Changes in central corneal thickness over time: the ocular hypertension treatment study

Ocular Biomechanics and Glaucoma

Biomechanics is a branch of biophysics that deals with mechanics applied to biology. Corneal biomechanics have an important role in managing patients with glaucoma. While evidence suggests that patients with thin and stiffer corneas have a higher risk of developing glaucoma, it also influences the accurate measurement of intraocular pressure. We reviewed the pertinent literature to help increase our understanding of the biomechanics of the cornea and other ocular structures and how they can help optimize clinical and surgical treatments, taking into consideration individual variabilities, improve the diagnosis of suspected patients, and help monitor the response to treatment.

Effects of topical prostaglandin therapy on corneal layers thickness in primary open-angle glaucoma patients using anterior segment optical coherence tomography

PURPOSE

To evaluate the effects of latanoprost, bimatoprost, and travoprost eye drops and their preservatives on each corneal layer thickness in patients with primary open-angle glaucoma (POAG).

METHODS

We retrospectively analyzed 79 eyes of 79 patients with POAG who were receiving prostaglandin therapy. Patients were divided into three subgroups according to monotherapy with latanoprost, bimatoprost, and travoprost during a mean of 43.14 ± 19.12 months follow-up period. In addition, the central corneal epithelial thickness (CET), central corneal stromal thickness (CST), and total central corneal thickness (CCT) were measured by anterior segment optical coherence tomography (AS-OCT) at baseline and every six months after treatment initiation at each visit between 9 and 12 o'clock in the morning. Furthermore, intraocular pressure (IOP) was measured with Goldmann applanation tonometry (GAT) after AS-OCT measurements at each visit.

RESULTS

All three groups were not significantly different in age, gender, follow-up period, and mean intraocular pressure values (p > 0.05 for all). The reduction of CCT in the latanoprost, bimatoprost, and travoprost groups was 6.53 ± 3.17, 18.59 ± 8.42, and 10.1 ± 1.13 µm, respectively. The decrease in CST values was 4.65 ± 1.54, 15.84 ± 7.47, 9.69 ± 1.45 µm, and CET values were 1.88 ± 1.66, 2.75 ± 0.73, 0.41 ± 0.54 µm in all groups, respectively. A statistically significant thinning was observed in all corneal layers (p < 0.05) except the CST values in the latanoprost group and CET values in the travoprost group. However, no significant difference was found in the average reduction of CET, CST, and CCT values among the three groups (p > 0.05).

CONCLUSION

Topical treatment with latanoprost, bimatoprost, and travoprost affects each layer of the cornea separately according to the active and protective substances contained in these eye drops. On the other hand, the thinning effect on the corneal layers was similar in these three drugs because there was no significant difference between the three groups in the total amount of thinning of the corneal layers during the follow-up period.

Clinical Ocular Biomechanics: Where Are We after 20 Years of Progress?

Purpose: Ocular biomechanics is an assessment of the response of the structures of the eye to forces that may lead to disease development and progression, or influence the response to surgical intervention. The goals of this review are (1) to introduce basic biomechanical principles and terminology, (2) to provide perspective on the progress made in the clinical study and assessment of ocular biomechanics, and (3) to highlight critical studies conducted in keratoconus, laser refractive surgery, and glaucoma in order to aid interpretation of biomechanical parameters in the laboratory and in the clinic.Methods: A literature review was first conducted of basic biomechanical studies related to ocular tissue. The subsequent review of ocular biomechanical studies was limited to those focusing on keratoconus, laser refractive surgery, or glaucoma using the only two commercially available devices that allow rapid assessment of biomechanical response in the clinic.Results: Foundational studies on ocular biomechanics used a combination of computer modeling and destructive forces on ex-vivo tissues. The knowledge gained from these studies could not be directly translated to clinical research and practice until the introduction of non-contact tonometers that quantified the deformation response of the cornea to an air puff, which represents a non-destructive, clinically appropriate load. The corneal response includes a contribution from the sclera which may limit corneal deformation. Two commercial devices are available, the Ocular Response Analyzer which produces viscoelastic parameters with a customized load for each eye, and the Corvis ST which produces elastic parameters with a consistent load for every eye. Neither device produces the classic biomechanical properties reported in basic studies, but rather biomechanical deformation response parameters which require careful interpretation.Conclusions: Research using clinical tools has enriched our understanding of how ocular disease alters ocular biomechanics, as well as how ocular biomechanics may influence the pathophysiology of ocular disease and response to surgical intervention.

Effect of Topical Prostaglandin Analogue Therapy on Central Corneal Thickness: A Systematic Review

To investigate whether prostaglandin analogue (PGA) eyedrops have a significant effect on central corneal thickness (CCT), we conducted a systematic search of literature published from 2000 to 2021. Among the studies conducted on topical PGA therapy in open-angle glaucoma or ocular hypertension patients over 18 years old, prospective studies with CCT change as an outcome were included. A single-arm meta-analysis was conducted to assess the overall effect on CCT, and subgroup analysis according to exposure time of PGA eyedrops was also performed. We counted the number of articles that reported on severe events (CCT reduction of 25 μm or more) and obtained their proportion. The methodological quality was assessed by the McHarm tool. Twenty-two reports of prospective studies were selected. The results of the single-arm meta-analysis showed very high heterogeneity. Still, in subgroup analysis, when PGA was used for more than 6 months, heterogeneity was low, and a significant decrease in CCT was observed. Severe events were reported in two reports and occurred in 3.8% to 14.8% of participants. PGA eyedrop use may cause a clinically significant CCT decrease, requiring CCT follow-up.

The Effect of Prostaglandin Analogs on Central Corneal Thickness of Patients with Glaucoma or Ocular Hypertension: A Systematic Review

BACKGROUND

Prostaglandin analogs (PGAs) are first-line antiglaucoma agents that appear to either decrease or increase central cornea thickness (CCT), creating controversy regarding the benefits of PGAs in treating CCT.

PURPOSE

We performed the first meta-analysis of observational studies to evaluate the effects of PGAs on CCT in patients with glaucoma or ocular hypertension (OHT).

METHODS

This systematic review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement. A literature search was performed of the PubMed, Embase, Cochrane Library, System for Information on Grey Literature in Europe (Open Grey), and ClinicalTrials.gov databases and the references of retrieved studies. Only observational studies were included in the meta-analysis. The final CCT of patients and 95% confidence interval (CI) of each study were extracted. Study quality was assessed using the Newcastle-Ottawa Scale (NOS) and the Agency for Healthcare Research and Quality (AHRQ). A fixed-effects model was used to calculate the weighted mean difference (WMD) and 95% CI. Subgroup analyses based on several stratified factors such as public bias (Begg's test) and sensitivity analyses were performed.

RESULTS

Five cohort, 5 case-control, and three cross-sectional studies including 2,722 subjects were included. The pooled effect of all thirteen studies showed that PGAs reduced the CCT of patients with glaucoma or OHT slightly but significantly (WMD = -9.37; 95% CI [-12.18, -6.57]; p = 0.00; I2 = 45.5%). Significant effects were also observed in all three study designs: cohort (WMD = -5.17; 95% CI [-9.52, -0.82]), case-control (WMD = -15.31; 95% CI [-22.66, -7.97]), and cross-sectional (WMD = -8.65; 95% CI [-17.30, -0.01]). In addition, subgroup analysis of exposure time showed the effect of PGAs to be more obvious in the first (WMD = -5.81; 95% CI [-9.49, -2.14]) and second (WMD = -13.73; 95% CI [-20.19, -7.28]) years.

