Correction factors for Goldmann Tonometry.

PURPOSE: The purpose of the study was to evaluate the in vitro accuracy of correction factors in decreasing the error in the intraocular pressure (IOP) measurements obtained using the Goldmann Applanation Tonometer (GAT). METHODS: Nineteen donor corneas, from individuals aged between 57 and 99 years (mean 75.7 years, standard deviation±11.4 years) were subjected to posterior pressure simulating in vivo true IOP (IOPT) using an inflation test rig. Central corneal thickness and corneal curvature were measured. The posterior pressure was set at 25 different pressure levels between 5 and 45 mm Hg and IOP was measured using the GAT. Five different correction equations were applied to the IOP measurements obtained using the GAT to determine corrected IOP. The multiparameter correction equations applied were derived by Elsheikh, Ehlers, Chihara, Shimmyo et al, and Orssengo and Pye. The differences between IOPT and the IOP measured using the GAT were recorded as uncorrected errors, whereas the differences between IOPT and each of the corrected IOP were the tonometry errors after correction. RESULTS: The mean and standard deviation of error in tonometry before correction was +2.25±0.62 mm Hg. The mean errors in tonometry after correction using the Elsheikh and Chihara equations were +0.78±0.62 and +1.08±0.61 mm Hg, respectively. The mean errors in tonometry for the Ehlers, Shimmyo et al, and Orssengo and Pye equations were negative, indicating an overcorrection; the values were -0.75±2.28, -1.27±1.85, and -0.77±1.83 mm Hg, respectively. CONCLUSIONS: The Elsheikh and the Chihara et al's equations considerably decreased error in IOP measurements obtained by the GAT when compared with IOPT and were more consistent than other correction equations. The 2 equations may be of clinical utility in obtaining estimates of IOPT.

Predicting the necessity of LASIK enhancement after cataract surgery in patients with multifocal IOL implantation.

PURPOSE: To investigate if the parameters measured routinely prior to cataract surgery with multifocal intraocular lens (IOL) implantation can predict the necessity of additional laser in situ keratomileusis (LASIK) to improve visual outcome. METHODS: Records of patients undergoing cataract surgery between January 2008 and December 2009 were reviewed. Individuals satisfied with visual outcome of cataract surgery and not satisfied were grouped (group 1 and 2, respectively). Preoperative data of refractive error, axial length, corneal astigmatism, intraocular pressure, and postoperative uncorrected visual acuity were recorded. Data was available for 62 patients (104 eyes), of which LASIK enhancement was deemed necessary in 21 eyes (20%; group 2). The receiver operator characteristic curves were used to discriminate between the groups and linear regression analysis was performed to predict the postoperative visual outcome. RESULTS: The astigmatism measured preoperatively using manifest refraction had an accuracy of 64% in discriminating between the groups. Age, spherical component of refraction, axial length, corneal astigmatism, and intraocular pressure were very close to chance prediction 59%, 57%, 56%, 51%, and 51%, respectively. The postoperative uncorrected visual acuity had an accuracy of 79% in discriminating the groups. Individuals with uncorrected visual acuity worse than 20/40 after cataract surgery were most likely to undergo LASIK enhancement; however, approximately 20% of group 2 underwent LASIK enhancement despite having visual acuity of 20/30 or better. When combined, preoperative visual acuity accounted for just 7% of variance in postoperative uncorrected visual acuity. CONCLUSION: Requirement of LASIK enhancement after cataract surgery with multifocal IOL implant is complex in nature, and parameters routinely measured before surgery cannot successfully identify the group requiring LASIK enhancement or predict postoperative uncorrected visual acuity.

Multiparameter correction equation for Goldmann applanation tonometry.

