Relationship between corneal biomechanical properties, central corneal thickness, and intraocular pressure across the spectrum of glaucoma

Association between corneal hysteresis and glaucoma in a Japanese population: the Hisayama Study

AIMS

To investigate the association between corneal hysteresis and the presence of glaucoma and its subtypes in a general Japanese population.

METHODS

We analysed the data of 2338 Japanese community-dwellers aged ≥40 years (1059 men, 1279 women) who underwent an eye examination in 2018 as part of the population-based, cross-sectional Hisayama Study. Participants were divided into quartile levels of corneal hysteresis, which had been measured with an ocular response analyzer. Glaucoma was defined based on the International Society of Geographical and Epidemiological Ophthalmology criteria. We conducted a logistic regression analysis to determine the ORs and their 95% CIs for the presence of outcomes according to the corneal hysteresis quartiles.

RESULTS

Glaucoma was diagnosed in 154 participants: primary open-angle glaucoma (POAG), n=115; primary angle-closure glaucoma, n=17; exfoliation glaucoma, n=21 and secondary glaucoma without exfoliation glaucoma, n=1. After adjustment for confounders, the OR for prevalent glaucoma was significantly increased in the participants in the first corneal-hysteresis quartile compared with those in the fourth quartile (OR: 1.80; 95% CI: 1.03 to 3.17). Regarding glaucoma subtypes, the first-quartile participants had significantly greater likelihoods of the presence of POAG (OR: 1.63; 95% CI: 1.02 to 2.61) and exfoliation glaucoma (OR: 6.49; 95% CI: 1.44 to 29.30) compared with those in the third and fourth quartiles after adjustment for potential confounders.

CONCLUSIONS

These results demonstrated a significant inverse association between corneal hysteresis and the likelihood of glaucoma, suggesting that the measurement of corneal hysteresis would provide useful information for elucidating the aetiology of glaucoma.

Comparing Corneal Biomechanic Changes between Solo Cataract Surgery and Microhook Ab Interno Trabeculotomy

Background/Objectives: This study aimed to examine the postoperative changes in the corneal biomechanical properties between solo cataract surgery and solo microhook ab interno trabeculotomy (LOT). Methods: This retrospective case-control study included 37 eyes belonging to 26 patients who underwent solo cataract surgery and 37 eyes belonging to 31 patients who underwent solo µLOT. These two groups were matched according to their preoperative intraocular pressure (IOP), axial length (AL), and age. Corneal Visualization Scheimpflug Technology (Corvis ST) was used to obtain four biomechanical parameters representing the corneal stiffness or corneal deformation at the highest concavity, including stiffness parameter A1 (SP-A1), stress-strain index (SSI), peak distance (PD), and deflection amplitude max (DefAmpMax). These parameters were compared preoperatively and 6 months postoperatively, and between the two surgical groups. Results: Preoperatively, the patients' IOP, age, and AL, as well as their results in four Corvis ST parameters, were similar between the two groups (p > 0.05). No significant difference was observed in SP-A1; however, PD and DefAmpMax were significantly larger, and SSI was significantly smaller postoperatively in the LOT group than in the cataract group. Conclusions: Corneal stiffness was reduced, and the cornea was more deformed with LOT than cataract surgery.

Progressive Paracentral Visual Field Loss at Low Intraocular Pressures Following LASIK

Intraocular pressure is currently the only known reliable, modifiable risk factor for the development and progression of glaucoma. Other risk factors for glaucoma include increasing age, myopia, decreased central corneal thickness, and low corneal hysteresis (CH) measurements. Photoablative keratorefractive surgery including laser assisted in situ keratomileusis (LASIK) has become a common way to treat refractive error, with over 25 million procedures performed in the United States alone. Though myopic LASIK has been associated with a decrease in CH measurements, relatively little is known about the risk of LASIK on glaucoma onset and progression. Here we present an observational study of 4 consecutive relatively young and otherwise healthy glaucoma patients with a history of myopic LASIK who showed progression of paracentral visual field deficits at intraocular pressures of 12 mm Hg or less while being carefully monitored. Therefore, these patients required lower targets of intraocular pressure, in the single-digit range, to slow or halt progression. In this cohort, the average corneal hysteresis was more than 2 standard deviations below normal values. This series suggests that additional study into the association of LASIK and glaucoma is warranted, including the potential risk contribution of diminished CH. These studies may be particularly relevant as patients who underwent LASIK procedures in the early 2000s may now be at increased risk of glaucoma due to the risk factor of age.

