Central corneal thickness measurements in patients with normal tension glaucoma, primary open angle glaucoma, pseudoexfoliation glaucoma, or ocular hypertension

Central Corneal Thickness in Individuals With Intellectual Disabilities

PURPOSE

To assess central corneal thickness (CCT) values in individuals with intellectual disabilities (ID).

METHODS

The study group was made up of 25 participants with ID (mean age, 36.9 +/- 8.7 years). The control group was made up of 25 healthy individuals (mean age, 37.1 +/- 10.1 years) with normal intellectual capacity and without any systemic or intraocular pathology. CCT value was measured by ultrasound pachymetry. Ten consecutive measurements were made at the center of the cornea of each eye.

RESULTS

In the ID group, mean CCT value was 554.0 +/- 39.7 microm in the right eye and 556.8 +/- 38.7 microm in the left eye. In the control group, mean CCT value was 535.7 +/- 24.2 microm in the right eye and 536.5 +/- 24.8 microm in the left eye. CCT value in the ID group was significantly greater than in the control group for both right (P < 0.05) and left eyes (P < 0.02).

CONCLUSIONS

CCT should be kept in mind during measurements of intraocular pressure (IOP) in individuals with ID because their CCTs may be greater than those in the general population.

Relationship between short wavelength perimetry and central corneal thickness values in ocular hypertensive subjects

PURPOSE

To evaluate the results of short wavelength perimetry (SWAP) of ocular hypertensive (OHT) patients and correlate these findings with central corneal thickness (CCT) measurements.

METHODS

Thirty-seven OHT patients with a mean age of 50.2+/-8.2 (SD) years and 30 control subjects with a mean age of 50.3+/-8.5 (SD) years were included in this study. A questionnaire was applied to patients to evaluate the demographic risk factors that may predict glaucoma development. After a detailed ophthalmologic examination, achromatic and short wavelength perimetries and ultrasonic pachymetry were performed and the results were compared between the two groups with Student t test and Mann-Whitney U test. A p value<0.05 is considered as statistically significant.

RESULTS

Mean CCT was higher in the OHT group (right eye; 558.13+/-28.39 microm and left eye; 558.94+/-27.30 microm) when compared with the control subjects (524.66+/-30.53 microm and 525.86+/-30.46 microm, respectively) (p<0.01). A significant positive correlation was found between CCT measurements and intraocular pressure (r=0.5, p<0.001). Four right eyes (10.8%) and five left eyes (13.5%) of OHT patients had defects in SWAP. OHT patients with SWAP abnormalities had significantly lower CCT measurements in right (527.25+/-17.34 microm) and left eye (528.80+/-13.60 microm) when compared with OHT patients without SWAP defects (561.87+/-27.29 microm and 563.65+/-25.92 microm, respectively) (p<0.05). Significant correlations were found between CCT and SWAP MD, PSD, and CPSD (p<0.05).

CONCLUSIONS

OHT patients with SWAP abnormalities had significantly lower CCT measurements than those without. CCT is considered as a risk factor for the development of glaucomatous damage in OHT patients.

Ultrasound and Partial Coherence Interferometry With Measurement of Central Corneal Thickness

PURPOSE

To compare noncontact pachymeter measurements with ultrasound pachymeter measurements and assess their reproducibility.

METHODS

Central corneal thickness was measured in 104 eyes of 56 patients with three laser interference pachymeters (OLCR [Haag Streit, Könitz, Switzerland], OCP [4optics AG, Lübeck, Germany], and ACMaster [Carl Zeiss Meditec, Jena, Germany]) and an ultrasound pachymeter (Tomey AL2000 [Tomey Corp, Nagoya, Japan]).

RESULTS

Compared to the ultrasound measurements, the mean difference for the laser interference pachymeter measurements were +8.8 microm (standard deviation [SD] 5.68) for the OLCR, -8.0 microm (SD 5.39) for the OCP, and -0.12 microm (SD 5.88) for the ACMaster. Reproducibility could only be estimated as not all of the devices allowed access to individual measurements. For all laser interference devices, reproducibility was estimated to be approximately 2 microm. Ultrasound measurements yielded a reproducibility of approximately 3.4 microm.

CONCLUSIONS

Although ultrasound pachymeter measurements differed significantly from OLCR and OCP measurements, agreement was considered good because the mean differences were <10 microm, and the results can be regarded as clinically interchangeable.

Comparison of rebound tonometry with Goldmann applanation tonometry and correlation with central corneal thickness

BACKGROUND/AIMS

Rebound tonometry (RT) is performed without anaesthesia with a hand held device. The primary aim was to compare RT with Goldmann applanation tonometry (GAT) and to correlate with central corneal thickness (CCT). The secondary aim was to prove tolerability and practicability of RT under "study conditions" and "routine practice conditions."

METHODS

In group 1 (52 eyes/28 patients), all measurements were taken by the same physician, in the same room and order: non-contact optical pachymetry, RT, slit lamp inspection, GAT. Patients were questioned about discomfort or pain. In group 2 (49 eyes/27 patients), tonometry was performed by three other physicians during routine examinations.

