Aachal Kotecha

The relative effects of corneal thickness and age on Goldmann applanation tonometry and dynamic contour tonometry.

Aims: To establish the effects of central corneal thickness (CCT) on intraocular pressure (IOP) measured with a prototype Pascal dynamic contour tonometer (DCT), to evaluate the effect of CCT and age on the agreement between IOP measured with the Pascal DCT and Goldmann applanation tonometer (GAT), and to compare the interobserver and intraobserver variation of the DCT with the GAT. Methods: GAT and DCT IOP measurements were made on 130 eyes of 130 patients and agreement was assessed by means of Bland-Altman plots. The effect of CCT and age on GAT/DCT IOP differences was assessed by linear regression analysis. Interobserver and intraobserver variations for GAT and DCT were assessed in 100 eyes of 100 patients. Results: The mean difference (95% limits of agreement) between GAT and DCT was −0.7 (−6.3 to 4.9) mm Hg. GAT/DCT IOP differences increased with thicker CCT (slope 0.017 mm Hg/μm, 95% CI 0.004 to 0.03, r2 = 0.05, p = 0.01), and with greater age, slope 0.05 mm Hg/year (95% CI 0.012 to 0.084, r2 = 0.05, p = 0.01). The intraobserver variability of GAT and DCT was 1.7 mm Hg and 3.2 mm Hg, respectively. The interobserver variability was (mean difference (95% limits of agreement)) 0.4 (−3.5 to 4.2) mm Hg for GAT and 0.2 (−4.9 to 5.3) mm Hg for DCT. Conclusions: GAT is significantly more affected than DCT by both CCT and subject age. The effect of age suggests an age related corneal biomechanical change that may induce measurement error additional to that of CCT. The prototype DCT has greater measurement variability than the GAT.

Intraocular Pressure Measurement Precision with the Goldmann Applanation, Dynamic Contour, and Ocular Response Analyzer Tonometers

OBJECTIVE To examine the repeatability and reproducibility of intraocular pressure (IOP) measurements obtained with the Goldmann applanation tonometer (GAT), the Pascal dynamic contour tonometer (DCT; Swiss Microtechnology AG, Port, Switzerland), and the Reichert Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Buffalo, NY). A secondary objective was to assess agreement between the devices. DESIGN Evaluation of technology. PARTICIPANTS One hundred participants; a mixture of glaucoma suspects, patients, and control volunteers. METHODS The IOP measurements were obtained with the GAT, DCT, and ORA by 2 of 3 experienced clinicians. Keratometry (CC) measurements were made using the IOLMaster (Carl Zeiss Meditech, AG, Jena, Germany). Three ORA corneal compensated IOP (IOPcc) measurements were obtained before the instillation of anesthesia, after which 2 GAT IOP and 3 DCT IOP measurements were obtained in a randomized order. Central corneal thickness (CCT) was measured using an ultrasound pachymeter. The average ORA corneal response factor (CRF) and the average DCT ocular pulse amplitude (OPA) were determined. Intraobserver variability was calculated by the repeatability coefficient. Interobserver variability (measurement reproducibility) and device agreement were calculated by Bland-Altman analysis (mean difference [bias] and 95% limits of agreement [LoA]). The effect of corneal characteristics (CC, CCT, and CRF) on the IOP measurement differences between tonometers also was determined. MAIN OUTCOME MEASURES Repeatability and reproducibility of the GAT, DCT, and ORA IOPcc and agreement between tonometers. RESULTS The repeatability coefficients for GAT, DCT, and ORA were 2.2, 2.3, and 4.3 mmHg, respectively. The intraobserver variability of ORA measurements was shown to be significantly associated with OPA and to a lesser degree with the quality of ORA waveform scans. The interobserver bias (95% LoA) was -0.8 (+/-3.9) mmHg for GAT -0.2 (+/-2.8) mmHg for DCT and -0.3 (+/-3.9) mmHg for ORA IOPcc. On average, GAT under-read both DCT and ORA IOP measurements by approximately 2 mmHg. The IOP measurement differences were better predicted by CRF than CCT. CONCLUSIONS The DCT shows excellent measurement precision, displaying the best repeatability and reproducibility of the 3 tonometers. Corneal stiffness, as defined using CRF, was associated significantly with agreement between devices. The IOP measurements with each device are not interchangeable. FINANCIAL DISCLOSURE(S) Proprietary or commercial disclosure may be found after the references.