CONCLUSIONS

The pooled effects of previously reported studies suggest that PGA use can reduce the CCT of patients with glaucoma or OHT slightly but significantly, with this effect more pronounced in the first 2 years. These findings suggest that clinicians must closely monitor changes in CCT in the first 2 years of PGA use to identify cases of intraocular pressure misestimation and the efficacy of PGAs.

Changes in Corneal Epithelial Thickness Induced by Topical Antiglaucoma Medications

Corneal thickness measurement is important for assessing intraocular pressure in patients with glaucoma. This study investigated the changes in corneal epithelial thickness (CET) induced by antiglaucoma medications and explored the factors affecting CET measurement. CET was measured over a 9.0 mm diameter area by using Fourier domain optical coherence tomography in 125 patients with primary open-angle glaucoma and 125 age-matched controls without glaucoma. The influence of sex, age, benzalkonium chloride (BAK)-containing instillations, disease severity, and types and numbers of medications was analyzed using simple and multiple regression analyses. CET over 25 sectors was smaller in the glaucoma group than in the control group (mean difference of 4.2 µm in the central 2.0 mm zone; 52.8 ± 3.6 vs. 48.5 ± 3.9, p < 0.001). Simple regression analysis revealed age, use of β-blockers, prostaglandin, carbonic anhydrase inhibitors, total number of medications, and number of daily BAK-containing instillations were associated with a thinner epithelium. Multiple regression analysis revealed β-blockers, prostaglandin, and number of BAK-containing instillations were significant factors. Use of β-blockers and number of BAK-containing instillations were also associated with a thinner epithelium in the monotherapy subgroup analysis. CET was significantly smaller in patients with glaucoma receiving topical medications and was affected by the use of β-blockers, prostaglandin, and BAK.

The Association Between Ocular Rigidity and Neuroretinal Damage in Glaucoma

Purpose

Ocular rigidity (OR) is an important biomechanical property, thought to be relevant in the pathophysiology of open-angle glaucoma (OAG). This study aims to evaluate the relationship between OR and neuroretinal damage caused by glaucoma.

Methods

One hundred eight subjects (22 with healthy eyes, 23 with suspect discs, and 63 with OAG) were included in this study. OR was measured using a noninvasive optical coherence tomography (OCT)-based method developed by our group. We also measured central corneal thickness (CCT), corneal hysteresis (CH), and corneal resistance factor (CRF). Pearson and partial correlations were performed to evaluate the relationship between OR and glaucomatous damage represented by ganglion cell complex (GCC), retinal nerve fiber layer (RNFL) thicknesses, and neuroretinal rim area.

Results

Significant positive correlations were found between OR and minimum GCC thickness (r = 0.325, P = 0.001), average GCC thickness (r = 0.320, P = 0.002), rim area (r = 0.344, P < 0.001), and RNFL thickness in the superior (r = 0.225, P = 0.023), and inferior (r = 0.281, P = 0.004) quadrants. These correlations were generally greater than those found for CCT, CH, and CRF. Furthermore, no correlation was found between OR and corneal biomechanical parameters. After adjusting for age, sex, and ethnicity, significant correlations were found between OR and minimum and average GCC thickness (r = 0.357, P = 0.001 and r = 0.344, P = 0.001, respectively), rim area (r = 0.327, P = 0.001), average RNFL thickness (r = 0.331, P = 0.001), and RNFL thickness in the superior (r = 0.296, P = 0.003) and inferior (r = 0.317, P = 0.001) quadrants.

Conclusions

In this study, we found a positive correlation between structural OCT-based parameters and OR, indicating more neuroretinal damage in eyes with lower OR. These findings could provide insight into the pathophysiology of OAG.

Rate of central corneal thickness changes in primary angle closure eyes: long-term follow-up results

Background

Central corneal thickness (CCT) and its association with intraocular pressure, which is a pivotal parameter in glaucoma management, has previously been reported. In this study, we intended to investigate the long-term change of CCT in terms of rate in eyes with primary angle-closure (PAC). Additionally, we aimed to analyze events that could affect CCT.

Methods

In this retrospective study, 26 patients with PAC who had a follow-up period of more than 5 years were analyzed. The rate of CCT changes from baseline was evaluated from the serial CCT measurements over the average follow-up period. The pattern of CCT change rate according to modes of treatment and history of angle-closure attack was analyzed using the repeated linear mixed model analysis.

Results

A total of 52 eyes were enrolled. The CCT reduction rate of the entire study population was − 0.72 ± 0.22 μm/yr (P = 0.001) with statistical significance. The CCT thinning rate of the laser peripheral iridotomy (PI) group was − 0.53 ± 0.25 μm/yr (P = 0.034) and that of the surgical trabeculectomy group was − 1.32 ± 0.43 μm/yr (P = 0.002), and it was not statistically significant (P = 0.112). The rate of CCT thinning in patients with a history of acute angle-closure attack was − 0.81 ± 0.31 μm/yr (P = 0.009) and that in patients without an attack was − 0.63 ± 0.30 μm/yr (P = 0.001), and it was not statistically significant (P = 0.680). Baseline CCT appeared to be the only significant factor affecting the rate of CCT changes (P < 0.001).

Conclusions

We found a significant reduction in CCT over a long observation period in PAC eyes. We also found that the rates of CCT reduction were not affected by different treatment modalities or acute angle-closure attacks. The analysis of long-term CCT changes in conjunction with baseline CCT would also be helpful in the clinical evaluation of the PAC patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12886-021-01908-4.

Heartbeat-Induced Corneal Axial Displacement and Strain Measured by High Frequency Ultrasound Elastography in Human Volunteers

Purpose

The purpose of this study was to establish in vivo data acquisition and processing protocols for repeatable measurements of heartbeat-induced corneal displacements and strains in human eyes, using a high-frequency ultrasound elastography method, termed ocular pulse elastography (OPE).

Methods

Twenty-four volunteers with no known ocular diseases were recruited for this study. Intraocular pressure (IOP) and ocular pulse amplitude (OPA) were measured using a PASCAL Dynamic Contour Tonometer (DCT). An in vivo OPE protocol was developed to measure heartbeat-induced corneal displacements. Videos of the central 5.7 mm of the cornea were acquired using a 50-MHz ultrasound probe at 128 frames per second. The radiofrequency data of 1000 frames were analyzed using an ultrasound speckle tracking algorithm to calculate corneal displacements and quantify spectral and temporal characteristics. The intrasession and intersession repeatability of OPE- and DCT-measured parameters were also analyzed.

Results

The in vivo OPE protocol and setup were successful in tracking heartbeat-induced corneal motion using high-frequency ultrasound. Corneal axial displacements showed a strong cardiac rhythm, with good intrasession and intersession repeatability, and high interocular symmetry. Corneal strain was calculated in two eyes of two subjects, showing substantially different responses.

Conclusions

We demonstrated the feasibility of high-frequency ultrasound elastography for noninvasive in vivo measurement of the cornea's biomechanical responses to the intrinsic ocular pulse. The high intrasession and intersession repeatability suggested a robust implementation of this technique to the in vivo setting.

Translational Relevance

OPE may offer a useful tool for clinical biomechanical evaluation of the cornea by quantifying its response to the intrinsic pulsation.

The Glaucoma Italian Pediatric Study (GIPSy): The Long-term Effect of Topical Latanoprost on Central Corneal Thickness

PRECIS

Central corneal thickness (CCT) may increase over time in children affected by primary congenital glaucoma and treated with latanoprost for at least 30 months.

PURPOSE

The purpose of this study was to investigate CCT modification over time in a population of primary pediatric glaucoma (PPG) patients prescribed a monotherapy of latanoprost.