RESULTS

RT was well tolerated and safe. Intraocular pressure (IOP) ranged between 6 mm Hg and 48 mm Hg. No different trends were found between the groups. RT tended to give slightly higher readings: n = 101, mean difference 1.0 (SD 2.17) mm Hg; 84.1% of RT readings within plus or minus 3 mm Hg of GAT; 95% confidence interval in the Bland-Altman analysis -3.2 mm Hg to +5.2 mm Hg. Both RT and GAT showed a weak positive correlation with CCT (r2 0.028 and 0.025, respectively).

CONCLUSIONS

RT can be considered a reliable alternative for clinical screening and in cases where positioning of the head at the slit lamp is impossible or topical preparations are to be avoided.

Comparison of ICare Tonometer with Goldmann Applanation Tonometer in Glaucoma Patients

PURPOSE

To compare the intraocular pressure (IOP) readings taken with the new ICare tonometer and with the Goldmann applanation tonometer (GAT) and to evaluate the influence of central corneal thickness (CCT) on the IOP measurements.

PATIENTS AND METHODS

One eye of 178 consecutive patients with primary open-angle glaucoma underwent ultrasonic CCT measurement, followed by IOP evaluation with the GAT and with the ICare tonometer. The deviation of ICare readings from GAT values, corrected according to the Doughty and Zaman formula, was calculated and correlated to CCT by a linear regression model. The agreement between the 2 devices was assessed by use of the Bland-Altman method.

RESULTS

The average CCT was 552+/-39 mum. The mean IOP and the mean corrected IOP with GAT were 19.4+/-5.4 mm Hg, and 18.5+/-5.7 mm Hg, respectively. The mean ICare IOP reading was 18.4+/-5.2 mm Hg. The deviations of ICare readings from corrected GAT values were highly correlated with CCT values (r=0.63, P<0.01). Linear regression analysis showed that a CCT change of 10 mum resulted in an ICare reading deviation of 0.7 mm Hg. The Bland-Altman scatter-plot showed a reasonable agreement between the 2 tonometers.

CONCLUSIONS

The ICare tonometer can be useful in a routine clinical setting. The IOP readings are quite in accordance with those obtained by GAT. The measurements seemed to be influenced by CCT variations, and thus pachymetry should always be taken into consideration.

Relationship Between Central Corneal Thickness and Localized Retinal Nerve Fiber Layer Defect in Normal-tension Glaucoma

PURPOSE

To determine whether central corneal thickness (CCT) is related to the extent of localized retinal nerve fiber layer (RNFL) defect at the initial examination of normal-tension glaucoma (NTG) patients.

PATIENTS AND METHODS

Seventy-five eyes of 75 NTG patients showing localized RNFL defects on RNFL photographs and corresponding visual field defects at the initial visit to a glaucoma specialist were selected for this study. All participants completed refraction, Goldmann applanation tonometry, CCT measurement, stereoscopic disc photography, RNFL photography, and automated perimetry. Each patient's age, spherical equivalent, intraocular pressure, CCT, approximation of the RNFL defect to the fovea (angle alpha), circumferential width of the RNFL defects (angle beta), horizontal and vertical cup-to-disc ratios, and mean deviation of visual field were analyzed.

RESULTS

In univariate and multivariate analyses, lower CCT was significantly associated with increased horizontal and vertical cup-to-disc ratios, decreased angle alpha, and increased angle beta. For a decrease of 10 microm of CCT, horizontal and vertical cup-to-disc ratios increased by 0.020, angle alpha decreased by 1.58 degrees, and angle beta increased by 1.71 degrees, respectively.

CONCLUSION

CCT is a significant factor in predicting the extent of localized RNFL defect at the initial examination of NTG patients.

Central corneal thickness and normal tension glaucoma: A cross-sectional study

BACKGROUND

Recently published evidence has identified thinner central corneal thickness (CCT) as a strong predictive factor for the conversion from ocular hypertension (OHT) to primary open-angle glaucoma (POAG). The association between CCT and development of normal-tension glaucoma (NTG), however, is less clear. Accordingly, we designed this cross-sectional study to further explore the relationship between CCT and NTG.

PATIENTS AND METHODS

All patients with a clinical diagnosis of NTG and NTG suspect (NTGS) who were seen from September 2002 through May 2003 at the Albuquerque VA Medical Center eye clinic were identified retrospectively. After eligible subjects were categorized into no, mild, moderate, and advanced visual field loss groups, analysis of variance (ANOVA) and regression analyses were used to determine group differences for several IOP variables, several systemic variables, and CCT. Additional analyses were completed after eligible subjects were recategorized into thin, intermediate, and thick CCT groups.

RESULTS

Eighty-four eyes in 84 NTGS subjects and 56 eyes in 56 NTG subjects were studied. Mean CCT was significantly thicker in the no field loss group (NTGS) when compared with all 3 groups with glaucomatous visual field loss (NTG). In multivariate regression analysis, the association between CCT and the presence of NTG-related visual field loss was robust and independent. Conversely, no relationship was found between CCT and severity of NTG-related visual field loss.

CONCLUSIONS

In eyes characterized by statistically normal intraocular pressure (IOP) measurements as measured by Goldmann applanation tonometry, we found a significant relationship between CCT and the presence, but not severity, of glaucomatous visual field loss. A prospective study is required to further explore and confirm these relationships.