Anterior chamber flare after trabeculectomy and after phacoemulsification

AIMS To evaluate and compare prospectively the anterior chamber inflammatory response after phacoemulsification cataract surgery and after trabeculectomy with peripheral iridectomy. METHODS Anterior chamber inflammation was measured using the Kowa FM-500 laser flare meter in 131 patients undergoing trabeculectomy and 148 patients undergoing phacoemulsification cataract extraction with intraocular lens implantation. Flare was measured before surgery and on each postoperative visit up to 12 months. RESULTS Before surgery there was no significant difference in flare readings between the two groups. Following trabeculectomy flare returned to baseline levels 4 weeks after surgery, while following phacoemulsification cataract extraction it remained significantly higher at week 6 (p<0.006) and month 3 (p<0.05). CONCLUSIONS Anterior chamber inflammation is more prolonged after cataract surgery than after trabeculectomy. This may have implications for the timing of trabeculectomy in relation to cataract surgery.

Cornea biomechanical characteristics and their correlates with refractive error in Singaporean children.

PURPOSE To determine corneal biomechanical parameters measured with the Reichert Ocular Response Analyser (ORA) in Singaporean children, and to assess their possible correlations with refractive error and biometry. METHODS This was a cross-sectional study of 271 subjects from the Singapore Cohort Study of Risk Factors for Myopia (SCORM). Corneal hysteresis (CH), corneal resistance factor (CRF), central corneal thickness (CCT), and cornea-compensated intraocular pressure (IOPcc) were measured with the ORA. Spherical equivalent refraction was assessed with an autokeratorefractometer and axial length by contact ultrasound A-scan biometry. Height, weight, and blood pressure were measured. RESULTS The mean age of the study population was 13.97 +/- 0.89 years, the distribution of the sexes was almost equal (138 boys, 50.9%), and most were Chinese (186 subjects, 68.6%). The mean (+/-SD) CH and CRF were 11.78 +/- 1.55 (range, 6.93-16.53) and 11.81 +/- 1.71 (range, 7.83-16.83) mm Hg. CH and CRF did not vary significantly with age (P = 0.24; 0.61), sex (P = 0.21; 0.08), or race (P = 0.23; 0.36). CH and CRF did not vary with myopia status (P = 0.79; 0.83) or axial length (Pearson correlation coefficient [r] = -0.11 and -0.05, P = 0.08 and 0.40). Multivariate analyses were performed with CH, CRF, or CCT as the dependent variable and age, sex, race, weight, IOPcc, CCT, SE refraction, and corneal curvature as covariates. CH was significantly associated with IOP (regression coefficients [beta] = -0.22 [95% confidence interval = -0.27 to -0.17]), CCT (beta = 0.03 [0.02-0.03]) and corneal curvature (beta = -1.13 [-2.08 to -0.19]). CRF was significantly associated with IOP, CCT, and corneal curvature (beta = 0.08 [0.02-0.14]; 0.03 [0.03-0.04], and -1.39 [-2.54 to -0.23], respectively). The only factor that was predictive of decreased CCT was Malay or Indian race (P = 0.03 and <0.001), compared with Chinese. CONCLUSIONS The CH and CRF values in our study on Singaporean children are slightly higher than in adult studies. CH and CRF are not associated with refractive error or axial length. Flatter corneas are associated with lower CH and CRF readings.

The Functional Consequences of Glaucoma for Eye–Hand Coordination

PURPOSE To examine whether patients with glaucoma exhibit differences in visually guided reaching-and-grasping (prehension) behavior compared with normally sighted control subjects. METHODS Sixteen patients with glaucoma and 16 control subjects with no ocular disease participated. Participants were required to reach out and precision grasp one of two cylindrical objects placed on a table top in front of them in laboratory conditions in three viewing conditions (binocular, right eye alone, left eye alone). Lightweight reflective markers were placed on the subjects preferred hand for recording its movement in three-dimensional space. Three motion capture units recorded the motion of these markers as the subjects reached out and precision grasped household objects. Various indices of prehension planning, execution, and control were quantified. Visual fields (VF) were measured using standard automated perimetry generating monocular mean deviation (MD) scores. Binocular VF sensitivity was estimated by using the integrated visual field (IVF). Stereoacuity was measured with the Frisby stereoacuity test. Significant differences in prehension movement between patients and control subjects in each viewing condition were investigated, and associations between prehension kinematics and VF sensitivity were examined. RESULTS The patients and control subjects were of a similar age (median [range]: patient group, 72.2 years [62.5-86.9]; control group, 69.0 years [64.3-78.3]). The patient group had asymmetrical disease and relatively minor binocular overlapping defects (better eye MD, -5.7 dB [-16.7 to +0.45 dB]; worse eye MD, -11.8 dB [-29.3 to -1.5 dB]; IVF score, 3 [0-36]). They exhibited slightly poorer stereoacuity levels than did the control subjects (patient group, 55 sec arc [40-110]; control group, 40 sec arc [20-80; Mann-Whitney U test, P < 0.05]). They also showed statistically significant delays in average movement onset (MO: approximately 100 ms delay, Mann-Whitney U test P < 0.0001) and overall movement time (OMD: approximately 140 ms delay; Mann-Whitney U test P < 0.05), suggesting impairments in initial movement planning and control. Deficits were exhibited in the reaching component, with data suggesting that glaucomatous patients made more tentative movements when reaching for the object. These deficits correlated with both increasing severity of VF defect and impaired stereoacuity. There were no differences in grasping characteristics between patients and control subjects in this sample. CONCLUSIONS This study provides evidence that patients with glaucoma exhibit deficits in eye-hand coordination compared with the age-matched normally sighted control. Further study is needed to assess the specific effect of field loss location on prehension kinematics.

