Dependence of dynamic contour and Goldmann applanation tonometries on peripheral corneal thickness.

AIM: To determine the effects of peripheral corneal thickness (PCT) on dynamic contour tonometry(DCT) and Goldmann applanation tonometry (GAT). METHODS: A cross-sectional study. We created a software which calculates the corneal contour (CC) as a function of the radius from the corneal apex to each pixel of the contour. The software generates a central circumference with a radius of 1 mm and the remainder of the cornea is segmented in 5 rings concentric with corneal apex being its diameter not constant around the corneal circumference as a consequence of the irregular CC but keeping constant the diameter of each ring in each direction of the contour. PCT was determined as the mean thickness of the most eccentric ring. Locally weighted scatterplot smoothing (LOWESS) regression was used to determine the pattern of the relationship between PCT and both DCT and GAT respectively. Thereafter, two multivariable linear regression models were constructed. In each of them, the dependant variable was intraocular pressure (IOP) as determined using GAT and DCT respectively. In both of the models the predictive variable was PCT though LOWESS regression pattern was used to model the relationship between the dependant variables and the predictor one. Age and sex were also introduced control variables along with their first-degree interactions with PCT. Main outcome measures include amount of IOP variation explained through regression models (R2) and regression coefficients (B). RESULTS: Subjects included 109 eyes of 109 healthy individuals. LOWESS regression suggested that a 2nd-degree polynomial would be suitable to model the relationship between both DCT and GAT with PCT. Hence PCT was introduced in both models as a linear and quadratic term. Neither age nor sex nor interactions were statistically significant in both models. For GAT model, R2 was 17.14% (F=9.02; P=0.0002), PCT linear term B was -1.163 (95% CI: -1.163, -0.617). PCT quadratic term B was 0.00081 (95% CI: 0.00043, 0.00118). For DCT model R2 was 14.28% (F=9.29; P=0.0002), PCT linear term B was -0.712 (95% CI: -1.052, -0.372), PCT quadratic term was B=0.0005 (95% CI: 0.0003, 0.0007). CONCLUSION: DCT and GAT measurements are conditioned by PCT though this effect, rather than linear, follows a 2nd-degree polynomial pattern.

Correlations between corneal and optic nerve head variables in healthy subjects and patients with primary open angle glaucoma.

AIM: To correlate corneal variables (determined using the Pentacam) with optic nerve head (ONH) variables determined using the Heidelberg retina tomograph (HRT) in healthy subjects and patients diagnosed with primary open angle glaucoma (POAG). METHODS: Measurements were made in 75 healthy eyes and 73 eyes with POAG and correlations examined through Pearson correlation coefficients between the two sets of variables in the two subject groups. The corneal variables determined were corneal volume (CVol), central corneal thickness (CCT), overall corneal thickness (OvCT), the mean thickness of a circular zone centered at the corneal apex of 1 mm radius (zone I) and the mean thickness of several concentric rings, also centered at the apex until the limbus, each of 1 mm width (zones II to VI respectively). The ONH variables were determined using the HRT. RESULTS: The following pairs of variables were correlated in the control group: CCT-disc area (DAr) (-0.48; P<0.0001), Zone I-DAr (-0.503; P<0.0001) and Zone II-DAr (-0.443; P<0.0001); and in the POAG group: CCT-cup-to-disc area ratio (CDRa) (-0.402; P<0.0001), Zone I-CDRa (-0.418; P<0.0001), Zone II-CDRa (-0.405; P=0.006), Zone I-cup shape measure (CSM) (-0.415; P=0.002), Zone II-CSM (-0.405; P=0.001), Zone IV-height variation contour (HVC) (0.378; P=0.002); Zone V-HVC (0.388, P<0.0001). CONCLUSIONS: In the healthy subjects, significant negative correlation was detected between central and paracentral corneal thickness and optic disc area. In contrast, the POAG patients showed significant negative correlation between central and paracentral corneal thickness and the cup-disc ratio and CSM, and positive correlation between peripheral corneal thickness and HVC.

Anatomical characterization of central, apical and minimal corneal thickness.

AIM: To anatomically locate the points of minimum corneal thickness and central corneal thickness (pupil center) in relation to the corneal apex. METHODS: Observational, cross-sectional study, 299 healthy volunteers. Thickness at the corneal apex (AT), minimum corneal thickness (MT) and corneal thickness at the pupil center (PT) were determined using the pentacam. Distances from the corneal apex to MT (MD) and PT (PD) were calculated and their quadrant position (taking the corneal apex as the reference) determined: point of minimum thickness (MC) and point of central thickness (PC) depending on the quadrant position. Two multivariate linear regression models were constructed to examine the influence of age, gender, power of the flattest and steepest corneal axes, position of the flattest axis, corneal volume (determined using the Pentacam) and PT on MD and PD. The effects of these variables on MC and PC were also determined in two multinomial regression models. RESULTS: MT was located at a mean distance of 0.909 mm from the apex (79.4% in the inferior-temporal quadrant). PT was located at a mean distance of 0.156 mm from the apex. The linear regression model for MD indicated it was significantly influenced by corneal volume (B=-0.024; 95%CI: -0.043 to -0.004). No significant relations were identified in the linear regression model for PD or the multinomial logistic regressions for MC and PC. CONCLUSION: MT was typically located at the inferior-temporal quadrant of the cornea and its distance to the corneal apex tended to decrease with the increment of corneal volume.