Intraocular Pressure Measurement-Comparison of Dynamic Contour Tonometry and Goldmann Applanation Tonometry

PURPOSE

The dynamic contour tonometer (DCT, Pascal tonometer, Swiss Microtechnology AG, Port, Switzerland) was recently introduced as a new method of intraocular pressure measurement, supposedly independent of corneal properties. In this study we analyzed the agreement and correlation of dynamic contour tonometry and Goldmann applanation tonometry (GAT) and investigated the influence of central corneal thickness (CCT) and corneal curvature. We also considered preferential patient groups for both methods.

METHODS

In a prospective study of 100 eyes without glaucoma, intraocular pressure was measured using dynamic contour tonometry and Goldmann applanation tonometry, followed by measurements of central corneal thickness and corneal curvature.

RESULTS

A clear correlation between dynamic contour tonometry and Goldmann applanation tonometry was found (r = 0.693; P < 0.001). Dynamic contour tonometry generally resulted in higher intraocular pressure measurements (median difference + 1.8 mm Hg, mean difference + 2.34 mm Hg). Unlike dynamic contour tonometry, Goldmann applanation tonometry was remarkably affected by central corneal thickness, but neither method was significantly influenced by corneal curvature. Bland-Altman graphs showed remarkable disagreement between dynamic contour tonometry and Goldmann applanation tonometry, which could be partially explained by the influence of central corneal thickness on Goldmann applanation tonometry. To obtain valid readings, dynamic contour tonometry required a more extensive selection of patients than Goldmann applanation tonometry.

CONCLUSIONS

Dynamic contour tonometry seems to be a reliable method for intraocular pressure measurement, which unlike Goldmann applanation tonometry is not influenced by central corneal thickness. In clinical practice, advantages from dynamic contour tonometry can be expected for cooperative patients, outpatients, and patients with sufficient bilateral ocular fixation, whereas Goldmann applanation tonometry measurements are more reliable in case of patients with inadequate cooperation, poor vision, or nystagmus.

Relationship between central corneal thickness and severity of glaucomatous visual field loss in a primary care population

BACKGROUND

Although the ability of central corneal thickness (CCT) to predict development of primary open-angle glaucoma has become increasingly well recognized, the ability of CCT to predict severity of glaucoma remains uncertain. This study was designed to expand the available knowledge about the relationship between CCT and glaucoma severity.

METHODS

Retrospective identification of all patients with a clinical diagnosis of either primary open angle glaucoma (POAG) or ocular hypertension who were seen from September 2002 through May 2003 at the Albuquerque VA Medical Center eye clinic was completed. Eligible subjects were segregated into no, mild, moderate, or advanced visual field loss groups based on Advanced Glaucoma Intervention Study (AGIS) visual field scoring criteria. Following statistical analyses comparing the visual field groups, the sample was divided into thin, intermediate, and thick CCT groups, and further analysis was performed.

RESULTS

Mean CCT was significantly higher in the no field loss group compared with all 3 groups with glaucomatous visual field loss. Mean CCT was not statistically different, however, between the mild, moderate, and advanced visual field loss groups. In linear regression analyses, no significant relationship was found between CCT and severity of visual field loss.

CONCLUSIONS

Although CCT was associated strongly with development of POAG-related visual field loss, CCT was not associated with severity of visual field loss in this study. These findings suggest that glaucoma patients with thinner corneas are just as likely to have advanced levels of field loss as glaucoma patients with thicker corneas. Prospective studies are needed to validate these findings.

Central Corneal Thickness Measurement in Clinical Practice

OBJECTIVE

To determine if a single measurement of central corneal thickness (CCT) is an adequate sample to aid in glaucoma risk assessment in clinical practice.

METHODS

Central corneal thickness was measured by ultrasound pachymetry (mean of 15 measurements for each eye) on two separate occasions at least one month apart (range, 33 to 610 days). Eyes with a history of prior incisional surgery or corneal pathology were excluded.

RESULTS

Ninety-eight eyes of 98 patients (43 male, 55 female) were enrolled. Mean age was 61.2 +/- 15.5 years. Mean inter-test period was 276 +/- 124 days. No significant difference in mean CCT was observed between the two visits (549 +/- 41 microm versus 548 +/- 42 microm, P = 0.4, two-tailed, paired t test). Measured CCT values differed by more than 20 microm in 20 eyes (20.4%), whereas CCT difference of at least 40 microm was seen in 5 eyes (5.1%). There was no correlation between the measured or absolute difference in CCT and IOP (r = -0.016, P > 0.43), inter-test time period (r = 0.072, P > 0.23), and glaucoma diagnosis.

CONCLUSION

Central corneal thickness measurements in this study differed by at least 20 microm in 20% of eyes. This has important implications for risk assessment, management, and follow-up of patients with glaucoma and related disorders. Factors affecting CCT measurement, such as examiner error or true alterations in corneal thickness, require continued investigation.

Central corneal thickness in primary open angle glaucoma, pseudoexfoliative glaucoma, ocular hypertension, and normal population

PURPOSE

This study was designed to determine the relationship between central corneal thickness (CCT) and intraocular pressure (IOP) measured by applanation tonometer in glaucomatous, ocular hypertensive, and normal eyes.