PURPOSE: To develop a correction factor to improve the accuracy of intraocular pressure (IOP) measurements made by the Goldmann applanation tonometer (GAT), which considers the combined effects of variations in central corneal thickness (CCT), central anterior curvature (R), age, and the IOP level itself. METHODS: Nonlinear numerical simulations based on the finite element method were used to represent corneal behavior under the effect of IOP and external tonometric pressure. The simulations considered various biomechanical corneal properties including the cornea's nonuniform thickness, elliptical topography, weak stromal interlamellar cohesion, low epithelial and endothelial stiffness, and hyperelastic and hysteretic material behavior. The simulations were used to model the GAT procedure on corneas to obtain a correction equation based on the values of CCT, R, age, and IOP measured using GAT (IOPG). The efficiency of the equation in reducing the effects of corneal parameters on IOPG measurements was also assessed using an independent clinical database. RESULTS: The individual effects of variations in CCT, R, and age were estimated at 1.66 mm Hg/100 µ of CCT, 0.89 mm Hg/1 mm of R, and 0.12 mm Hg/decade of age. The correction equation reduced the association between clinical IOP measurements and corneal parameters with r2 reducing from 11.8 to 0.02%. CONCLUSIONS: The GAT correction factor can consider the combined effect of variations in corneal thickness, curvature, age, and IOP. The factor could significantly reduce the reliance of IOPG measurements on corneal stiffness parameters.

Measuring accurate IOPs: Does correction factor help or hurt?

PURPOSE: To evaluate if using the Ehlers correction factor on the intraocular pressure (IOP) measured using the Goldmann applanation tonometer (GAT) improves its agreement with the PASCAL dynamic contour tonometer (DCT). PATIENTS AND METHODS: A total of 120 eyes of 120 individuals were examined. Participants underwent IOP measurement with both the DCT and the GAT and central corneal thickness measurement. The Ehlers correction factor was applied on the GAT IOP measurements to calculate Ehlers-corrected GAT IOP. The agreement between the DCT and GAT, and DCT and Ehlers-corrected GAT IOP was analyzed. The analyses were repeated by stratifying the data by race. RESULTS: The mean IOP of the GAT, DCT, and the Ehlers-corrected GAT was 15.30, 16.78, and 14.68 mmHg, respectively. The agreement as assessed by Bland-Altman plot for the GAT with the DCT and DCT and Ehlers-corrected GAT IOP was +4.1 to -6.9 and +4.15 to -8.25 mmHg, respectively. The results were similar even when stratifying the data by race. CONCLUSION: Using Ehlers correction factor to account for the effect of corneal parameters on the IOP measured by the GAT worsens the agreement with the DCT. This effect remains even when stratifying the data by race.

Comparison of shape-based analysis of retinal nerve fiber layer data obtained From OCT and GDx-VCC.

PURPOSE: To directly compare in 1 population: (1) the performance of Optical Coherence Tomograph (OCT) and GDx-Variable Corneal Compensator (VCC) when using Wavelet-Fourier Analysis (WFA) and Fast-Fourier Analysis (FFA), (2) the performance of these shape-based and standard metrics, and (3) the shape of the retinal nerve fiber layer (RNFL) temporal, superior, nasal, inferior, temporal (TSNIT) curves obtained by the 2 different devices. METHODS: RNFL estimates were obtained from 136 eyes of 136 individuals (73 healthy and 63 mild glaucoma). WFA and FFA with and without asymmetry measures were performed on the TSNIT RNFL estimates to identify glaucoma from healthy eyes. Performance of WFA, FFA, and the standard metrics of OCT (Inferior Average) and GDX-VCC (Nerve Fiber Indicator) was evaluated by calculating receiver operating characteristic area. Measurements were obtained at a custom radius (33 to 41 pixels) for GDx-VCC to match the OCT radius (1.73 mm). RESULTS: WFA and FFA shape analysis significantly improved performance of both OCT (0.937) and GDx-VCC (0.913) compared with Inferior Average and Nerve Fiber Indicator (0.852 and 0.833, respectively). With either shape-based or standard metrics, OCT performance was slightly, but not significantly, better than GDx-VCC performance. Comparison of RNFL curves revealed that the GDx-VCC curves were more jagged and the peaks shifted more nasally when compared with the OCT RNFL curves. CONCLUSIONS: Performance of both OCT and GDx-VCC devices are improved by shape-based analysis methods. Classification performance was greater when using WFA for the OCT, and greater with FFA for the GDx-VCC. Significant differences between the machines exist in the measured TSNIT thicknesses, possibly because of GDx-VCC's measurements being affected by polarization magnitude varying with angle.

Atypical retardation pattern: can performance of classification be improved?