MATERIALS AND METHODS

The present paper reports the results of a post hoc analysis on patients enrolled in the Glaucoma Italian Pediatric Study (GIPSy). Children affected by PPG, with a postsurgical intraocular pressure between 22 and 26 mm Hg and treated with latanoprost monotherapy for at least 30 months were eligible for the analysis. CCT variation from baseline was investigated over the follow-up using univariable and multivariable longitudinal linear mixed models. The impact of age, sex, and intraocular pressure on CCT variation were evaluated taking into account the interaction of each variable with time.

RESULTS

Twenty-seven eyes (20 patients) were included in the analysis. Mean duration of latanoprost treatment was 36.6 months (SD 2.5) and mean CCT at baseline was 551 μm (SD 37.7). A significant increase of CCT over time was revealed by multivariable analysis, taking into account the impact of age at baseline and its interaction with time (P=0.03). The interaction between age and time was significant (P=0.04), indicating that older age at baseline was associated with lower increase of CCT over time. No variation of CCT was found in univariable analysis (P=0.28).

CONCLUSION

In this population of PPG patients treated with latanoprost for at least 30 months, CCT significantly increased over time, when the impact of age and its interaction with time were considered.

Interocular Asymmetry of Visual Field Loss, Intraocular Pressure and Corneal Parameters in Primary Open-angle Glaucoma

OBJECTIVES

To assess the association between the corneal biomechanical parameters and visual field (VF) loss in patients with asymmetric primary open-angle glaucoma (POAG).

METHODS

A total of 89 POAG patients (50 males, 56.2%) with asymmetric VF loss, aged 65.2 ± 13.3 years old, were enrolled in this study. Asymmetric VF loss was defined as an interocular difference of the global index mean deviation (MD) >2dB. Intraocular pressure (IOP), central corneal thickness (CCT) and corneal biomechanical parameters such as maximum amplitude at the apex of highest concavity (Def ampl HC) were measured. The worse eye was defined as the eye with a smaller MD.

RESULTS

The worse eyes had lower MD (-11.9 ± 6.7 dB vs. -5.3 ± 5.0 dB; P<0.001) and higher IOP (14.6 ± 3.3 vs.13.9 ± 2.6 mmHg, P=0.04) than the better eyes. There was no significant difference between the two groups for CCT. The interocular difference of MD (IDMD) was negatively correlated with the interocular difference of IOP (r=-0.22, P=0.04), while positively correlated with the interocular difference of Def ampl HC (r=0.27, P=0.01). In patients with moderate asymmetric VF loss (IDMD ≥6 dB), the Def ampl HC of the worse eyes group (1.07 ± 0.12 mm) were significantly lower than the better eyes group (1.10 ± 0.11 mm, p=0.02).

CONCLUSION

Asymmetric POAG was associated with asymmetry in IOP and corneal biomechanical parameters but not in CCT. Lower deflection amplitude and higher IOP was found in eyes with more severe visual field damage in POAG patients.

Twelve-Year Incidence and Baseline Risk Factors for Pseudoexfoliation: The Thessaloniki Eye Study (An American Ophthalmological Society Thesis)

PURPOSE

To determine the 12-year incidence of pseudoexfoliation (PEX), baseline risk factors for incident PEX and risk factors for incident pseudoexfoliative glaucoma (PEXG) among those with PEX in an elderly white population.

METHODS

Longitudinal, population-based study in Thessaloniki, the major urban center in Northern Greece. The baseline cohort included 2554 participants ≥60 years old. The surviving cohort was re-examined 12 years later using the same methodology. PEX was defined as typical fibrillar material at the pupil margin and/or on the lens capsule. Glaucoma was defined as both structural and functional damage, irrespective of intraocular pressure (IOP).

RESULTS

Of 1468 eligible subjects in the surviving cohort, 1092 (74%) participated in the follow-up study. The mean age ± standard deviation (SD) at baseline was 68.9 ± 4.6 years. The mean follow-up time was 11.6 ± 1.6 years. The 12-year incidence of PEX was 19.6% (95% confidence interval (CI), 17.1-22.2), with women more likely to be affected than men (Fisher's exact test, P = .0197). Higher axial length was associated with lower odds of incident PEX (odds ratio [OR], 0.72 per mm; 95% CI, 0.57-0.92). PEX at baseline was not associated with an increased likelihood of major vascular disease (P = .9038). Higher baseline IOP (OR, 1.26 per mm Hg; 95% CI, 1.07-1.48) and history of heart attack at baseline (OR, 13.49; 95% CI, 2.85-63.87) were associated with a greater likelihood of developing PEXG among those with PEX. A history of alcohol consumption at baseline was protective of individuals developing PEXG if they had PEX at baseline.

CONCLUSION

This is one of the very few longitudinal population-based studies that has specifically assessed the incidence of PEX. The association with axial length was previously found only in a cross-sectional study. The associations with heart attack and alcohol consumption are new findings. In individuals with baseline PEX, higher IOP at baseline, history of heart attack at baseline, and no alcohol consumption were associated with a greater likelihood of developing glaucomatous damage approximately 12 years later. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Effects of aging on corneal parameters measured with Pentacam in healthy subjects

Our purpose was to prospectively analyze the age-related changes of corneal Scheimpflug parameters in healthy subjects. Thirty-five eyes of 35 volunteers (age 14-67 years) were investigated with an average interval of 3.6 years. Changes of corneal parameters (flattest keratometric reading at anterior (K1F) and posterior surface (K1B), steepest keratometric reading at anterior (K2F) and posterior surface, anterior astigmatism, posterior astigmatism (AstigB), flat axis of anterior and posterior astigmatism (AxisB), thinnest pachymetric value (PachyMin), corneal volume (CV10-mm)) were analyzed. K1F and K2F decreased significantly during observation and showed stronger decrease in younger than in older individuals. Higher values proved to be more stable. K1B decreased significantly and the degree of decrease was dependent on its baseline value and age: in young subjects low values increased, high values decreased. AstigB decreased significantly and showed a baseline-dependent significant increase from lower and a significant decrease from higher initial values. Over time, the mean AxisB shifted significantly. PachyMin and CV decreased significantly with age, especially from higher baseline values in younger subjects. The results of this longitudinal study suggest that both corneal surfaces change significantly with age. We demonstrate for the first time that age and baseline values influence age-related changes of corneal parameters.

Ten-year reinvestigation of ocular manifestations in Marfan syndrome

IMPORTANCE

Long-term follow-up of Marfan syndrome (MFS) patients.

BACKGROUND

Investigate changes in ocular features in MFS patients fulfilling the Ghent-2 criteria following a period of 10 years.

DESIGN

Repeated cross-sectional study with two observations.

PARTICIPANTS

Eighty-four MFS patients were investigated in 2003-2004 (baseline). Forty-four of these patients (52%) were examined after 10 years.

METHODS

A comprehensive ocular examination performed at baseline and follow-up.

MAIN OUTCOME MEASURES

Development or progression of ectopia lentis (EL).

RESULTS

At follow-up, mean age was 50.1 ± 11.9 years (range: 30-80 years), 74% were female and 70% of the patients were diagnosed with EL compared to 66% at baseline. Two patients (3 eyes) had developed EL over the decade, representing a 13% risk. Furthermore, one eye had progressed from a subtle tilt of the lens to dislocation. We found no significant change in the axial length (P = 0.96), the corneal curvature (P = 0.64) or the spherical equivalent (P = 0.23). Best corrected visual acuity was improved at follow-up (P = 0.02). There were 7% and 33% risks for development of retinal detachment and cataract between baseline and follow-up, respectively.

CONCLUSIONS AND RELEVANCE

Our study indicates that even though EL typically occurs at an early stage in most MFS patients, there is still a risk of developing EL in adulthood. The risk of developing vision-threatening complications such as retinal detachment and cataract was much higher than in the normal population, but even so, the visual potential of the MFS patients was relatively good.