METHODS

A total of 125 subjects were included in the study. Twenty-six had primary open angle glaucoma (POAG), 25 had pseudoexfoliative glaucoma (PXG), 24 had ocular hypertension (OHT), and 50 of them were normal. IOP values were measured by Goldmann applanation tonometer whereas CCT values were measured by ultrasonic pachymeter.

RESULTS

CCT values in the OHT group (595.75+/-22.52 microm) were greater than the CCT values of the POAG group (539.92+/-21.50 microm), the PXG group (526.28+/-31.73 microm), and the normal group (533.96+/-29.25 microm) (p<0.05). Eight patients who were diagnosed with OHT showed IOP values of 21 mm Hg or lower with corrected IOP values according to CCT.

CONCLUSIONS

Increased CCT may lead to falsely high values of IOP measured with Goldmann applanation tonometer. In this study, when IOP values of the OHT group were redefined according to the formulae regarding the CCT, the authors noted that one third of them were normal. Determination of the CCT in OHT cases is crucial since it has great impact on IOP values, measured with applanation tonometer, which is the main parameter in the diagnosis and follow-up of glaucoma.

Central and paracentral corneal pachymetry in patients with normal tension glaucoma and ocular hypertension

PURPOSE

The difference in central corneal thickness among subgroups of glaucoma patients, as well as its influence on Goldmann applanation tonometry, has been well documented in several clinical trials. In the present study, possible similarities and differences between central corneal thickness and corneal thickness of paracentral quadrants in patients with normal tension glaucoma (NTG) and ocular hypertension (OHT) were investigated.

METHODS

Central and paracentral corneal thickness was measured by optical slit scan pachymetry (Orbscan II). Fourteen patients (28 eyes) with NTG and 11 patients (22 eyes) with OHT were included in this study. t-Test was performed for statistical analysis. To evaluate overall corneal topography, the mean and SD values of the differences between the central corneal thickness and each peripheral quadrant were analysed.

RESULTS

The following data was obtained (microm): (central, upper, temporal, nasal, inferior paracentral quadrant): OHT group 617-695-663-687-660. NTG group 568-629-593-612-616. Corneal thickness of all four paracentral quadrants differed significantly between the OHT and NTG groups. There was a more heterogeneous intraindividual pattern of overall corneal topography in the OHT group, and a more heterogeneous pattern of corneal topography among the individuals of the NTG group (interindividual heterogeneity).

CONCLUSIONS

A comparison of central corneal thickness and paracentral corneal thickness revealed clinically relevant differences between the OHT and NTG groups. The presented data underlines the importance of correlating the site of applanation with the corresponding corneal thickness, especially in OHT patients. It further substantiates the necessity to obtain individual pachymetric data for each NTG patient.

Central Corneal Mechanical Sensitivity in Pseudoexfoliation Syndrome

PURPOSE

Corneal involvement and disturbances of the tear film have been reported in pseudoexfoliation syndrome (PEX). Tear film deficiencies are correlated with changes in corneal sensitivity. The present study aims at evaluating central corneal mechanical sensitivity (CCMS) in PEX.

METHODS

Patients with unilateral or bilateral PEX findings constituted the study group (SG). Age- and gender-matched patients without PEX in either eye constituted the control group (CG). Patients with conditions affecting corneal sensitivity were excluded. CCMS and central corneal thickness (CCT) were measured (using a Cochet-Bonnet esthesiometer and an ultrasonic pachymeter, respectively). Schirmer (ST) and break-up time (BUT) tests were also performed.

RESULTS

The SG included 40 patients (24 male, 60%). The CG included 38 patients (25 male, 65.78%). ST scores were significantly lower in the SG compared with the CG (11.45 +/- 2.52 mm and 14.27 +/- 1.18 mm, respectively, P = 0.04). BUT scores were also significantly lower in the SG compared with the CG (7.64 +/- 2.37 mm and 12.43 +/- 3.14 mm, respectively, P = 0.03). CCMS was significantly (P = 0.02) reduced in the SG compared with CG (4.54 +/- 0.23 cm and 5.73 +/- 0.44 cm, respectively). Differences in CCT between the SG and the CG were not statistically significant.

CONCLUSIONS

The decrease in CCMS in PEX eyes may be related to decreased BUT and ST scores in PEX, as previously described. However, a direct involvement of sensory nerves may also participate. The reduction in CCMS may have implications for the clinical management of PEX patients.

Central corneal thickness in high myopia

PURPOSE

To examine if central corneal thickness (CCT) is different in emmetropia and high myopia.

METHODS

57 emmetropic subjects (0 to + 1.5 D) and 48 high myopes (all more than - 6 D in spherical equivalent refraction) were studied. CCT was measured by a Haag-Streit Optical Low-Coherence Reflectometry (OLCR) pachymeter, a recently developed high precision pachymeter with a standard deviation (SD) for repeated measurements of 1 microm.

RESULTS

Mean CCT for the emmetropic group was 538.6 microm (SD = 32.1), and for the myopic group 527.7 microm (SD = 35.0). Neither the mean CCT nor the variance from the two groups showed a statistically significant different (p > 0.05).

CONCLUSION

CCT is not systematically altered in myopia. The process by which the myopia progresses does not to a measurable degree influence the central cornea.