Evaluation of Goldmann Applanation Tonometry Using a Nonlinear Finite Element Ocular Model

Goldmann applanation tonometry (GAT) is the internationally accepted standard for intra-ocular pressure (IOP) measurement, which is important for the diagnosis of glaucoma. The technique does not consider the effect of the natural variation in the corneal thickness, curvature and material properties. As these parameters affect the structural resistance of the cornea, their variation is expected to lead to inaccuracies in IOP determination. Numerical Analysis based on the finite element method has been used to simulate the loading conditions experienced in GAT and hence assess the effect of variation in corneal parameters on GAT IOP measurements. The analysis is highly nonlinear and considers the hyper-elastic J-shaped stress–strain properties of corneal tissue observed in laboratory tests. The results reveal a clear association between both the corneal thickness and material properties, and the measured IOP. Corneal curvature has a considerably lower effect. Similar trends have been found from analysis of clinical data involving 532 patients referred to the Glaucoma Unit at Moorfields Hospital, and from earlier mathematical analyses. Nonlinear modelling is shown to trace the behaviour of the cornea under both IOP and tonometric pressure, and to be able to provide additional, and potentially useful, information on the distribution of stress, strain, contact pressure and gap closure.

The Relationship between Diurnal Variations in Intraocular Pressure Measurements and Central Corneal Thickness and Corneal Hysteresis

PURPOSE To examine the relationship between office-hour changes in IOP, measured with the Goldmann applanation tonometer (GAT) and dynamic contour tonometer (DCT), and the corneal characteristics central corneal thickness (CCT) and corneal hysteresis (CH). METHODS Sixty-two eyes of 62 untreated normal subjects and patients with untreated glaucoma had IOP measurements performed with the GAT (mm Hg) and DCT (mm Hg) over an 8-hour period at 2-hour intervals beginning at 9 AM. CCT (micrometers) was measured using a noncontact optical low-coherence reflectometry (OLCR) pachymeter, and CH (mm Hg) was measured with an ocular response analyzer (ORA). The associations between IOP measurements and corneal characteristics for each patient over the measurement period were assessed by using multilevel modeling. RESULTS GAT and DCT IOP and CCT changed significantly during office hours (ANOVA; GAT: F = 19.9, P < 0.001; DCT: F = 4.6, P = 0.001; CCT: F = 16.4; P < 0.001). No significant changes were observed in CH (ANOVA; F = 1.8, P = 0.13). Multilevel modeling analysis of the interrelationships between CCT, CH, and age on IOP measurements revealed that both CCT and CH changes were significantly associated with GAT IOP changes (GAT IOP/CCT slope, 0.04 mm Hg/microm; 95% confidence intervals [CIs], 0.02-0.06; GAT IOP/CH slope, 0.20 mm Hg/mm Hg; 95% CI, 0.01-0.39). CCT, but not CH, changes were significantly associated with DCT IOP changes (DCT IOP/CCT slope, 0.03 mm Hg/microm; 95% CI, 0.00-0.05). However, although the association between CCT and GAT IOP was relatively uniform between subjects, association between CCT and DCT IOP showed greater intersubject variability. Age had no effect on the diurnal variation of IOP measured with either device. CONCLUSIONS Measured IOP and corneal characteristics covary during office hours. Changes in CCT and CH are associated with changes in GAT IOP and, less consistently, with DCT IOP. The data suggest that variations in corneal characteristics explain a small proportion of the change in IOP measurements made with the GAT during office hours.

Optic disc changes following trabeculectomy: longitudinal and localisation of change.