PURPOSE: (1) To evaluate and compare the classification performance of Wavelet-Fourier analysis (WFA), Fast-Fourier analysis (FFA), and the standard GDx-variable corneal compensator (VCC) output in identifying glaucomatous eyes from a mixed group of healthy and glaucomatous eyes with atypical retardation pattern (ARP). (2) To investigate if classification performance improves when only the superior and inferior quadrants are used for WFA and FFA. (3) To evaluate the classification performance as a function of severity of ARP. METHODS: Retinal nerve fiber layer (RNFL) estimates were obtained from 445 eyes of 240 individuals. On the basis of typical scan score (TSS), 348 eyes had typical retardation pattern (TRP) and 97 had ARP (78% TRP and 22% ARP). The classification performance of WFA and FFA classifiers was tested using three different ways: (1) Classifiers were trained on the TRP data, and tested on ARP data. (2) Classifiers were trained on TRP and 90% ARP data using 10-fold cross validation technique and tested on ARP data (10%). (3) Classifiers were trained and tested using the ARP data using 10-fold cross validation technique. Sensitivity, specificity, and Receiver Operating Characteristic Curve (ROC) areas were calculated. The classification performance was also assessed for the standard parameters of GDx-VCC. RESULTS: Of the standard GDx-VCC parameters, the nerve fiber indicator (NFI) had the highest ROC area (0.80). Of the shape-based analyses, WFA and FFA of the complete temporal, superior, nasal, inferior, and temporal curve had the highest ROC area (0.85 and 0.82, respectively). The difference in the ROC areas did not reach the statistically significant level (p = 0.07). On eyes with severe ARP (TSS < 60) all metrics performed similarly, but in case of moderate ARP (TSS 60 to 79), the ROC area of WFA and FFA were both greater than that of NFI (the difference was 9% and 7%, respectively). CONCLUSION: Although the WFA and FFA classification performance was greater than NFI as assessed by ROC area the difference was not statistically significant.

Relationships between central corneal thickness and optic disc topography in eyes with glaucoma, suspicion of glaucoma, or ocular hypertension.

PURPOSE: To identify relationships between central corneal thickness (CCT) and optic disc topography, as determined by scanning laser ophthalmoscopy (SLO), for patients seen in a specialist glaucoma service. METHODS: 272 eyes of 144 patients with primary open angle glaucoma (POAG; n = 71), normal tension glaucoma (NTG; n = 50), ocular hypertension (OH; n = 48) and those considered to be suspicious for glaucoma (GS; n = 103) underwent ultrasonic pachymetry and optic disc topography by SLO. Correlations between CCT and SLO parameter values were identified. A Bonferroni correction for multiple comparisons was performed and a p value of <0.0042 was considered significant. RESULTS: Mean CCT values were 533 mum (POAG), 530 mum (NTG), 550 mum (GS), and 565 mum (OH). As a group the GS and OH eyes had significantly thicker CCT values than eyes with POAG. In addition, the NTG eyes had significantly thinner CCT values than GS and OH eyes. Overall multiple SLO parameters correlated with CCT even after accounting for co-variance with age, refraction and inclusion of both eyes. Sub-group analysis indicated that 'optic disc rim area' positively correlated with CCT (r = 0.378) and 'cup to disc area ratio' negatively correlated with CCT (r = -0.370) in the POAG group. In the GS group the parameter 'area below reference' (a measure of cup volume) and 'mean cup depth' had negative correlations with CCT (r = -0.297 and -0.323) indicating that eyes with thinner than average corneal thickness measurements had larger and deeper cups. CONCLUSION: Thinner corneas appear to be associated with larger and deeper optic disc cups in the eyes of patients seen in a specialist glaucoma service.

Application of shape-based analysis methods to OCT retinal nerve fiber layer data in glaucoma.