[The risk of glaucoma and corneal thickness]

Intraocular pressure is still the most important risk factor for the development of glaucomatous optic nerve damage. There is growing evidence that corneal thickness is a risk factor for the development of glaucoma. This might be caused by the effect of corneal thickness on intraocular pressure (IOP) measurements. Goldmann applanation tonometry measurements are correlated with corneal thickness. Thick corneas lead to false high readings whereas thin corneas lead to false low readings. If corneal thickness as a risk factor for glaucoma is only related to the dependency of IOP measurements on corneal thickness or is related to possible different biomechanical tissue properties in glaucomatous eyes is not known. However, a large proportion of the corneal thickness effect seems to be related to the effect on IOP readings by applanation tonometry. Neglecting corneal thickness can lead to false measurements of IOP with consequent misdiagnosis and false treatment. Therefore, measurements of corneal thickness should be performed in glaucoma patients and suspects.

The relationship between central corneal thickness-adjusted intraocular pressure and glaucomatous visual-field loss

BACKGROUND

Although measurement of central corneal thickness (CCT) is increasingly becoming an important component of glaucoma risk analysis, significant controversy exists regarding the benefit of calculating a corrected intraocular pressure (IOP) value from measured IOP and CCT data.

METHODS

Three hundred forty-four male subjects were identified from a VA eye clinic with one of the following clinical diagnoses: ocular hypertension (OHT), primary open-angle glaucoma (POAG), normal tension glaucoma (NTG), and normal tension glaucoma suspect (NTGS). Using one eye per subject, multivariate logistic regression and correlational analyses were performed to determine relationships between glaucomatous visual-field loss and several glaucoma risk factors, including adjusted IOP values.

RESULTS

Multivariate logistic regression analysis did not identify CCT-adjusted IOP values as independent risk factors for development of either NTG or POAG-related glaucomatous visual-field loss. CCT, however, was found to be strongly associated with both NTG and POAG-related visual-field loss. Correlational analysis revealed a weak correlation between Ehlers-adjusted pre-treatment IOP and severity of POAG-related visual-field loss, but no other adjusted IOP values significantly correlated with severity of visual-field loss in either POAG or NTG.

CONCLUSIONS

Our results suggest that adjusted IOP, as calculated using current algorithms, is not useful within glaucoma risk analysis, since adjusted IOP was unable to predict either presence or severity of glaucomatous visual-field loss in this study. CCT, conversely, was found to be a robust and independent predictor of glaucomatous visual-field loss. These findings, while supporting routine CCT measurements for all glaucoma suspects, do not support routine clinical computation of adjusted IOP values using current algorithms.

Relationship between corneal thickness and optic disc damage in glaucoma

BACKGROUND

Central corneal thickness (CCT) has been found to be a powerful predictor for the development of glaucoma in patients with ocular hypertension. This study aimed to determine whether CCT was also related to glaucoma severity.

METHODS

The vertical cup/disc ratio (VCDR) was used as a single objective measure of glaucoma severity. Vertical cup diameters and vertical disc diameters were measured using a graticule attached to a 60 D Volk lens, in consecutive patients presenting to a single ophthalmologist. Measurements were corrected for magnification. The difference between corrected VCDR and 95% probability of VCDR normality was compared to CCT, which was measured with an ultrasonic pachymeter.

RESULTS

During the study period 554 eyes from 281 patients with glaucoma were reviewed. The mean CCT was 519.0 microm, with the mean corrected VCDR being 0.71. A univariate linear regression model revealed that an increase of 10 microm in CCT resulted in a 0.009 decrease in the difference between true VCDR and the predicted normal VCDR. The R2 value for the regression was 0.04 (P < 0.0001).

CONCLUSION

There is a highly statistically significant, albeit small, negative correlation between corneal thickness and glaucoma severity. This study supports the notion that CCT should be measured in the assessment of patients with glaucoma. Progressive thinning or presence of a thin cornea may have pathogenic or prognostic roles in some types of glaucoma.

Glaucoma studies from 1996 to 1999 in peer-reviewed journals

BACKGROUND

Studies on glaucoma therapy were analyzed with regard to study design, diagnostic parameters and success criteria.

MATERIALS AND METHODS

In 11 frequently read peer-reviewed periodicals, 101 studies published between 1996 and 1999 were chosen according to specific criteria. Design parameters, diagnostic measures and success criteria were investigated.

RESULTS

Thirty-seven studies were retrospective and 64 prospective. Twenty-five studies were multicenter studies. Thirty-seven studies dealt with drug therapy, 12 with laser therapy, and 52 with surgical therapy. The study duration in 51% of the studies (n=52) was up to 1 year, in 28% (n=28) from 1 to 2 years, in 17% (n=17) over 2 years, and in 5% (n=5) in excess of 4 years. Four studies gave insufficient data. Forty-one studies recruited less than 50 patients and 57 studies recruited less than 75 patients. Thirty-four studies included more than 100 patients and 11 studies more than 250 patients. Sixty-one studies included race as a parameter. All 101 studies measured intraocular pressure (IOP). Forty-five studies explicitly described the method of tonometry. Thirteen studies measured a diurnal IOP. Forty-one studies examined the visual field, of which 30 named the method of perimetry. Only 28 studies examined the optic disc morphology. They all employed ophthalmoscopy, and two additionally employed optic disc photography. Sixty-three studies explicitly defined success criteria, establishing 95 different definitions: 74 definitions (78%) used a specified value for IOP as a success criterion. Out of the 70 definitions that gave an absolute value for IOP, 12 definitions (17%) used IOP values between 14 and 16 mmHg as the upper limit, 4 definitions (6%) used IOP values between 17 and 19 mmHg, and 54 definitions (77%) used IOP values between 20 and 22 mmHg. Out of the 26 definitions that specified a percentage IOP reduction, it was judged to be a success when the IOP was lowered by 20% or more from the starting value in ten definitions (38%), by 25% or more in three definitions (12%), and by 30% or more in 13 definitions (50%).