Non-contact tonometry: predicting intraocular pressure using a material-corneal thickness-independent methodology

Introduction: Glaucoma, a leading cause of blindness worldwide, is primarily caused by elevated intraocular pressure (IOP). Accurate and reliable IOP measurements are the key to diagnose the pathology in time and to provide for effective treatment strategies. The currently available methods for measuring IOP include contact and non contact tonometers (NCT), which estimate IOP based on the corneal deformation caused by an external load, that in the case of NCT is an air pulse. The deformation of the cornea during the tonometry is the result of the coupling between the IOP, the mechanical properties of the corneal tissue, the corneal thickness, and the external force applied. Therefore, there is the need to decouple the four contributions to estimate the IOP more reliably. Methods: This paper aims to propose a new methodology to estimate the IOP based on the analysis of the mechanical work performed by the air jet and by the IOP during the NCT test. A numerical eye model is presented, initially deformed by the action of a falling mass to study the energy balance. Subsequently, Fluid-Structure Interaction (FSI) simulations are conducted to simulate the action of Corvis ST. Results and discussion: The new IOP estimation procedure is proposed based on the results of the simulations. The methodology is centred on the analysis of the time of maximum apex velocity rather than the point of first applanation leading to a new IOP estimation not influenced by the geometrical and mechanical corneal factors.

Central visual field in glaucoma: An updated review

Evaluation of central vision in glaucoma is important due to its impact on patients' quality of life and activities of daily living such as reading, driving, and walking. The 10-2 visual field (VF) assessment remains a mainstay in the functional analysis of central vision in glaucoma diagnosis and progression. However, it may be underutilized in clinical practice. Monitoring of disease progression especially in advanced cases, glaucoma evaluation in certain ocular disorders such as high myopia, disc hemorrhage, low corneal hysteresis, and certain optic disc phenotypes, as well as earlier detection of central VF damage, are certain conditions where additional monitoring with the 10-2 pattern may provide complementary clinical information to the commonly utilized 24-2 pattern. In addition, the development of artificial intelligence techniques may assist clinicians to most effectively allocate limited resources by identifying more risk factors to central VF damage. In this study, we aimed to determine specific patient characteristics that make central VF damage more likely and to assess the benefit of incorporating the 10-2 VF in various clinical settings.

Biomechanical and Vascular Metrics Between Eyes of Patients With Asymmetric Glaucoma and Symmetric Glaucoma

PRCIS

Corneal hysteresis (CH) and pulsatile ocular blood volume (POBV) were significantly lower in the eye with greater damage in asymmetric glaucoma, without a difference in intraocular pressure (IOP) or central corneal thickness (CCT), and no difference in elastic parameters.

OBJECTIVE

To compare biomechanical and vascular metrics between the eyes of patients with asymmetric glaucoma (ASYMM) and those with symmetric glaucoma (SYMM).

PATIENTS AND METHODS

Forty-five patients were prospectively recruited and divided into ASYMM, defined as cup-to-disc (C/D) ratio difference >0.1 between eyes and SYMM, with C/D difference ≤0.1. For ASYMM, the smaller C/D was defined as the best eye ("best") and the fellow eye was defined as the worst eye ("worse"). All metrics were subtracted as "worse" minus "best," including the viscoelastic parameter CH, and elastic parameters from the Corvis ST, including stiffness parameter at first applanation, stiffness parameter at highest concavity, integrated inverse radius, deformation amplitude ratio, IOP, CCT, mean deviation (MD), ganglion cell complex (GCC), and POBV were included. Paired t tests were performed between eyes in both groups. Statistical analyses were performed with SAS using a significance threshold of P <0.05.