PURPOSE: (1) To evaluate the performance of shape-based analysis [wavelet-Fourier analysis (WFA) and fast Fourier analysis (FFA)] applied to retinal nerve fiber layer (RNFL) thickness values obtained from the optical coherence tomograph (OCT) to discriminate healthy and glaucomatous eyes. (2) To compare the performance of the shape-based metrics to that of the standard OCT output measures (Inferior Average and Average Thickness). METHODS: RNFL values were obtained from 152 eyes of 152 individuals (83 healthy and 69 "mild"-stage perimetric glaucoma). WFA and FFA were performed on the RNFL values and linear discriminant functions for both were obtained using Fisher linear discriminant analysis. Performance was evaluated by calculating sensitivity, specificity, and area under the receiver operating characteristic (ROC) curve (ROC area). RESULTS: The ROC area of the shape-based methods [0.94 (WFA) and 0.88 (FFA)] was greater than that of OCT metrics [0.81 (Inferior Average) and 0.74 (Average Thickness)]. Specifically, WFAs performance was significantly better than both the FFA (P=0.009) and the Inferior Average (P=0.001). Inferior average performed significantly better than Average Thickness (P=0.006). CONCLUSIONS: The ability to differentiate glaucomatous from healthy eyes using stratus OCT measurements is improved by using these analysis methods that emphasize the shape of the RNFL thickness pattern.

Predicting visual field loss in ocular hypertensive patients using wavelet-fourier analysis of GDx scanning laser polarimetry.

PURPOSE: To predict which ocular hypertensive (OHT) patients later develop a visual field loss by applying shape-based analysis (wavelet-Fourier analysis, WFA) to retinal nerve fiber layer (RNFL) thickness estimates. METHODS: Visual field information and scanning laser polarimetry (SLP) RNFL estimates were obtained from both eyes of 122 patients (73 glaucoma and 49 OHT) and 102 healthy individuals. WFA was applied to RNFL temporal, superior nasal, inferior, and temporal (TSNIT) curves (28 points) of the glaucoma and healthy eyes to obtain a classifier. Without modification, the classifier was then applied to the OHT eyes (16 OHTconverters and 33 OHTnonconverters). The visual fields of the OHT patients (6-month follow-up for a mean period of 4 years) were analyzed using the Advanced Glaucoma Intervention Study (AGIS) criteria to identify eyes which developed subsequent field loss in this period of time (OHT converters) and those that did not (OHT nonconverters). For the OHT converters, the classifier was applied to scans from each of three points in time before the initial visual field damage. For the OHT nonconverters, the last available scan was used. The accuracy of the WFA metric in predicting conversion of OHT eyes was assessed by calculating area under the receiver operating characteristic (ROC) curve (area under the ROC curve, AUC), sensitivity at 80% specificity, and likelihood ratio. RESULTS: The performance (AUC) of WFA in predicting conversion of the OHT eyes from scans taken just before visual field loss was 0.83 with a sensitivity (SD)/specificity (SD) of 0.76 +/- 0.11/0.80 +/- 0.07 and likelihood ratio (+LR +/- SD) of 3.8 +/- 1.4. Performance for scans obtained 6 months before the first signs of visual field defects was 0.77 (AUC), 0.71 +/- 0.11/0.80 +/- 0.07 (sensitivity/specificity), and 3.5 +/- 1.4 (+LR +/- SD). Performance was 0.73 (AUC), 0.59 +/- 0.12/0.8 +/- 0.07 (sensitivity/specificity), and 3.0 +/- 0.12 (+LR +/- SD) using the earliest available RNFL estimates. CONCLUSION: The WFA method of temporal, superior nasal, inferior, and temporal shape analysis offers a means of predicting progression in OHT patients before visual field loss.

Evaluation of some factors affecting the agreement between the Proview Eye Pressure Monitor and the Goldmann applanation tonometer measurements.

BACKGROUND: Our aim was to examine whether training level and ocular factors could account for part of the difference in intraocular pressure (IOP) measured using the Goldmann applanation tonometer (GAT) and Proview Eye Pressure Monitor (PPT). METHODS: One hundred and nineteen individuals (238 eyes) were enrolled in the study. The mean age was 35.8 years (range 21 to 79). All study participants obtained IOP measurements using the PPT after hearing instructions on how to perform PPT. Glaucoma patients obtained additional IOP measurements using PPT after viewing an instructional video and after 30 days of home use. IOP was also measured using the GAT at each experimental session. RESULTS: The difference in IOP measured by the GAT and the PPT was 0.55 +/- 3.38 mmHg, 0.17 +/- 3.79 mmHg and -1.30 +/- 3.79 mmHg for myopic, emmetropic and hypermetropic groups, respectively, which were statistically significant (ANCOVA; p = 0.014). The difference in IOP between GAT and PPT was not significantly different for measurements obtained after verbal instructions, instructional video or after 30 days of home use (Repeated-ANCOVA; p = 0.30). The overall agreement between the GAT and the PPT was poor. Intra-class correlation coefficient was 0.575, and the 95% confidence interval (CI) of agreement was -6.93 to +6.73 mmHg. CONCLUSION: There was a small systematic difference in IOP measured by the GAT and PPT when comparing the different refraction groups; however, this level of difference between the groups is unlikely to be of clinical significance. The level of training in using the PPT did not influence its measurements. The limits of agreement between the PPT and the GAT were wide and long-term use of PPT did not improve its agreement.