CONCLUSIONS

Examples of prestigious studies show the necessity of observation periods of several years and demonstrate the need for a high number of participants, necessitating the cooperation of many study centers. Beside a more precise characterization of the patient collectives, clinical studies should always specify the measurement method clearly. Investigating a diurnal IOP profile to recognize changes in IOP and IOP peaks, as well as the routine determination of central corneal thickness, would be desirable. New diagnostic techniques for improved assessment of the functional and morphologic damage will gain in relevance in the future. The lack of a common definition of success reveals the complexity of the disease. However, an IOP reduction based on the degree of damage and ascertaining the target pressure seems sensible.

Dynamic corneal imaging

PURPOSE

To determine the clinical practicability of in vivo dynamic corneal imaging (DCI) to assess the individual elastic properties of normal human eyes, eyes with abnormal findings, and eyes after refractive surgery.

SETTING

University Eye Clinic, Paracelsus Private Medical University, Salzburg, Austria.

METHODS

The DCI method uses sagittal, stepwise, central indentation of the cornea with electronically controlled microprecision motors and sequential registration of videotopography images. The indentation steps are preselected and range from 50 to 800 mum. The computerized analysis of the videotopography images captured during the process uses Zernike polynomials to establish a newly defined flexing curve for normal eyes and eyes with abnormal findings.

RESULTS

Dynamic corneal imaging was done in 187 eyes of 103 patients who had clinically healthy corneas, distinct keratoconus, or previous refractive surgery. The method rapidly evaluated artificially and reversibly induced changes in corneal topography in a clinical setting using a modified Placido disk-based computer-assisted videokeratography system with a small cone. In early analysis, the flexing curve showed a significant correlation with the applied indentation depth. Factors influencing the shape of the curve were central corneal thickness, intraocular pressure, and patient age. The DCI method also allowed easy examination of keratoconic corneas and corneas after refractive surgery.

CONCLUSIONS

Dynamic corneal imaging induced a reproducible and reversible change in corneal topography corresponding to the different indentation depths. The results indicate that several clinical parameters are correlated with corneal elastic behavior in vivo and that the technology could increase the predictability of refractive corneal surgery and help in the early diagnosis of corneal diseases and with newly developed therapies.

Central corneal thickness and its relationship to intraocular pressure in children

OBJECTIVE

Central corneal thickness (CCT) has emerged as an important predictive factor for the development of glaucomatous damage. Although a child's cornea reaches adult thickness by the age of 3, little has been reported about normal CCT measurements in eyes of children. We wished to test the hypotheses that (1) there is a correlation between increasing CCT and increasing intraocular pressure (IOP) in the eyes of children, (2) the CCT in the eyes of children with ocular hypertension is greater than that in eyes of normal pediatric subjects, and (3) the average CCT of black children is less than that of white children.

METHODS

We performed a retrospective chart review of 69 pediatric patients seen in the office of one ophthalmologist between January 1997 and December 2001 in whom CCT was measured by ultrasound pachymetry. We categorized the subjects into diagnostic groups of controls, glaucoma, glaucoma suspects, and ocular hypertension based on IOP, cup-to-disc ratio, and visual field parameters.

RESULTS

The average CCT for the control patients was 555+/-37 microm; for patients with glaucoma, it was 563+/-33 microm; for glaucoma suspects, 559+/-39 microm; and for those with ocular hypertension, 595+/-39 microm. The difference between the control and ocular hypertensive groups was significant (P<0.02). The difference in CCT between the black control subjects and the white ones (537+/-36 microm vs. 564+/-28 microm) was not statistically significant (P = 0.125).

CONCLUSIONS

Central corneal thickness is greater in children with ocular hypertension than in control subjects or those with glaucoma, and the values for CCT in these children correlate closely with values reported for adults. Corneas of black children may be thinner than the corneas of white children.

Central corneal thickness of Caucasians, Chinese, Hispanics, Filipinos, African Americans, and Japanese in a glaucoma clinic

OBJECTIVE

To characterize the central corneal thickness (CCT) of Asian (Chinese, Japanese, and Filipino), Caucasian, Hispanic, and African American patients in a multiethnic glaucoma practice.

DESIGN

Retrospective study (chart review).

PARTICIPANTS

Glaucomatous (n = 600) and nonglaucomatous (n = 201) eyes of 801 patients examined in a San Francisco glaucoma clinic from June 2002 to April 2004 who met inclusion criteria were included in the study. The 6 racial (ethnic) groups represented in the study were Caucasian (n = 186, 23.2%), Chinese (n = 157, 19.6%), Japanese (n = 121, 15.1%), Hispanic (n = 116, 14.5%), Filipino (n = 114, 14.2%), and African American (n = 107, 13.4%).