RESULTS

For ASYMM (16 patients), "worse" showed significantly lower CH (-0.76 ± 1.22), POBV (-0.38 ± 0.305), MD (-3.66 ± 6.55), and GCC (-7.9 ± 12.2) compared with "best." No other parameters were significantly different. For SYMM (29 patients), there were no significantly different metrics between eyes.

CONCLUSIONS

Lower CH, POBV, GCC, and worse MD were associated with greater glaucomatous damage in asymmetric glaucoma without a difference in IOP or CCT. Lower CH and GCC are consistent with previous studies. POBV, a new clinical parameter that may indicate reduced blood flow, is also associated with greater damage.

Central corneal thickness in new cases of dry eyes: A case-control study

SIGNIFICANCE

Loss of homeostasis and chronic inflammation result in ocular surface damage in dry eyes, which is also associated with corneal thinning in established cases. Yet, the correlation between corneal thickness and new cases of dry eyes remains inadequately supported by evidence.

PURPOSE

This study aimed to compare the central corneal thickness of new cases of dry eyes to that of age- and gender-matched controls.

METHODS

A total of 45 dry eye patients were compared with 61 age- and gender-matched non-dry eye individuals. The Ocular Surface Disease Index (OSDI) questionnaire was used to evaluate symptoms, and the central corneal thickness was measured with a Nidek CEM-530 specular microscope. Patients were grouped based on disease severity (OSDI scores), and the clinical findings were compared between groups for slit-lamp examinations, Schirmer's I test, and tear film breakup time.

RESULTS

The median age of patients was 25.0 (interquartile range [IQR], 20.0 to 32.0) and 27.0 (IQR, 20.0 to 32.0) years in the control and dry eye groups, respectively (p=0.63). The median (IQR) values of the OSDI scores, tear film breakup time scores, and Schirmer's test measurements in the control groups were 10.4 (8.3 to 10.4), 12.0 (11.0 to 14.0) seconds, and 16.0 (13.5 to 19.5) mm, respectively, which differed from the dry eye groups (p<0.0001). These values in the dry eye group were 29.1 (25.0 to 39.5), 4.0 (3.0 to 8.0) seconds, and 8.0 (3.5 to 11.0) mm, respectively. Patients with dry eyes had lower central corneal thickness than controls (p<0.01). The mean ± standard deviation central corneal thicknesses in patients with dry eyes and the control group were 520.3 ± 26.8 and 545.3 ± 18.8 μm, respectively.

CONCLUSIONS

The central corneal thickness in dry eyes was significantly reduced compared with the control group. These findings may be useful in monitoring and managing dry eyes and should be considered in intraocular pressure measurements and refractive surgical procedures.

Changes in corneal biomechanics in patients with glaucoma: a systematic review and meta-analysis

INTRODUCTION

Corneal biomechanics has been implicated in a variety of ocular diseases. The purpose of this study was to evaluate the relationship between the glaucoma and corneal biomechanical properties, and exploring the value of corneal biomechanics in the diagnosis and follow-up of glaucoma diseases.

METHODS

We searched studies in PubMed, EMBASE, Web of Science and clinicaltrials.gov., as of October 8, 2022. Only English studies were included, without publication time limit. We also searched the reference lists of published reviews. This meta-analysis was conducted with random-effects models, we used mean difference(MD) to evaluate the outcome, and the heterogeneity was assessed with the I2 statistic. Subgroup analyses were performed under the appearance of high heterogeneity. We used 11 items to describe the characteristics of included studies, publication bias was performed through the Egger's test. The quality assessment were evaluated by Newcastle-Ottawa Scale(NOS) items.