Longitudinal Change in Central Corneal Thickness in the Tema Eye Survey

PURPOSE

To determine the change and rate of change in central corneal thickness (CCT) and their determinants.

DESIGN

Longitudinal observational population-based study.

METHODS

A total of 758 normal and 58 glaucomatous subjects underwent complete eye examination, with CCT measurements at 2 separate visits. Change and rate of change in CCT were determined. Univariate and multivariate linear regression analyses were performed to determine the factors associated with change and rate of change.

RESULTS

The mean follow-up duration was 8.4 ± 0.7 years. The overall change was -8.9 ± 16.7 μm in OD and -9.8 ± 16.2 μm in OS, both P < .0001. Changes in glaucomatous and normal subjects were -14.1 ± 2.2 μm vs -8.6 ± 0.6 μm in OD (P = .02) and -14.5 ± 2.2 μm vs -9.5 ± 0.6 μm in OS (P = .03), respectively. The overall rate of thinning was -1.1 μm/year (OD) and -1.2 μm/year (OS). Rates in glaucomatous and normal eyes were -1.7 ± 0.3 μm/year vs -1.0 ± 0.1 μm/year in OD (P = .02) and -1.7 ± 0.3 μm/year vs -1.1 ± 0.1 μm/year in OS (P = .03), respectively. Change and rate of change were associated with baseline CCT (ß = -0.1 to -0.09 and -0.011, respectively, all P < .001) and glaucoma (ß = -6.8 to -5.6, P ≤ .009, and -0.75 to -0.69, P ≤ .007, respectively).

CONCLUSION

CCT decreased significantly over time. The change and rate of change were greater in glaucomatous than normal eyes, and were greater than described in cross-sectional studies.

Reproducibility of Central Corneal Thickness Measurements in Healthy and Glaucomatous Eyes

PURPOSE

The purpose of this study is to characterize intraday and interday variation in central corneal thickness (CCT) in healthy and glaucomatous subjects.

METHODS

In this prospective cohort study, 40 healthy subjects and 42 subjects with primary open-angle glaucoma underwent CCT measurements by ultrasonic pachymetry on 5 days over 1 year: first at baseline, then at 1 week and 1, 6, and 12 months after baseline. On 1visit, CCT was measured every 2 hours from 08:00 to 20:00. Intraday CCT variance was compared with interday CCT variance.

RESULTS

Mean CCT at all visits and time points ranged from 561 to 574 µm in healthy eyes and from 548 to 563 µm in glaucomatous eyes. The mean intraday CCT range (highest minus lowest readings) was 21±10 µm in healthy participants and 21±10 µm in glaucoma patients. The mean interday CCT range was 27±13 µm in healthy participants and 24±14 µm in glaucoma patients. Within-subject variance was significantly larger among the interday CCT measurements than the intraday CCT measurements in both healthy subjects and glaucoma patients (P<0.0001 for both) demonstrating that measurements taken on separate days are more different than measurements taken on the same day. Interday CCT differences of ≥30 µm were seen in 27.7% of normal subjects and 30% of glaucoma patients.

CONCLUSIONS

CCT measurements exhibit intraday and interday variation, with the latter being significantly greater than the former. A single CCT assessment inadequately characterizes CCT and may impact risk assessment in patients with suspect and/or diagnosed glaucoma.

Central corneal thickness in glaucoma

PURPOSE OF REVIEW

The purpose of this review is to summarize the role of central corneal thickness (CCT) in the clinical management of a glaucoma patient.

RECENT FINDINGS

The prognostic value of CCT is well recognized in patients with ocular hypertension. However, its predictive value in other glaucoma suspects and patients with established glaucoma is less certain. Tonometry artefacts can result from variations in CCT. However, an adequately validated correction algorithm for Goldmann applanation tonometry measurements does not exist. Newer methods of tonometry are potentially less influenced by CCT but are limited in their clinical use. There may also be biological and genetic associations between corneal thickness and glaucoma. Demographics, environmental factors, glaucoma treatment and the measurement device used have a significant influence on CCT, and should be considered when interpreting the effect of cornea thickness in patients with glaucoma. New measurements of the biomechanical properties of the cornea are likely to be better approximations of the globe biomechanics than CCT, but these require further evaluation.

SUMMARY

The clinical significance of CCT is well recognized in the context of glaucoma diagnosis and management, though the extent of its importance remains debatable. Corneal biomechanical properties may be more significantly associated with glaucoma than CCT.

Structural and biomechanical corneal differences between patients suffering from primary congenital glaucoma and healthy volunteers

PURPOSE

To determine whether a set of ocular morphometric and biomechanical variables are able to discriminate between healthy volunteers and patients suffering from primary congenital glaucoma (PCG).

METHODS

Case-control study in which 66 patients with PCG and 94 age-matched healthy subjects were evaluated using ocular response analyser (ORA) to record corneal biomechanical properties. Topographic corneal variables were obtained using the Pentacam in both groups. To determine the ability to discern between both groups, a multivariate binary logistic model was constructed. The outcome was the diagnosis of PCG and the predictors; the corneal variables analysed along with their first-term interactions. Sensitivity and specificity of this model along with the area under the receiver characteristic operating curve (AUC of ROC) were determined.

RESULTS

The best model to discriminate between both groups included the following predictors: corneal hysteresis (CH), corneal resistance factor (CRF), posterior maximum elevation (PME), anterior maximum elevation (AME) and central corneal thickness (CCT). This model, for a cut-point of 50%, presents a sensitivity of 86.67%, a specificity of 86.89% and an AUC of the ROC curve of 93.16% [95% confidence interval (CI): 88.97-97.35]. The adjusted odds ratios of those predictors which showed a significant discriminating capacity were as follows: for CH, 0.27 (95% confidence interval: 0.15-0.46); for CRF, 2.13 (95% CI: 1.33-3.40); for PME, 1.06 (95% CI: 1.01-1.12); and for AME, 1.35 (95% CI: 1.10-1.66).

CONCLUSION

Corneal hysteresis (CH), CRF, PME and AME are able to discern between patients with PCG and healthy controls. This fact suggests that there are structural and biomechanical differences between these groups.

Comparison of ocular response analyzer parameters in primary open angle glaucoma and exfoliative glaucoma patients

AIM

We sought to identify differences in the following measures of the ocular response analyser (ORA) between primary open angle glaucoma (POAG) and exfoliative glaucoma (EXG) patients: Corneal hysteresis (CH), corneal resistance factor (CRF), corneal-compensated intraocular pressure (IOPcc) and Goldmann-correlated intraocular pressure (IOPg). We also sought to relate these ORA measures with central corneal thickness (CCT).

MATERIALS AND METHODS

This cross-sectional study was conducted on a total of 162 individuals (46 EXG patients, 66 POAG patients and 50 healthy subjects without any ocular and systemic disease). ORA measurements were performed, and a minimum of three readings were obtained from each test subject. Groups were compared according to their ORA parameters.

RESULTS

The mean CH levels of the EXG, POAG and healthy subjects were 7.6 ± 2.1, 9.1 ± 1.9 and 9.6 ± 1.7 mmHg, respectively. CH was significantly lower in the EXG patients compared to the other groups (P < 0.001).The mean CRF levels of the EXG, POAG and healthy subjects were 9.0 ± 2.0, 10.1 ± 1.7 and 9.8 ± 1.8mmHg, respectively. CRF levels in the eyes of the EXG patients were significantly lower compared to those of either the POAG patients (P = 0.005) or the healthy subjects (P = 0.03), but there was no significant difference in CRF levels between the POAG patients and the healthy subjects (P = 0.59). There was a significant positive correlation between CH and CCT in the EXG patients and healthy subjects (P < 0.001), but this correlation was not present in the POAG patients (P = 0.70).