Effect of Proview self-tonometry on pharmaceutical compliance.

AIM: To evaluate changes in patient compliance with medical treatment while using the Proview Eye Pressure Monitor. METHODS: A crossover study design was used to compare the compliance of patients with established use of topical medication to lower intraocular pressure in the treatment of primary open-angle glaucoma. Thirty-two patients currently managed with latanoprost 0.005%, brimonidine 0.15%, travoprost 0.004% or bimatoprost 0.03% as monotherapy or in combination were randomly assigned to two study groups. Group 1 was instructed in the use of Proview Eye Pressure Monitor three times daily for 30 days as an adjunct to the glaucoma regimen. Group 2 was observed with no change in the patients' treatment regimen during this phase of study. A crossover occurred at 30 days. Compliance was monitored by assigning new bottles of topical medication during each phase of study. Bottles were weighed with a Mettler balance (Mettler Toledo Co.) at the initiation and completion of each phase. The changes in bottle-weight determined the amount of medication consumed by each patient for each phase of the study. The weights were analysed to estimate changes in compliance. RESULTS: A paired samples Student t-test compared the consumed bottle weights with and without Proview Eye Pressure Monitor usage. No statistical significance or trend was identifiable (p = 0.98). Use of the Proview Eye Pressure Monitor did not significantly change compliance with adjunct eye drop medication. CONCLUSION: The use of the Proview Eye Pressure Monitor use did not improve but appeared to hinder compliance with glaucoma treatment in this study.

Evaluation of tonometric correction factors.

PURPOSE: To investigate the efficacy of currently available correction factors in correcting intraocular pressure (IOP) measurements for the errors induced by the normal variations in corneal structural characteristics. MATERIALS AND METHODS: Central corneal thickness (CCT) and corneal radius of curvature were measured on 324 individuals (175 normal: group 1 and 149 had either open angle glaucoma or ocular hypertension: group 2). IOP was measured in all normal subjects with the Goldmann applanation tonometer and the highest recorded IOP was obtained from patient charts for subjects with either open angle glaucoma or ocular hypertension. Regression analysis was performed on IOP, CCT, and corneal radius of curvature. The corrected IOP was also calculated using the models proposed by Ehlers and Orssengo and Pye. Linear regression analysis was used to calculate the residual association between corneal parameters and corrected IOP. RESULTS: There was a significant positive correlation between IOP measured using Goldmann applanation tonometer and the CCT in both groups. There was no significant correlation between corneal radius of curvature and IOP in either group. There was a significant negative correlation in both the groups between CCT and corrected IOP calculated using the models of Ehlers and Orssengo and Pye. This indicates that the Ehlers and Orssengo and Pye models may significantly overestimate the effect of CCT on IOP measurement. CONCLUSION: The effect of CCT and IOP as observed in the present study and by other studies in literature is less than predicted by both the Ehlers formula and the Orssengo and Pye model. Correcting IOP for the effect of CCT using these models could be erroneous and lead to overcorrection of IOP, thus resulting in erroneously low corrected IOP eyes with thicker cornea and erroneously high corrected IOP in eyes with thinner cornea.

Analysis of GDx-VCC polarimetry data by Wavelet-Fourier analysis across glaucoma stages.