METHODS

Central corneal thickness was measured by means of ultrasound pachymetry in Asian (Chinese, Japanese, and Filipino), Caucasian, Hispanic, and African American participants with glaucomatous and normal eyes. The relationship between CCT and race was investigated using multivariate regression analyses, controlling for confounders. One eye of each of 801 participants was included for analysis.

MAIN OUTCOME MEASURES

Correlation of mean CCT with race, glaucoma diagnosis, age, spherical equivalent, gender, and history of ocular surgery.

RESULTS

The mean CCT of all participants was 542.9 mum. Central corneal thicknesses of Chinese (555.6 microm), Caucasian (550.4 microm), Filipino (550.6 microm), and Hispanic (548.1 microm) participants did not significantly differ. The CCT of Japanese participants (531.7 microm) was significantly less than that of Caucasians, Chinese, Filipinos, and Hispanics (all, P< or =0.001) and greater than that of African Americans (P = 0.03). African Americans had a CCT (521.0.0 microm) less than that of all races (P< or =0.05). Glaucoma suspects and patients with normal tension glaucoma (NTG), primary open-angle glaucoma (POAG), pseudoexfoliation glaucoma (PEX), and chronic angle-closure glaucoma (CACG) had corneas significantly thinner than those of normal participants (P< or =0.004), whereas ocular hypertensives had significantly thicker corneas (P<0.0001). Among all participants, decreasing values of CCT were significantly related to older age (P<0.01). Less negative or more positive refractive errors, gender, and history of ocular surgery were not associated with changes in CCT (P = 0.38, P = 0.50, and P = 0.97, respectively).

CONCLUSIONS

Studies examining individual Asian subpopulations in isolation suggest that differences in CCT may exist among different Asian groups. The results of this study indicate that CCT does, in fact, vary among Asian subpopulations; Japanese have thinner corneas than Chinese and Filipinos. Caucasians, Chinese, Hispanics, and Filipinos have comparable CCT measurements, whereas the corneas of African Americans are significantly thinner. Additionally, older individuals; glaucoma suspects; and participants with NTG, POAG, PEX, and CACG have thinner corneas. Ocular hypertensives, however, have thicker corneas.

Corneal thickness measurement by confocal microscopy, ultrasound, and scanning slit methods

PURPOSE

To measure corneal thickness by using a calibrated confocal microscope and to compare this measurement to thickness determined by ultrasonic and noncontact scanning slit pachymetry.

DESIGN

Comparison of corneal thickness measured by using four instruments in normal subjects.

METHODS

Thickness measured by a clinical confocal microscope (Tandem Scanning) was calibrated from measurements of polymethylmethacrylate contact lenses with known thickness. Corneal thickness was measured in one eye of 24 normal subjects by using this instrument, two ultrasonic pachymeters (DHG-1000 and Sonogage), and a noncontact optical scanning slit pachymeter (Orbscan II).

RESULTS

Mean corneal thickness measured by confocal microscopy was 516 +/- 30 microm (+/-SD). This was less than the mean thickness measured by both ultrasonic pachymeters, 554 +/- 28 microm by the DGH, and 555 +/- 28 microm by the Sonogage (P <.001). Thickness measured by the Orbscan II pachymeter was 540 +/- 35 microm (P <.001, compared with either confocal or ultrasound) after applying an "acoustic factor" of 0.92, a default correction of the software.

CONCLUSION

Corneal thickness measured by calibrated confocal microscopy is approximately 39 microm (7.0%) less than thickness measured by two commonly used ultrasonic pachymeters and approximately 24 microm (4.4%) less than thickness measured by the corrected Orbscan II pachymeter. These differences are important for planning and measuring the effects of refractive and other surgical procedures. The precision of confocal microscopy is limited by corneal motion in an anterior-posterior direction. The difference between instruments suggests that verification of clinical ultrasonic pachymeters should be revisited.

Effect of corneal drying on central corneal thickness

PURPOSE

To determine the amount of corneal thinning induced by corneal drying, and to discuss its possible consequences on applanation pressure.

PATIENTS AND METHODS

Two sets of central corneal thickness measurement from 100 eyes of 100 patients were taken by ultrasonic pachymeter at time settings of 0, 15, 30, 45, and 60 seconds. During the first set (group 1), patients were asked to blink voluntarily between the measurements. During the second set (group 2), the same patients were prevented from blinking during the measurements.

RESULTS

Mean central corneal thickness at 0 second in groups 1 and 2 were 547.0 +/- 37.4 microm, and 546.7 +/- 37.6 microm, respectively. Corneal thickness did not change significantly in group 1, but did significantly decrease in group 2. Rate of thinning increased linearly during the first 30 seconds, and then leveled around 0.3 microm/s up to 60 seconds. Mean corneal thickness decreased 3.0% during the 60 second drying. The decrease in thickness was significant when each time point in group 2 was compared with the preceding one (P < 0.01). Neither age nor 0 second corneal thickness in group 2 showed a significant correlation with the amount of corneal thinning at 60 seconds.