RESULTS

A total of 27 eligible studies were identified for data synthesis and assessment. The result of meta-analysis showed that in the comparison of included indicators, the corneal biomechanics values of glaucoma patients were statistically lower than those of normal subjects in a similar age range. The covered indicators included central corneal thickness(CCT) (MD = -8.34, 95% CI: [-11.74, -4.94]; P < 0.001), corneal hysteresis(CH)(MD = -1.54, 95% CI: [-1.88, -1.20]; P < 0.001), corneal resistance factor(CRF)( MD = -0.82, 95% CI: [-1.21, -0.44]; P < 0.001), and intraocular pressure(IOP)( corneal-compensated intraocular pressure (IOPcc): MD = 2.45, 95% CI: [1.51, 3.38]; P < 0.001); Goldmann-correlated intraocular pressure (IOPg): MD = 1.30, 95% CI: [0.41, 2.20]; P = 0.004), they all showed statistical difference. While the value of axial length(AL) did not show statistically different(MD = 0.13, 95% CI: [-0.24, 0.50]; P = 0.48).

CONCLUSION

Corneal biomechanics are associated with glaucoma. The findings can be useful for the design of glaucoma screening, treatment and prognosis.

Corneal characteristics and OCT-angiography findings in pediatric glaucoma and glaucoma suspects

PURPOSE

To analyze corneal biomechanics, specular microscopy, and optical coherence tomography-angiography findings in children with glaucoma.

METHODS

Pediatric patients (<18 years of age) with glaucoma (n = 38), increased cup:disk ratio and normal intraocular pressure (IOP) glaucoma suspects (n = 36), and controls (n = 67) were prospectively enrolled. Patients underwent testing with Ocular Response Analyzer, CellChek Specular Microscope, and Heidelberg OCT-A.

RESULTS

Average age of participants was 12.4 ± 3.5 years, with no difference between groups (P = 0.71). Glaucoma patients had undergone more intraocular surgeries (P < 0.0001) and showed worse logMAR visual acuity (P < 0.0001) than suspects or controls. Central corneal thickness (CCT) was greater in glaucoma patients (642.8 ± 85.9 μm [P < 0.0001]) and suspects (588 ± 43.7 μm [P = 0.003]) compared with controls (561 ± 39.9 μm). Corneal hysteresis (CH) was decreased in eyes with glaucoma (10.4 ± 3.0) compared with controls (11.7 ± 1.5 [P = 0.006]), but not suspects (11.3 ± 2.0 [P = 0.1]). Glaucoma patients had lower endothelial cell density (2028.4 ± 862.7 [P < 0.0001]) and greater average cell area (547.2 ± 332.4 [P < 0.0005]) compared with suspects (2919.3 ± 319.1, 347.5 ± 46.2) and controls (2913.7 ± 399.2, 350.8 ± 57.7), but there was no difference in polymegathism (P = 0.12) or pleomorphism (P = 0.85). No differences in vessel density or vessel skeletal density in the retinal vascular complex (P = 0.3077, P = 0.6471) or choroidal vascular complex (P = 0.3816, P = 0.7306) were detected.

CONCLUSIONS

Children with glaucoma showed thicker corneas with lower endothelial cell density and greater cell area, but no difference in retinal/choroidal vascular densities compared with suspects and controls.

Comparing corneal biomechanic changes among solo cataract surgery, microhook ab interno trabeculotomy and iStent implantation