PURPOSE: The purpose of this study was to apply shape-based analysis techniques of retinal nerve fiber layer (RNFL) thickness to GDx-VCC (variable corneal and lens compensator; Laser Diagnostic Technologies, Inc., San Diego, CA) polarimetry data and to evaluate the techniques' ability to detect glaucoma in its earliest stages. Wavelet-based (wavelet-Fourier analysis [WFA]), Fourier-based (fast Fourier analysis [FFA]), and several previous variations of shape-based analysis were considered, as well as the standard metric nerve fiber indicator (NFI), and all were compared as a function of disease stage. METHODS: GDx-VCC scans of one eye of each of 67 patients with glaucoma and each of 67 healthy age-matched subjects provided RNFL thickness estimates at a fixed distance from the optic disc. Severity of disease was graded according to the Glaucoma Staging System and also by mean deviation (MD) from standard automated perimetry. WFA, FFA, and NFI procedures were performed including the following variations: use of signed or unsigned phase, inclusion of interocular or intraocular asymmetry of analysis parameters, and combination of features by principle components analysis or Wilks lambda. Independent samples (k-fold variation) were used for training and testing. Sensitivity, specificity, and receiver operating characteristic (ROC) area were obtained. RESULTS: Classification performance of WFA (ROC = 0.978) was significantly better than FFA (ROC = 0.938) and NFI (ROC = 0.900). This difference was largest for the earliest stages of glaucoma. Shape-based analysis methods performed better than NFI overall. Adding between-eye asymmetry measures helped FFA but not WFA. CONCLUSIONS: Shape-based analysis, and WFA in particular, makes an important improvement in detecting earliest glaucoma with polarimetry.

Predicting subsequent visual field loss in glaucomatous subjects with disc hemorrhage using retinal nerve fiber layer polarimetry.

PURPOSE: To predict progression of visual field loss after an episode of disc hemorrhage in glaucoma patients on the basis of retinal nerve fiber layer (RNFL) GDx polarimetry measurements analyzed by wavelet-Fourier analysis (WFA). METHODS: Retrospective GDx data from 16 subjects (10 progressors and 6 non-progressors based on visual fields) obtained near the time of disc hemorrhage were analyzed to predict which patients would have visual field progression. Polarimetry scans throughout a follow-up period (31 months average) were also analyzed to compare field progression to RNFL thickness change after the hemorrhage. Mean RNFL thickness inferred from the polarimetry data at sixteen 22.5 degrees sectors at distances of 1.6, 1.7, and 1.8 disc diameters were used. Data were analyzed by applying to appropriate regions of disc hemorrhage patients a structural analysis (WFA) we had developed previously. A linear discriminant function (Fischer) was produced and a leave-one-out method using separate training and test data was used to assure validity of the results. RESULTS: Patients who subsequently progressed were successfully predicted with moderate success (sensitivity / specificity was 0.77 / 0.88 with ROC area = 0.858). A separate analysis comparing pre- and post-hemorrhage RNFL sector thickness revealed clear evidence of RNFL thinning at the inferior and superior sectors before progression of visual field. The thinning of RNFL thickness was not restricted to regions corresponding to the location of the hemorrhage. CONCLUSION: Wavelet-Fourier analysis can differentiate progressors from non-progressors with moderate accuracy. Comparison to a prior study of this same cohort emphasizes that relatively small regions must be considered (as opposed to larger quadrants) to see these significant changes in RNFL.

Repeatability and effects of sequential measurements with POBF tonograph.

AIMS: (1) To investigate the effect of performing sequential measurements using a Pulsatile Ocular Blood Flow (POBF) Tonograph on POBF and intraocular pressure (IOP) measurements; (2) to determine any effect on IOP caused by performing POBF measurements on the fellow eye; and (3) to assess repeatability of measurements obtained using the POBF Tonograph. METHODS: Forty-six normal adult subjects (13 men, 33 women; age range, 18 to 55 years) took part in the present study. Twenty-one subjects underwent sequential measurements with a POBF Tonograph on five different occasions with five different time intervals (1, 2, 5, 10, and 15 minutes) between recordings. Twenty subjects underwent a baseline measurement on one eye with the POBF Tonograph, followed by repeat measurements in both eyes after 15 minutes. An additional 25 subjects underwent sequential measurements on the same eye with an interval of 15 minutes between readings. The intraclass correlation coefficient and Bland-Altman plots were used to assess repeatability of the instrument for all 46 subjects for the 15-minute time interval. RESULTS: In all the groups, repeated measurement resulted in a lower recorded IOP. A multivariate analysis of variance indicated that there was a significant decrease in IOP when measurements were repeated within 15 minutes (p=0.024), but there was no significant change in the POBF values (p=0.76). The recorded IOP was significantly lower when sequential measurements were performed with an interval of 1 minute (p <0.01) compared with longer time intervals. The decrease in IOP between baseline and the repeat measurement was significantly greater in the 1-minute interval group compared with the 15-minute interval group (p <0.005). There was no significant difference in POBF values between the baseline and repeat measurements in any group. There was no significant change in IOP induced by a single measurement on the fellow eye (p >0.05). The intraclass correlation coefficient value indicated good agreement between the values of the baseline and repeat measurements using the POBF Tonograph but had wide upper and lower limits of agreement. CONCLUSION: If IOP measurements have to be repeated using the POBF Tonograph, they are best done after an interval of at least 2 minutes and preferably after 15 minutes. Use of the POBF Tonograph had no significant immediate effect on the IOP or POBF values obtained from a fellow eye.