CONCLUSION

Corneal thickness may significantly decrease during 1 minute of drying. Decreased thickness may result in underestimation of applanated pressure as well as central corneal thickness.

Corneal thickness and endothelial cell density measured by non-contact specular microscopy and pachymetry in Rhesus macaques (Macaca mulatta) with laser-induced ocular hypertension

PURPOSE

Sustained increase in intraocular pressure (IOP) in humans results in a loss of corneal endothelial cells and an increase of corneal thickness. The effects of chronically elevated IOP on the corneal endothelium of monkeys with laser-induced ocular hypertension, a commonly used animal model of human glaucoma have not been documented. This study examined the central corneal thickness (CCT), the corneal endothelial cell density (ECD), and the corneal endothelial cell size (ACS) in Rhesus monkeys with experimental ocular hypertension. Materials and methods. Ten male monkeys with argon laser-induced ocular hypertension in one eye for an average duration of 2.4+/-0.7 years, were sedated with ketamine hydrochloride, and the CCT, ECD, and ACS measured at the center of the cornea of both eyes with a Topcon SP-2000P non-contact specular microscope (Topcon America Corporation((R)), Paramus, NJ, USA). CCT was also measured using a DHG-500 Pachette ultrasonic pachymeter (DHG Technology Inc., Exton, PA, USA). Mean and standard deviation (S.D.) of CCT, ECD and ACS for each eye was calculated and statistically compared.Results. Mean CCT in the hypertensive and normal eyes measured by specular microscopy was 0.477+/-0.023mm and 0.468+/-0.020 mm, respectively. Mean ECD in the hypertensive and normal eyes was 2601.7+/-631.8 and 3990.2+/-402.9 cells mm(-2), respectively. The mean size of the endothelial cells was 252.4+/-23.9 micro m(2) in the normal eye and 408.7+/-115.0 microm m(2) in the hypertensive eye. No significant difference in the measurement of CCT was observed between the specular microscope and the pachymeter (p=0.46). No significant difference in the mean CCT was observed between the two eyes (p=0.4820), whereas the mean ECD was significantly lower in the hypertensive eye than in the normal eye (p<0.001). The ECD was inversely related to the length of IOP elevation (p<0.001).

CONCLUSIONS

No difference in the corneal thickness measurement was observed between the specular microscopy and the pachymetry techniques. Chronic ocular hypertension did not significantly affect the CCT, but caused a significant loss of endothelial cells in the center of the cornea of the laser treated eyes compared to the normotensive eyes. The duration of elevated IOP was the most important factor affecting the ECD.

Assessment of reproducibility of measures of intraocular pressure and central corneal thickness in young white adults over a 16-h time period

PURPOSE

The aims of this study were to obtain clinical measures of intraocular pressure (IOP) and central corneal thickness (CCT) and during waking hours to further assess both the nature of diurnal changes, and the reproducibility of any possible association between the two measures.

METHODS

Fifteen white (Norwegian) healthy young adult subjects aged 20-29 years had IOP and CCT measures made, using a non-contact specular microscope and non-contact tonometry (NCT), every hour over a period of 16 h, starting at 06.30 hours. The experiment was repeated on two successive days.

RESULTS

The average NCT readings were 12.5 and 11.4 mmHg for days 1 and 2, while pachometry averages were 0.518 and 0.514 mm. The range of NCT values, for any set of readings, averaged 2 mmHg, while this range was 0.014 mm for CCT measures. The reproducibility of these measures, as estimated by the coefficient of variation (COV) was 7.2% for NCT and 1.0% for pachometry measures. The COV for tonometry was inversely proportional to the actual values of these measures. A significant time-related decline in tonometry values was seen over day 1 but not day 2. Pachometry values declined very rapidly over the first hour in both sessions and were essentially constant for the rest of the sessions. The strongest association between pachometry data and NCT measures was noted around midday (12.30 hours) on both sessions.

CONCLUSIONS

These studies confirm that the reproducibility (repeatability) of NCT and pachometry are substantially different. It is suggested that this aspect of tonometry needs to be taken into account, especially where there are lower values, when considering the possible impact of corneal thickness on tonometry measures and their interpretation. These types of effects need to be further assessed in older patients with ocular hypertension, or other glaucoma suspects.

The corneal thickness and intraocular pressure story: where are we now?

A review of the current literature was conducted regarding the effect of corneal thickness on the diagnosis of glaucoma, and the influence of excimer laser refractive surgery on intraocular pressure (IOP) measurement with Goldmann applanation tonometry. In general, normals and primary open angle glaucoma patients have a similar distribution of corneal thickness; however, there is a wide variation, ranging from 427 to 716 micro m. Normal tension glaucoma patients have a tendency towards thinner corneas than normals; however, there is an overlap of thickness measurements of more than two-thirds in 95% of patients. There is a trend for ocular hypertensives to have thicker corneas than normals, but again there is an overlap of about one-third in 95% of patients. The general trend after excimer laser refractive surgery is for a decrease in IOP, with a mean fall in IOP measured of 0.63 mmHg per dioptre correction. There is, however, a large scatter of values with some patients having the same or lower IOP post-laser, but with other patients measuring higher pressures. Corneal thickness can influence IOP measurement by Goldmann applanation tonometry; however, the magnitude of the effect is subject to much individual variation.