Minimally invasive glaucoma surgery has expanded the surgical treatment options in glaucoma, particularly when combined with cataract surgery. It is clinically relevant to understand the associated postoperative changes in biomechanical properties because they are influential on the measurement of intraocular pressure (IOP) and play an important role in the pathogenesis of open-angle glaucoma (OAG). This retrospective case-control study included OAG patients who underwent cataract surgery combined with microhook ab interno trabeculotomy (µLOT group: 53 eyes of 36 patients) or iStent implantation (iStent group: 59 eyes of 37 patients) and 62 eyes of 42 solo cataract patients without glaucoma as a control group. Changes in ten biomechanical parameters measured with the Ocular Response Analyzer and Corneal Visualization Scheimpflug Technology (Corvis ST) at 3 and 6 months postoperatively relative to baseline were compared among the 3 groups. In all the groups, IOP significantly decreased postoperatively. In the µLOT and control groups, significant changes in Corvis ST-related parameters, including stiffness parameter A1 and stress‒strain index, indicated that the cornea became softer postoperatively. In contrast, these parameters were unchanged in the iStent group. Apart from IOP reduction, the results show variations in corneal biomechanical changes from minimally invasive glaucoma surgery combined with cataract surgery.

The influence of topical anesthetic and fluorescein on non-contact tonometry measurements using ultra-high-speed dynamic Scheimpflug

This study aimed to investigate the effects of topical anesthetic and fluorescein drops on intraocular pressure (IOP), central corneal thickness (CCT) and biomechanical properties as measured by Corvis ST (CST-Oculus; Wezlar, Germany) in healthy eyes. A cross-sectional observational study was conducted on 46 healthy patients. The CST measurements were obtained before and immediately after the instillation of topical anesthetic and fluorescein drops. Pre-post instillation data were statistically analyzed. IOP measurements were compared to Goldmann's Applanation Tonometry (GAT), which was also performed after drops instillation. Biomechanical parameters analyzed included applanation 1 velocity, applanation 2 velocity, applanation 1 time, applanation 2 time, whole eye movement, deflection amplitude, and stiffness parameter at first applanation. A statistically significant difference in IOP, both for non-corrected IOP (IOPnct) and biomechanically corrected IOP (bIOP), was observed before and after the instillation of eyedrops. Despite this statistical significance, the observed difference lacked clinical relevance. The IOPnct demonstrated a significant difference pre and post-anesthetic and fluorescein instillation compared to GAT (14.99 ± 2.27 mmHg pre-instillation and 14.62 ± 2.50 mmHg post-instillation, versus 13.98 ± 2.04 mmHg, with p-values of 0.0014 and 0.0490, respectively). Comparable findings were noted when justaposing bIOP to GAT (14.53 ± 2.10 mmHg pre-instillation and 13.15 ± 2.25 mmHg post-instillation, against 13.98 ± 2.04 mmHg, with p-values of 0.0391 and 0.0022, respectively). Additionally, CCT measurements revealed a statistically significant elevation following the administration of topical anesthetic and fluorescein drops (from 544.64 ± 39.85 µm to 586.74 ± 41.71 µm, p < 0.01. None of the analyzed biomechanical parameters showed statistically significant differences after drops instillation. While the administration of topical anesthetic and fluorescein drops induced a statistically significant alteration in both IOPnct and bIOP readings, these changes were not clinically consequential. Furthermore, a notable statistical rise was observed in CCT measurements post-drops instillation, as determined by CST. Yet, corneal biomechanical parameters remained unaffected.

Dependence of corneal hysteresis on non-central corneal thickness in healthy subjects

OBJECTIVE

To evaluate the dependence of corneal hysteresis (CH) on non-central corneal thickness.

METHODS

Cross-sectional study of 1561 eyes of 1561 healthy volunteers with IOP less than 21mmHg, open angles on gonioscopy and no prior eye surgeries or local or systemic diseases. Pentacam-Scheimpflug technology was employed to segment the cornea into 6 circular zones centered on the apex (zones 1-6) and to determine the mean corneal thickness of these areas. CH was measured with ORA. Univariate and multivariate linear regression models adjusted for age and sex were created to model the dependence of CH on corneal thickness in zones 1 to 6.

RESULTS

In the univariate linear regression models, we found that CH was dependent on mean corneal thickness of zone 1 (B=0,004; R2=0.95%; P<0.001), zone 2 (B=0,004; R2=0.57%; P=0.002), zone 4 (B=0,005; R2=1.50%; P<0.001) and zone 6 (B=0,003; R2=0.92%; P<0.001). Similar results were obtained in the multivariate model (R2=3.46%; P<0.001).