CONCLUSIONS

In this study, CH and CRF were found to be significantly reduced in the eyes of EXG patients compared to both the POAG patients and healthy subjects. Reduced CH in EXG patients might result in decreased support of peripapillary scleral structure and increased damage to the optic nerve during IOP increase.

Effect of selective laser trabeculoplasty on corneal biomechanics

PURPOSE

To evaluate the effect of selective laser trabeculoplasty (SLT) on ocular biomechanics as measured with the ocular response analyzer (ORA, AMETEK Inc. and Reichert Inc.).

METHODS

In a prospective case series, 52 eyes of 52 patients (aged 66.6 ± 9.6 years) with insufficient intraocular pressure (IOP) control despite maximum tolerated medical therapy underwent SLT (Solo(™) SLT, Ellex Inc.) for further IOP reduction. Goldmann-correlated IOP (IOPg), corneal-compensated IOP (IOPcc), corneal hysteresis (CH) and the corneal resistance factor (CRF) were measured with the ORA prior to and at least 4 weeks following SLT.

RESULTS

IOPg decreased statistically significantly from 18.0 ± 6.4 to 14.8 ± 3.8 mmHg and IOPcc from 20.2 ± 6.5 to 16.7 ± 3.4 mmHg (p < 0.001). CH increased from 8.53 ± 2.03 to 9.12 ± 1.83 mmHg (p = 0.028) and CRF decreased from 9.58 ± 2.18 to 9.1 ± 2.1 mmHg (p = 0.037), which was statistically significant. Correcting the CH and CRF data for the influence of IOP reduction with a covariance analysis, however, resulted in an unchanged CH (IOPg adjusted: p = 0.318; IOPcc adjusted: p = 0.468) and CRF (IOPg adjusted: p = 0.320; IOPcc adjusted: p = 0.464) after SLT.

CONCLUSION

Selective laser trabeculoplasty does not change corneal biomechanical properties as measured with the ORA in already pretreated patients with glaucoma. The measured increase in CH and decrease in CRF can be solely explained by IOP reduction.

Central Corneal Thickness and its Associations With Ocular and Systemic Factors in an Urban West African Population

PURPOSE

To assess the associations of central corneal thickness (CCT) with ocular and systemic factors in a West African population.

DESIGN

Population-based cross-sectional study.

METHODS

Participants aged ≥40 years in the Tema Eye Survey who had clinically normal corneas were included in this study. CCT was determined bilaterally using handheld ultrasound pachymetry. The association between CCT and ocular or systemic factors was analyzed with univariable linear regression. Multivariable linear regression analysis was performed for variables significantly associated with CCT in the univariable analysis. Main outcome measures were CCT (μm) and its associations with age, sex, IOP, cup-to-disc ratio (CDR), glaucoma, hypertension, and diabetes.

RESULTS

A total of 6806 eligible subjects were identified, of whom 5603 (82.3%) participated in the Tema Eye Survey. A total of 4737 participants (84.5% of participants) of West African descent and clinically normal corneas were included. The mean age ± standard deviation (SD) of participants was 51.2 ± 9.7 years and 38.7% were male. The mean ± SD CCT in the population was 533.9 ± 34.0 μm. In the multivariable linear regression analysis, increased CCT was significantly associated with younger age, male sex, and higher IOP (all P < .001). There were no significant associations between CCT and CDR, glaucoma, hypertension, and diabetes.

CONCLUSIONS

This population-based cross-sectional survey of West African adults found a significant association between increased CCT and younger age, male sex, and higher IOP but not glaucoma or CDR. Variations in corneal thickness may influence the accuracy of IOP measurements in African persons.

Longitudinal Evaluation of Central Corneal Thickness in Patients With Systemic Sclerosis

PURPOSE

To investigate the longitudinal change of central corneal thickness (CCT) in patients with systemic sclerosis (SSc) and to elucidate whether it contributes to misinterpretation of intraocular pressure (IOP) in this group of patients.

METHODS

Twenty patients with SSc and 20 sex- and age-matched controls were examined at 2 visits 5 years apart. Age, sex, race, subtype of SSc, disease duration, autoantibody profile, use of disease-modifying antirheumatic drugs (DMARDs), best-corrected visual acuity, spherical equivalent refraction, IOP, and CCT were recorded. IOP was assessed by applanation tonometry and CCT by ultrasonic pachymetry.

RESULTS

CCT decreased by 7.2 μm [95% confidence interval (CI), -2.1 to -12.2 μm] between the first and second measurements (P = 0.008) in patients with SSc and by 2.4 μm (P = 0.39, 95% CI, -8.0 to 3.3 μm) in the control group. Considering patients with SSc, CCT decreased by a mean of 11.6 μm [95% CI, -4.3 to -19.0 μm (P = 0.007)] among those taking DMARDs at the second visit and by 4.2 μm [95% CI, -3.0 to -11.5 μm (P = 0.2)] in patients not taking any DMARDs. There was no statistically significant change in IOP between the 2 visits for either the SSc group (P = 0.84) or the control group (P = 0.29). Mean change in CCT was not associated with either IOP at first visit or with change in IOP in SSc patients.

CONCLUSIONS

CCT decreased with time in SSc. However, the slight rates of thinning observed are unlikely to considerably influence applanation tonometry or clinical decision-making over the short to intermediate term.

In vitro Effects of Prostaglandin Analogs on Cultured Astrocytes Obtained from the Lamina Cribrosa

PURPOSE

To evaluate the effects of prostaglandin analogs (PGAs) on cell viability and apoptosis in cultured astrocytes obtained from the lamina cribrosa (LC) of the human optic nerve head (ONH).

METHODS

Astrocytes were cultured from LC samples obtained from human donor ONH and treated with three kinds of acid form of PGAs: latanoprost (LAT-A), tafluprost (TAF-A), and bimatoprost (BIM-A) (0.1, 1, 10, 50 and 100 ug/mL). Cell viability was assessed using the WST-1 assay. Cell apoptosis was measured using the deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labeling (TUNEL) assay. Apoptotic protein expression was evaluated using western blot analysis.

RESULTS

ONH astrocytes expressed FP receptor in western blot analysis. In the presence of 0.1 ug/mL of LAT-A, BIM-A, and TAF-A, the cell viability was 85%, 85% and 82%, respectively. WST-1 assay revealed about 50% of cell viability following treatment with 50 ug/mL of all PGAs. After exposing astrocytes to 10 ug/mL of each PGA for 24 hours, apoptotic cells were stained in TUNEL assay. Western blot analysis revealed that the PGAs up-regulated Bax (pro-apoptotic protein) and down-regulated Bcl-xL (anti-apoptotic protein) in the astrocytes.

CONCLUSIONS

PGAs affected cell viability in cultured astrocytes obtained from human ONH LC. PGA treatment may induce apoptosis in ONH astrocytes.

Decreased keratocyte density and central corneal thickness in primary open-angle glaucoma patients undergoing treatment with topical prostaglandin analogues

Purpose:

To evaluate whether prostaglandin (PG) analogue use is associated with alterations in keratocyte density and central corneal thickness (CCT) in subjects with primary open-angle glaucoma (POAG).

Materials and Methods:

Thirty-five POAG patients treated with PG analogues for >2 years and 35 control subjects without glaucoma were included in this cross-sectional study. All subjects were underwent CCT measurements using ultrasound pachymetry. Keratocyte densities of each stromal layer were determined by in vivo confocal microscopy. Student's t-test and Chi-square test were used for statistical evaluations. Correlations between keratocyte densities and CCT were analyzed using Pearson's correlation analysis.