Characterization of the thickness of different corneal zones in glaucoma: effect on dynamic contour, Goldmann and rebound tonometries.

PURPOSE: To characterize five models of corneal thickness circular zoning in a sample of healthy controls and a sample of patients with primary open-angle glaucoma (POAG) and to determine their effect on Goldmann (GAT), dynamic contour (DCT) and rebound tonometers (RT). METHODS: The study participants were 122 controls and 129 cases. Five corneal thickness zoning models (A, B, C, D and E) were constructed. The partitioning pattern consisted of a circle centred at the corneal apex and several concentric rings, until the limbus; the contours of each ring followed the geometry of the corneal contour of each participant. In Model A, the central circle was 1 mm in diameter and five concentric rings were established. Mean was obtained for each zone for both samples and compared between them using a t-test. The effect on the tonometers of central cornel thickness (CCT) and mean thickness of the zones generated was determined through several linear regression models (one per tonometer and per sample). RESULTS: According to a t-test, cases and controls differ in zones I [mean difference (MD): 17.93 µm], V (MD: 25.52 µm) and VI (MD: 31.78 µm) of model A (higher values in the cases sample). RT was affected by CCT (controls: B = 0.089; cases: B = 0.081). DCT was affected by zone IV of model A (controls: B = -0.029; cases: B = -0.012). GAT was affected by CCT (controls: B = 0.043; cases: B = 0.025) and zone III of model A (controls: B = -0.045; cases: B = -0.033). CONCLUSION: Our results highlight the importance of the thickness of other regions of the cornea different from its main centre in discriminating between healthy controls and patients with POAG and in IOP measurements made using DCT, GAT and RT.

Effect of corneal morphometry on dynamic contour and Goldmann applanation tonometry.

OBJECTIVES: To determine the effects on dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT) of the power of the flattest and steepest corneal meridians, their orientation, central corneal thickness (CCT), mean overall corneal thickness, and the mean thickness of a circular zone centered at the corneal apex of 1 mm radius (zone I) and the mean thicknesses of several concentric rings also centered at the apex of width 1 mm (zones II to VI, respectively). METHODS: A total of 136 consecutive healthy eyes were examined. Two multivariate linear regression models were constructed, 1 for each tonometry system. In both models, the predictive variables were: keratometric power of the flattest and steepest axes, flattest axis (as one of the categories 0 to 30, 30 to 60, 60 to 90, 90 to 120, 120 to 150, and 150 to 180 degrees), CCT, mean overall corneal thickness (determined using the Pentacam), and mean thicknesses of corneal zones I, II, III, IV, V, and VI (determined using the Pentacam). RESULTS: The multivariate regression analysis (adjusted R=0.11; P=0.04) revealed that GAT was influenced by CCT [B=0.042; 95% confidence interval (CI), 0.002-0.085] and the mean thicknesses of zones I (B=0.996; 95% CI, 0.105-1.729), II (B=-1.688; 95% CI, -3.171 to -0.204), and III (B=0.718; 95% CI, 0.028-1.407), whereas DCT was solely affected by the mean thickness of zone II (B=-0.372; 95% CI, -0.728 to -0.016) (adjusted R2=0.13; P=0.02). CONCLUSIONS: Although DCT is only affected by the mean thickness of zone II, GAT is influenced by CCT and the mean thickness of zones I, II, and III.

Comparing corneal variables in healthy subjects and patients with primary open-angle glaucoma.

PURPOSE: This study was designed to identify possible differences between healthy subjects and patients with primary open-angle glaucoma (POAG) in keratometry, central corneal thickness, overall corneal thickness, mean thickness of a circular zone centered at the corneal apex of 1-mm radius (zone I), and mean thickness of several concentric rings also centered at the apex of 1-mm width (zones II to VI, respectively). METHODS: These variables were recorded in 126 healthy subjects and 130 patients with POAG. Corneal thicknesses and the power of the flattest and steepest axes were compared between the two populations using a t-test and the position of the flattest axis using a Mann-Whitney U test. A binary logistic regression procedure was used to determine the diagnostic capacity of the corneal variables using the area under the receiver operator characteristic curve (AUC) to select the best regression equation. RESULTS: Significant differences between subjects and patients were detected in mean corneal thickness and in mean thicknesses of zones I to VI. The logistic regression model included as predictors the mean corneal thickness and the mean thicknesses of zones IV and VI; for this model, the AUC was 0.711, sensitivity was 67.7%, and specificity was 65.5%. CONCLUSIONS: Healthy subjects and glaucoma patients differ significantly in terms of mean overall corneal thickness and thicknesses of the corneal zones I to VI defined here. The variables mean corneal thickness and mean thicknesses of zones IV and VI are able to discriminate between subjects with or without glaucoma.