CONCLUSION

This study suggests a significant dependence of CH on non-central corneal thickness. The model of corneal thickness segmentation into circular zones centered on the corneal apex is able to explain 3.47% of the variation in CH measurements.

Corneal Biomechanical Measures for Glaucoma: A Clinical Approach

Over the last two decades, there has been growing interest in assessing corneal biomechanics in different diseases, such as keratoconus, glaucoma, and corneal disorders. Given the interaction and structural continuity between the cornea and sclera, evaluating corneal biomechanics may give us further insights into the pathogenesis, diagnosis, progression, and management of glaucoma. Therefore, some authorities have recommended baseline evaluations of corneal biomechanics in all glaucoma and glaucoma suspects patients. Currently, two devices (Ocular Response Analyzer and Corneal Visualization Schiempflug Technology) are commercially available for evaluating corneal biomechanics; however, each device reports different parameters, and there is a weak to moderate agreement between the reported parameters. Studies are further limited by the inclusion of glaucoma subjects taking topical prostaglandin analogues, which may alter corneal biomechanics and contribute to contradicting results, lack of proper stratification of patients, and misinterpretation of the results based on factors that are confounded by intraocular pressure changes. This review aims to summarize the recent evidence on corneal biomechanics in glaucoma patients and insights for future studies to address the current limitations of the literature studying corneal biomechanics.

Relationship between Inter-Eye Asymmetries in Corneal Hysteresis and Visual Field Severity in Patients with Primary Open-Angle Glaucoma

This study investigated the influence of asymmetric corneal hysteresis (CH) on asymmetric visual field impairment between right and left eyes in patients with primary open-angle glaucoma (POAG) without a history of intraocular surgery. CH, corneal resistance factor (CRF), and corneal compensated intraocular pressure (IOPcc) were measured using the Ocular Response Analyzer. Differences between the eyes (right eye-left eye: DIF_RL) and CH-based and in target parameters (higher CH eye-lower CH eye: DIF_CH) were calculated in the same patient. In 242 phakic eyes of 121 patients, older age (p < 0.001), lower CH (p = 0.001), and lower CRF (p = 0.007) were significantly associated with worse standard automated perimetry (SAP) 24-2 mean deviation (MD). The DIFs_RL in axial length (p = 0.003), IOPcc (p = 0.028), and CH (p = 0.001) were significantly associated with the DIF_RL in SAP24-2 MD, but not in central corneal thickness (CCT), Goldmann applanation tonometry (GAT) measurement, and CRF. When dividing the patients into two groups based on the median of the CH DIFs_CH (0.46), the DIFs_CH in CRF (p < 0.001), IOPcc (p < 0.001), CCT (p = 0.004), SAP24-2 MD (p < 0.001), and SAP10-2 MD (p = 0.010) were significantly different between the groups. Large inter-eye asymmetry in CH is an important explanatory factor for disease worsening in patients with POAG.

Corneal Hysteresis for the Diagnosis of Glaucoma and Assessment of Progression Risk: A Report by the American Academy of Ophthalmology

PURPOSE

To review the current published literature on the utility of corneal hysteresis (CH) to assist the clinician in the diagnosis of glaucoma or in the assessment of risk for disease progression in existing glaucoma patients.

METHODS

Searches of the peer-reviewed literature in the PubMed database were performed through July 2022. The abstracts of 423 identified articles were examined to exclude reviews and non-English articles. After inclusion and exclusion criteria were applied, 19 articles were selected, and the panel methodologist rated them for level of evidence. Eight articles were rated level I, and 5 articles were rated level II. The 6 articles rated level III were excluded.