Results:

Keratocyte densities in each stromal layer were significantly lower in glaucoma patients receiving PG analogues as compared to those of controls (P < 0.001). The mean CCT was also lower in glaucoma patients (515.2 ± 18.8 μ) than control subjects (549.6 ± 21.1 μ, P < 0.001). A positive correlation between keratocyte densities in each stromal layer and CCT was observed in POAG patients.

Conclusions:

Long-term administration of topical PG analogues may adversely influence keratocyte densities and CCT. Further prospective studies are required clarify the relationship between PG analogues and their effects on the cornea.

TNF-α Gene Polymorphisms and Primary Open Angle Glaucoma in Romanian Population / Polimorfisme ale genei TNF-α în populația română cu glaucom primar cu unghi deschis

Primary open-angle glaucoma (POAG) represents the most common form of a heterogeneous group of glaucomatous optic neuropathies which are a worldwide cause of irreversible blindness. Immune dysregulation and the genetic background are considered important risk factors. The influence on susceptibility to POAG of single nucleotide polymorphisms (SNPs) of tumor necrosis factor-α (TNF-α) was intensively studied, mostly on Asian population. We investigated the possible association of two TNF-α SNPs (-308G/A and -857C/T) with susceptibility to POAG and its clinical characteristics. A case-control association study of aforementioned TNF-α SNPs was performed on 197 POAG patients (divided into two subgroups: high-tension and normal-tension glaucoma - HTG/ NTG) versus 208 ethnically matched controls. This is the first study performed on Romanian population. No significant differences were found in terms of allelic frequencies, genotype distribution of the studied SNPs, or their haplotypes between POAG and healthy control groups. In the subgroup analysis, TT genotype of TNF-α -857T-allele was found to be associated with higher values of central corneal thickness (CCT) in NTG subgroup (p-value 0.032). In order to confirm the association between -857C/T SNP of TNF-α and CCT in NTG subgroup of POAG patients, additional studies on different populations should be performed.

Long-term change in central corneal thickness from a glaucoma perspective

AIM

To investigate the longitudinal change in central corneal thickness (CCT) over 3 years in patients with glaucoma.

MATERIALS AND METHODS

The Chennai Glaucoma Follow-up Study, an offshoot of the Chennai Glaucoma Study, was designed to evaluate the progression of glaucoma. A cohort of participants in the Chennai Glaucoma Study that were suffering from glaucoma or were at a higher risk for glaucoma underwent comprehensive ophthalmic evaluation at the base hospital at 6-month intervals during the years 2004 to 2007. The CCT (average of 10 readings) was measured between 11 am and 1 pm on any given day using an ultrasonic pachymeter. Patients with a history of ocular surgery, corneal disease and usage of topical carbonic anhydrase inhibitor were excluded. No patient was a contact lens wearer.

RESULTS

One hundred and ninety-six patients (84 male, 112 female) met the inclusion criteria. We analyzed data from the right eye. The mean age of the patients was 59.97 ± 9.06 years. Fifty-nine (30.1%) of the patients were diabetic. The mean change in CCT (CCT at first patient visit--CCT at last patient visit) was 3.46 ± 7.63 μm. The mean change in CCT was 0.75 μm per year (R² = 0.00). Age, gender, intraocular pressure at the first patient visit and diabetic status had no significant influence on the magnitude of change in CCT.

CONCLUSION

A carefully obtained CCT reading by a trained examiner need not be repeated for at least 3 years as long as the ocular and systemic factors known to affect the measurement of CCT are constant.

Central corneal thickness measurements in premature infants

AIM

To evaluate the central corneal thickness (CCT) measurements of premature infants.

METHODS

The premature infants who were born between 25-34wk of gestational age (GA) were retrieved for the study. All CCT measurements were performed within the first 2d of their life under topical anesthesia by using an ultrasonic pachymeter.

RESULTS

The mean CCT of 200 eyes was 647.79±63.94 µm. The mean CCT of the right and left eyes were similar (647.30±64.72 µm and 648.29±63.47 µm consecutively). The mean CCT of the girls was 642.08±54.89 µm and the boys was 653.07±71.14 µm, with no statistically significant difference (P>0.05). But there was a negative correlation of the CCT with GA and birth weight (BW) (P<0.01).

CONCLUSION

The CCT values of premature babies were negatively correlated with GA and BW.

Optic disc planimetry, corneal hysteresis, central corneal thickness, and intraocular pressure as risk factors for glaucoma

PURPOSE

To determine whether corneal hysteresis and central corneal thickness are independent risk factors for glaucoma.

DESIGN

A cross-sectional population-based cohort study.

METHODS

Associations were tested between corneal hysteresis, measured in 1754 population-based subjects from the TwinsUK cohort, and glaucoma-related endophenotypes, including intraocular pressure (IOP), vertical cup-to-disc ratio, optic disc area, and optic disc cup area. Corneal hysteresis, IOP, and central corneal thickness (CCT) were measured; optic disc photographs were analyzed; and multivariable linear regression analysis was performed.

RESULTS

Data were available on 1645 individuals. Multiple regression analysis showed corneal hysteresis to be significantly negatively associated with age (beta coefficient = -0.03, P < .00005) and IOP (beta coefficient = -0.06, P < .00005). Corneal hysteresis was also found to be associated with CCT (beta coefficient = 0.02, P < .0005). There was no significant association between corneal hysteresis and optic disc area (P = .6), cup area (P = .77), vertical cup-to-disc ratio (P = .51), or spherical equivalent (P = .08). CCT was also found to be significantly associated with IOP (beta coefficient = 3.3, P < .0005) and corneal hysteresis (beta coefficient = 9.4, P < .0005), but not with age (P = .59) or spherical equivalent (P = .16).

CONCLUSION

In this large cohort of healthy British twins, we found no relationship between corneal hysteresis or CCT and quantitative measures of optic disc cupping, suggesting that corneal hysteresis and CCT are not independent risk factors for glaucoma.

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

PURPOSE

To characterize five models of corneal thickness circular zoning in a sample of healthy controls and a sample of patients with primary open-angle glaucoma (POAG) and to determine their effect on Goldmann (GAT), dynamic contour (DCT) and rebound tonometers (RT).

METHODS

The study participants were 122 controls and 129 cases. Five corneal thickness zoning models (A, B, C, D and E) were constructed. The partitioning pattern consisted of a circle centred at the corneal apex and several concentric rings, until the limbus; the contours of each ring followed the geometry of the corneal contour of each participant. In Model A, the central circle was 1 mm in diameter and five concentric rings were established. Mean was obtained for each zone for both samples and compared between them using a t-test. The effect on the tonometers of central cornel thickness (CCT) and mean thickness of the zones generated was determined through several linear regression models (one per tonometer and per sample).

RESULTS

According to a t-test, cases and controls differ in zones I [mean difference (MD): 17.93 μm], V (MD: 25.52 μm) and VI (MD: 31.78 μm) of model A (higher values in the cases sample). RT was affected by CCT (controls: B = 0.089; cases: B = 0.081). DCT was affected by zone IV of model A (controls: B = -0.029; cases: B = -0.012). GAT was affected by CCT (controls: B = 0.043; cases: B = 0.025) and zone III of model A (controls: B = -0.045; cases: B = -0.033).

CONCLUSION

Our results highlight the importance of the thickness of other regions of the cornea different from its main centre in discriminating between healthy controls and patients with POAG and in IOP measurements made using DCT, GAT and RT.

The rate of structural change: the confocal scanning laser ophthalmoscopy ancillary study to the ocular hypertension treatment study

PURPOSE

To compare rates of topographic change in ocular hypertensive eyes that develop primary open-angle glaucoma (POAG) compared to eyes that do not, and to identify factors that influence the rate of change.