The effect of central corneal thickness on estimates of the anterior chamber depth.

BACKGROUND: The slitlamp can be used to estimate the anterior chamber depth (ACD). The length of a slit object is increased until the corneal and iris/lens images appear to just touch. Multiplying the just-touching-slit-length (JTSL) by a conversion factor gives an estimate of the ACD as measured by ultrasonography. The purpose of this study was to determine if central corneal thickness (CCT) affects the accuracy of this technique. METHODS: The ACD of 50 subjects was measured by A-scan ultrasonography and estimated by the slitlamp technique. CCT was measured by ultrasonic pachometry. The refractive error was determined subjectively. RESULTS: The average ultrasonographic ACD for all subjects was 3.32 +/- 0.65 mm. The average JTSL was 2.46 +/- 0.38 mm. The conversion ratio between the ultrasonographic ACD and the average JTSL was 1.35. The predicted ACD using the regression equation of JTSL on the ultrasound anterior chamber depth (USACD) was 3.32 +/- 0.54 mm. The corresponding value using the regression equation of JTSL and CCT on USACD was exactly the same, that is, 3.32 +/- 0.54 mm. CONCLUSION: Incorporation of CCT into a regression equation does not improve the accuracy of the Smith technique.

Vertical cup-to-disc ratio: agreement between direct ophthalmoscopic estimation, fundus biomicroscopic estimation, and scanning laser ophthalmoscopic measurement.

PURPOSE: Ophthalmoscopic estimation of the vertical cup-to-disc ratio (VCDR) of the optic nerve head is important in the management of patients with glaucoma or who are glaucoma suspects. The purpose of this study was to compare the accuracy of estimation of VCDR obtained with direct ophthalmoscopy with that obtained with fundus biomicroscopy through undilated pupils. Measurements of VCDR obtained with the Heidelberg Retina Tomograph (HRT), a confocal scanning laser ophthalmoscope, were used as a standard. METHODS: Thirty young, healthy adults had their optic discs photographed and then imaged and analyzed with the HRT. Due to HRT software limitations, the VCDR could not be calculated automatically, and so a validated VCDR measurement was derived. This was used as the standard against which ophthalmoscopic estimations were compared. Two months later, the subjects had their VCDRs estimated using direct ophthalmoscopy and fundus biomicroscopy performed in random order after varying time intervals. Agreement between ophthalmoscopic VCDR estimation and HRT-derived VCDR measurement was assessed by means of bias plots and the weighted kappa statistic. RESULTS: Compared with the HRT-derived VCDR measurement, both ophthalmoscopic techniques tended to underestimate VCDR. The bias with direct ophthalmoscopy was statistically significant. There were also wide variations in VCDR estimation with direct ophthalmoscopy and fundus biomicroscopy, which were more pronounced with direct ophthalmoscopy. The weighted kappa statistic indicated moderate agreement between fundus biomicroscopy and HRT-derived VCDR measurement. There was poor to fair agreement between direct ophthalmoscopy and HRT-derived VCDR measurement. The level of disagreement was independent of HRT-derived VCDR, optic nerve head size and pupil size for both direct ophthalmoscopy and fundus biomicroscopy. CONCLUSIONS: Fundus biomicroscopy enables more accurate, less variable VCDR estimation than direct ophthalmoscopy. The clinician should record which method they used to examine the optic nerve head so that subsequent clinical decisions are not influenced by apparent VCDR changes.