomographic parameters, which go beyond curvature and central corneal thickness, are critical for clinical decisions when screening for ectasia risk after laser vision correction.2 Advanced analysis from the raw data generated by the Oculyzer allowed for the calculation of Ambrósio’s Relational Thickness3 and the deviation value from the Belin-Ambrósio Enhanced Ectasia Display (BAD-D), which demonstrates enhanced accuracy for detecting milder forms of ectasia, defined as the fellow eye with relatively normal curvature maps from patients with asymmetric keratoconus (considered as forme fruste keratoconus) (Figure 1).4 BAD-D in the right eye was higher than 1.22, the cut-off value for forme fruste keratoconus (93.62% sensitivity and 94.56% specificity).4 Ambrósio’s Relational Thickness average was lower than 521 μm in both eyes, the cut-off value for forme fruste keratoconus (91.94% sensitivity and 93.05 specificity).4 Ambrósio’s Relational Thickness maximum in the right eye was lower than 386 μm, the cut-off for keratoconus detection (99.17% sensitivity and 97.28% specificity).4 The patient was 21 years old at the time of the Oculyzer examination. Therefore, the Ectasia Risk Score System should be recalculated to 5 for the right eye and 4 for the left eye. Although the consideration of age may be an important factor for the lack of specificity of the Ectasia Risk Score System, there is compelling evidence that supports the younger age being related to lower biomechanical properties of the cornea.2 In a retrospective study involving preoperative clinical and Pentacam data from 23 cases that developed ectasia after LASIK and 266 stable LASIK cases (more than 12 months of follow-up), age was a strong predictor of ectasia risk, being combined in a stepwise logistic regression analysis with BAD-D and residual stromal bed to calculate the Ectasia Susceptibility Score-1. The Ectasia Susceptibility Score-1 represents the percentage of risk for developing ectasia, with a threshold of 6.45%. This allows for 100% sensitivity and 94% specificity, which is better than BAD-D, the best isolated parameter with a cut-off of 1.29 (87% sensitivity and 92.1% specificity).5 Considering the calculated residual stromal bed of 281 and 301 μm,1 the Ectasia Susceptibility Score-1 would be 96.75% and 79% for the right and left eye, respectively. Also, minimal re-