DESIGN

Longitudinal, randomized clinical trial.

METHODS

Four hundred forty-one participants (832 eyes) in the Confocal Scanning Laser Ophthalmoscopy Ancillary Study to the Ocular Hypertension Treatment Study were included. POAG was defined as repeatable visual field, photography-based optic disc changes, or both. The rate of topographic change in the 52 participants (66 eyes) who developed POAG was compared with that of participants who did not develop POAG using multivariable mixed effects models.

RESULTS

In both univariate and multivariate analyses, the rate of rim area loss was significantly faster in eyes in which POAG developed than in eyes in which it did not (univariate mean, -0.0131 mm(2)/year and -0.0026 mm(2)/year, respectively). The significantly faster rate of rim area loss in black persons found in the univariate analysis did not remain significant when baseline disc area was included in the model. In multivariate analyses, the rate of rim area loss and other topographic parameters also was significantly faster in eyes with worse baseline visual field pattern standard deviation and higher intraocular pressure during follow-up. Moreover, a significant rate of rim area loss was detected in eyes in which POAG did not develop (P < .0001). The rate of rim area loss in eyes with an optic disc POAG endpoint was significantly faster than in those with a visual field POAG endpoint.

CONCLUSIONS

The rate of rim area loss is approximately 5 times faster in eyes in which POAG developed compared with eyes in which it did not. These results suggest that measuring the rate of structural change can provide important information for the clinical management of ocular hypertensive patients. Additional follow-up is needed to determine whether the statistically significant change in the eyes in which POAG did not develop represents normal aging or glaucomatous change not detected by conventional methods.

Corneal hysteresis as a risk factor for glaucoma progression: a prospective longitudinal study

PURPOSE

To evaluate the role of corneal hysteresis (CH) as a risk factor for the rate of visual field progression in a cohort of patients with glaucoma followed prospectively over time.

DESIGN

Prospective observational cohort study.

PARTICIPANTS

The study group included 114 eyes of 68 patients with glaucoma followed for an average of 4.0 ± 1.1 years. Visual fields were obtained with standard automated perimetry. Included eyes had a median number of 7 (range, 5-12) tests during follow-up.

METHODS

The CH measurements were acquired at baseline using the Ocular Response Analyzer (Reichert Instruments, Depew, NY). Evaluation of rates of visual field change during follow-up was performed using the visual field index (VFI). Linear mixed models were used to investigate the relationship between rates of visual field loss and baseline CH, baseline intraocular pressure (IOP), and central corneal thickness (CCT), while adjusting for potentially confounding factors. An interaction term between IOP and CH was included in the model to investigate whether the effect of IOP on rates of progression depended on the level of CH.

MAIN OUTCOME MEASURES

Effects of CH, IOP, and CCT on rates of VFI loss over time.

RESULTS

The CH had a significant effect on rates of visual field progression over time. In the univariable model including only CH as a predictive factor along with time and their interaction, each 1 mmHg lower CH was associated with a 0.25%/year faster rate of VFI decline over time (P<0.001). The multivariable model showed that the effect of IOP on rates of progression depended on CH. Eyes with high IOP and low CH were at increased risk for having fast rates of disease progression. The CH explained a larger proportion of the variation in slopes of VFI change than CCT (17.4% vs. 5.2%, respectively).

CONCLUSIONS

The CH measurements were significantly associated with risk of glaucoma progression. Eyes with lower CH had faster rates of visual field loss than those with higher CH. The prospective longitudinal design of this study supports the role of CH as an important factor to be considered in the assessment of the risk of progression in patients with glaucoma.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

The relationship between measurement method and corneal structure on apparent intraocular pressure in glaucoma and ocular hypertension

PURPOSE

To analyse the relationship between measured intraocular pressure (IOP) and central corneal thickness (CCT), corneal hysteresis (CH) and corneal resistance factor (CRF) in ocular hypertension (OHT), primary open-angle (POAG) and normal tension glaucoma (NTG) eyes using multiple tonometry devices.

METHODS

Right eyes of patients diagnosed with OHT (n=47), normal tension glaucoma (n=17) and POAG (n=50) were assessed. IOP was measured in random order with four devices: Goldmann applanation tonometry (GAT); Pascal dynamic contour tonometer (DCT); Reichert ocular response analyser (ORA); and Tono-Pen XL. CCT was then measured using a hand-held ultrasonic pachymeter. CH and CRF were derived from the air pressure to corneal reflectance relationship of the ORA data.

RESULTS

Compared to the GAT, the Tonopen and ORA Goldmann equivalent (IOPg) and corneal compensated (IOPcc) measured higher IOP readings (F=19.351, p<0.001), particularly in NTG (F=12.604, p<0.001). DCT was closest to Goldmann IOP and had the lowest variance. CCT was significantly different (F=8.305, p<0.001) between the 3 conditions as was CH (F=6.854, p=0.002) and CRF (F=19.653, p<0.001). IOPcc measures were not affected by CCT. The DCT was generally not affected by corneal biomechanical factors.

CONCLUSION

This study suggests that as the true pressure of the eye cannot be determined non-invasively, measurements from any tonometer should be interpreted with care, particularly when alterations in the corneal tissue are suspected.

Comparison of dynamic contour tonometry and Goldmann applanation tonometry in relation to central corneal thickness in primary congenital glaucoma

BACKGROUND

To compare intraocular pressure (IOP) measurements obtained with dynamic contour tonometer (DCT) and Goldmann applanation tonometer (GAT), and to investigate their relationship to central corneal thickness (CCT) in primary congenital glaucoma (PCG) eyes.

METHODS

Thirty-one eyes of 31 PCG patients (25.7 ± 7.2 years old) were examined. PCG was defined as elevated IOP, enlarged corneal diameter (buphthalmos), Haab's striae and abnormal findings at gonioscopy. The mean of three measurements of GAT, DCT (quality scores 1 and 2), and CCT were obtained and assessed for agreement by means of Bland-Altman plot and for Spearman correlation test.

RESULTS

Mean CCT was 534 ± 72.3 μm (range: 430 to 610 μm). Mean IOP measurements were 15.1 ± 4.2 mmHg (range: 5.5 to 22.7 mmHg) for DCT and 14.5 ± 5.6 mmHg (range: 7.0 to 34.0 mmHg) for GAT (P = 0.244). Spearman correlation tests showed that IOP difference (DCT - GAT) was not correlated with CCT (r (2) = 0.023, P = 0.417). IOP measurements by DCT were weakly but statistically correlated with those obtained with GAT (r(2) = 0.213, P = 0.0089). Bland-Altman analysis revealed poor agreement between DCT and GAT readings, considering the 95 % confidence intervals of ± 10.45 mmHg.

CONCLUSIONS

The differences between DCT and GAT readings were not influenced by CCT in this series of patients. Considering the weak correlation and the poor agreement observed between GAT and DCT measurements and that they both may be affected by corneal biomechanical changes, these methods should not be used interchangeably, and may possibly give no meaningful IOP values in PCG patients.

Corneal biomechanics and biomaterials

The ability to clearly observe one's environment in the visible spectrum provides a tremendous evolutionary advantage in most of the world's habitats. The complex optical processing system that has evolved in higher vertebrate animals gathers, focuses, detects, transduces, and interprets incoming visible light. The cornea resides at the front end of this imaging system, where it provides a clear optical aperture, substantial refractive power, and the structural stability required to protect the fragile intraocular components. Nature has resolved these simultaneous design requirements through an exceedingly clever manipulation of common extracellular-matrix structural materials (e.g., collagen and proteoglycans). In this review, we (a) examine the biophysical and optical roles of the cornea, (b) discuss increasingly popular approaches to altering its natural refractive properties with an emphasis on biomechanics, and (c) investigate the fast-rising science of corneal replacement via synthetic biomaterials. We close by considering relevant open problems that would benefit from the increased attention of bioengineers.