AIMS To determine whether there were any changes in the optic disc at 2 years after trabeculectomy. To determine the factors that most influenced change and whether change was localised to any region of the optic disc. METHODS 95 patients undergoing routine trabeculectomy as part of the ongoing Moorfields/MRC 5-fluorouracil trial were recruited into the study. Eyes were imaged preoperatively (4 (SD 3) weeks) with the Heidelberg retina tomograph (HRT, Heidelberg Engineering), and at 3 months (SD 2 weeks), 1 year (SD 1 month), and 2 years (SD 1 month) after surgery. Parameters investigated for change were rim area, rim volume, and maximum cup depth. The predefined segment analysis available on the HRT analysis software was used to determine segmental change. RESULTS The images of 70 patients were analysed. Intraocular pressure reduced from 22.25 (SD 3.76) mm Hg, at the time of preoperative imaging to 15.27 (SD 4.96) mm Hg at 3 months, 14.38 (SD 3.89) mm Hg at 1 year, and 13.80 (SD 3.54) mm Hg at 2 years after trabeculectomy. An increase in rim area and rim volume was present at all time points after surgery, but was only found to be statistically significant at 2 years after surgery. Maximum depth of cup reduced by month 3 and month 12, but showed a slight increase at 2 years after surgery, although this was still lower than the preoperative measure. Segmental analysis found a significant change in rim volume in the nasal, inferonasal, superonasal, and superotemporal regions at 2 years after surgery. No significant regional localisation for change was found at any other time point or in any other parameter investigated. CONCLUSIONS Reversal of disc cupping is present at 2 years after trabeculectomy. The factor most influencing change is reduction of intraocular pressure. Segmental analysis showed that change in rim volume was greatest in the nasal, inferonasal, superonasal and superotemporal regions at 2 years.

Corneal biomechanical characteristics in patients with diabetes mellitus

PURPOSE: To compare the corneal biomechanical properties in eyes of patients with diabetes mellitus and in those of subjects without diabetes mellitus. SETTING: Hospital eye clinic. DESIGN: Comparative case series. METHODS: Corneal hysteresis (CH) and corneal response factor (CRF) were measured in diabetic patients and nondiabetic subjects using the Ocular Response Analyzer. Central corneal thickness (CCT) and intraocular pressure (IOP) were also measured. Differences in corneal biomechanical properties were determined using a 1‐way analysis of variance. Interassociations between ocular and diabetic parameters were also evaluated. RESULTS: Sixty‐one eyes of 61 diabetic patients and 123 eyes of 123 nondiabetic subjects were evaluated. The CRF was significantly greater in the eyes of diabetic patients (mean difference, 1.09 mm Hg; 95% confidence interval [CI], 0.49‐1.69; P = .001). There were no significant differences in CCT or CH between groups (CH: mean difference, 0.38 mm Hg; 95% CI, −0.21 to 0.97 mm Hg; P = .21; CCT: mean difference, 0.13 μm; 95% CI, −10.6 to 10.8 μm; P =.98). Corneal hysteresis and CRF were weakly correlated with blood glucose concentration (slopes: CH: 0.28; 95% CI, 0.03–0.50; P = .03; CRF: 0.27; 95% CI 0.02‐0.49; P = .04). In a multiple regression analysis, the effects of blood glucose concentration were reduced and age and CCT became significant predictors of CH and CRF. CONCLUSIONS: The eyes in diabetic patients displayed altered corneal biomechanics that may be related to blood glucose concentration. Further studies are required to establish the effects of long‐term poor glucose control on corneal biomechanical properties and how this might affect the diabetic patient’s response to refractive surgery procedures. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned.

Glaucomatous Progression in Series of Stereoscopic Photographs and Heidelberg Retina Tomograph Images

OBJECTIVE To compare optic disc changes using automated analysis of Heidelberg retina tomograph (HRT) images with assessments, by glaucoma specialists, of change in stereoscopic photographs. METHODS Baseline and follow-up stereophotographs and corresponding HRT I series of 91 eyes from 56 patients were selected. The selection criteria were sufficiently long, good-quality HRT series (7 visits in > or =70 months of follow-up) and follow-up photographs contemporaneous with the final HRT image. Topographic change analysis (TCA), statistic image mapping (SIM), and linear regression of rim area (RALR) across time were applied to HRT series. Glaucomatous change determined from stereophotographs by expert observers was used as the reference standard. RESULTS Expert observers identified 33 eyes (36%) as exhibiting glaucomatous change. Altering HRT progression criteria such that 36% of eyes progressed according to each method resulted in concordance between HRT methods and stereophotograph assessment of 54% for TCA, 65% for SIM, and 67% for RALR (Cohen kappa = 0.05, 0.23, and 0.30, respectively). Receiver operating characteristic curves of the HRT analyses revealed poor precision of HRT analyses to predict stereophotograph-assessed change: areas under the curve were 0.61 for TCA, 0.62 for SIM, and 0.66 for RALR. CONCLUSIONS Statistical methods for detecting structural changes in HRT images exhibit only moderate agreement with each other and have poor agreement with expert-assessed change in optic disc stereophotographs.