The influence of central corneal thickness on response to topical prostaglandin analogue therapy

OBJECTIVE

Central corneal thickness (CCT) affects intraocular pressure (IOP) readings; however, CCT influence on topical medication efficacy is unknown. We evaluated the IOP-lowering effect of topical prostaglandin analogues (PGAs) in relation to CCT.

DESIGN

Post hoc analysis of a randomized prospective trial.

METHOD

Subjects randomized to a PGA were followed for 24 weeks and were analyzed for relationship between CCT and IOP lowering.

PARTICIPANTS

Patients with either newly diagnosed ocular hypertension or open-angle glaucoma.

RESULTS

75 subjects were enrolled. The mean age was 62.7 ± 10.5 years; 48 were Caucasian. The mean CCT was 562.4 ± 41.4 μ. At repeated measures, ANCOVA analysis showed a significant effect of both baseline IOP (p < 0.0001) and CCT (p = 0.003) on IOP. At week 12, a regression analysis of the effect of CCT on baseline IOP showed that for every 10 μ increase in CCT there was 0.3 mm Hg less IOP decrease from baseline.

CONCLUSIONS

We found a statistically significantly association between a lower mean IOP and a thinner cornea when baseline IOP is controlled for. The magnitude of the relationship is small but may be clinically significant in patients with either very thin or very thick corneas.

Randomized crossover study of latanoprost and travoprost in eyes with open-angle glaucoma

INTRODUCTION

To compare the intraocular pressure (IOP)-lowering effects of 0.005% latanoprost to that of 0.004% travoprost in eyes with open-angle glaucoma (OAG).

METHODS

Forty-two patients with OAG who received either latanoprost or travoprost every evening for 12 weeks, and then switched to the other medication for another 12 weeks. The IOP measurements were made with a Goldmann applanation tonometer (GAT) at the baseline, and at 1, 3, 4, and 6 months after the treatment. The IOP at the untreated baseline and at the end of each treatment period was measured at 10:00, 12:00, and 16:00 hours. The central corneal thickness (CCT) was measured at each visit using an ultrasonic pachymeter.

RESULTS

The mean baseline IOP was 13.9 ± 2.5 mmHg, and the CCT was 536.7 ± 30.5 μm. Latanoprost reduced the IOP by 2.5 ± 1.7 mmHg and travoprost by 2.6 ± 1.5 mmHg from the baseline (p = 0.6807). The CCT decreased significantly to 531.9 ± 30.3 at 3 months (p = 0.0160) and to 529.4 ± 30.5 μm at 6 months (p = 0.0002) after the therapy. The decrease was significantly greater in eyes after travoprost (p = 0.0049).

CONCLUSIONS

Travoprost has similar effect as latanoprost in reducing the IOP in glaucoma patients with relatively low IOPs. The use of prostaglandin analogs can decrease the CCT, and this change should be considered when the IOPs obtained by GAT are analyzed.

Evaluation of corneal biomechanical properties with the Reichert Ocular Response Analyzer

PURPOSE

To compare corneal hysteresis (CH) and corneal resistance factor (CRF) measured with the Ocular Response Analyzer (ORA) in patients with primary open-angle glaucoma (POAG), patients with ocular hypertension (OHT), and normal subjects (NL); and to assess correlations of CH and CRF with corneal-compensated intraocular pressure (IOPcc), Goldmann applanation tonometry (GAT), glaucoma type, central corneal thickness (CCT), previous filtering procedure, and antiglaucoma medications.

METHODS

A total of 108 POAG, 22 OHT, and 24 NL were enrolled in this observational study. Goldmann applanation tonometry and ORA were performed in a randomized sequence followed by pachymetry. One eye per subject was selected at random for analysis. Chi-square, Wilcoxon, and Kruskal-Wallis tests were used for comparison and Spearman coefficient for assessing correlations.

RESULTS

Mean CH and CRF were significantly lower in POAG than in OHT and NL. Ocular Response Analyzer IOPcc overestimated IOP compared to GAT only in POAG. This difference increased with higher GAT. Goldmann applanation tonometry and IOPcc were correlated. Corneal hysteresis was negatively correlated with age in POAG. Corneal resistance factor and CH were positively correlated with CCT in POAG and OHT. Unlike CRF, CH was not correlated with GAT in POAG and OHT. Corneal-compensated intraocular pressure was not correlated with CCT. Difference between GAT and IOPcc was not CCT dependent. Corneal hysteresis and CRF were comparable in POAG and NTG, unchanged after filtering procedure. Corneal hysteresis was not altered by topical medications. Corneal resistance factor was significantly lower in treated eyes and those receiving prostaglandin analogues with no correlation with the treatment duration.

CONCLUSIONS

Patients with glaucoma seem to have distinctive corneal biomechanical properties compared to OHT and NL. They may be influenced by many other unknown subparameters.

Laser refractive surgery in glaucoma patients

Glaucoma may not be an absolute contraindication to Laser-Assisted in situ Keratomileusis (LASIK), but so far it is a relative one. People who are glaucoma suspects or who have glaucoma are just as likely as any other to seek laser refractive surgery. LASIK is a popular ocular procedure, relatively pain free and it is carried out with an extremely precise computer-controlled excimer laser emission. On the other hand, glaucoma is a group of diseases manifested by optic nerve damage with visual field changes. Patients that undergo a transient but significant rise in intraocular pressure during LASIK procedure have risk of further optic nerve damage. Furthermore, steroids which are typically used after refractive surgery can increase intraocular pressure (IOP) especially in steroid responders, who are more prevalent among glaucoma patient. Glaucoma patients interested in LASIK surgery may visit a glaucoma specialist or another LASIK surgeon who has had experience with performing LASIK in glaucoma patients. PRK (photorefractive keratectomy), LASEK (laser epithelial keratomileusis) are good alternatives for glaucoma patients. Refractive surgeons might want to consider giving patients a photo of their optic nerve, or a drawing, or an objective record of their preoperative examination.

Ocular findings in Quarter Horses with hereditary equine regional dermal asthenia

OBJECTIVE

To compare ocular structures of Quarter Horses homozygous for hereditary equine regional dermal asthenia (HERDA) with those of Quarter Horses not affected by HERDA (control horses) and to determine the frequency of new corneal ulcers for horses with and without HERDA during a 4-year period.

DESIGN

Cohort study of ocular structures and retrospective case series of horses with and without HERDA.

ANIMALS

The cohort portion of the study involved 10 Quarter Horses with HERDA and 10 Quarter Horses without HERDA; the retrospective case series involved 28 horses with HERDA and 291 horses without HERDA.

PROCEDURES

Ophthalmic examinations, Schirmer tear tests, tonometry, corneal pachymetry, histologic examinations, and scanning electron microscopy (SEM) were performed in cohorts of Quarter Horses with and without HERDA. Records were reviewed to determine the incidence of corneal ulcers in horses with and without HERDA during a 4-year period.

RESULTS

Corneal thickness of horses with HERDA was significantly less than that of control horses, but tear production of horses with HERDA was significantly greater than that of control horses. Results of SEM revealed zones of disorganized, haphazardly arranged collagen fibrils in corneas of horses with HERDA that were not evident in corneas of control horses. The incidence of corneal ulcers was significantly greater for horses with HERDA than for horses without HERDA during the 4-year period.

CONCLUSIONS AND CLINICAL RELEVANCE

Alterations in corneal thickness, arrangement of collagen fibers, and incidence of corneal ulcers indicated that abnormalities in horses with HERDA were not limited to the skin.