RESULTS

Corneal hysteresis is lower in patients with primary open-angle glaucoma, primary angle-closure glaucoma, pseudoexfoliative glaucoma, and pseudoexfoliation syndrome compared with normal subjects. Interpretation of low CH in patients with high intraocular pressure (IOP) or on topical hypotensive medications is complicated by the influence of these parameters on CH measurements. However, CH is also lower in treatment-naïve, normal-tension glaucoma patients compared with normal subjects who have a similar IOP. In addition, lower CH is associated with an increased risk of progression of glaucoma based on visual fields or structural markers in open-angle glaucoma patients, including those with apparently well-controlled IOP.

CONCLUSIONS

Corneal hysteresis is lower in glaucoma patients compared with normal subjects, and lower CH is associated with an increased risk of disease progression. However, a causal relationship remains to be demonstrated. Nevertheless, measurement of CH complements current structural and functional assessments in determining disease risk in glaucoma suspects and patients.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

Effect of Corneal Hysteresis on the Rates of Microvasculature Loss in Glaucoma

PURPOSE

To investigate the association between corneal hysteresis (CH) and rates of optic nerve head whole image capillary density (wiCD) loss over time in open-angle glaucoma (OAG).

DESIGN

Observational cohort.

PARTICIPANTS

One hundred seventy-four eyes (122 OAG and 52 glaucoma suspect eyes) from 112 patients over more than 2 years and 4 visits or more.

METHODS

Baseline CH measurements were acquired with the Ocular Response Analyzer. Linear mixed-effect models were designed to investigate the effect of CH, average intraocular pressure (IOP) during follow-up, and baseline visual field (VF) mean deviation (MD) on the rates of wiCD loss and circumpapillary retinal nerve fiber layer (cpRNFL) thinning over time, while adjusting for confounders. Interaction between CH or baseline MD and average IOP during follow-up were included in final models to evaluate the effect of baseline MD or average IOP during follow-up on structural changes for different values of CH.

MAIN OUTCOME MEASURE

Effect of CH, IOP, and baseline MD on the rates of wiCD loss and cpRNFL thinning over time.

RESULTS

The average follow-up time was 3.9 years. In the multivariable model, non-Black race, higher average IOP during follow-up, lower baseline CH, lower baseline VF MD, and higher numbers of IOP-lowering medications were associated with faster rates of wiCD loss over time. For CH values 6 mmHg and 12 mmHg, every 1-mmHg increase in average IOP during follow-up was associated with 0.23% per year faster and 0.07% per year slower rates of wiCD loss over time, respectively. While every 1-mmHg decrease in CH was associated with 1.89% per year faster rate of wiCD loss for MD of -12 dB, it was associated with 0.81% per year faster rate of wiCD loss for MD of -3 dB.

CONCLUSION

Lower CH values were significantly associated with faster rates of wiCD loss over time. In eyes with lower CH, both higher average IOP during follow-up and more severe glaucoma damage at baseline were associated with faster rates of wiCD loss and cpRNFL thinning. These results support CH as a useful parameter for risk assessment of glaucoma progression.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

Corneal Biomechanics in Primary Open Angle Glaucoma and Ocular Hypertension: A Systematic Review and Meta-analysis

PRCIS

Normal tension glaucoma patients had softer corneas than normal controls, whereas high-tension glaucoma and ocular hypertension patients had stiffer corneas.

PURPOSE

To comprehensively identify the corneal biomechanical differences of patients with primary open angle glaucoma (POAG) and ocular hypertension (OHT) using the Ocular Response Analyzer or the Corvis ST.

METHODS

The electronic databases PubMed, Embase, and Web of Science were comprehensively searched for studies comparing corneal biomechanical differences between POAG and OHT patients with normal controls by Ocular Response Analyzer or Corvis ST. The weighted mean differences and 95% confidence intervals (CIs) were calculated. Subgroup analyses were performed according to the subtypes of POAG, including high-tension glaucoma (HTG) and normal tension glaucoma (NTG).

RESULTS

Thirty-one case-control studies were ultimately included, with 2462 POAG patients, 345 OHT patients, and 3281 normal controls. The corneal hysteresis (CH), corneal resistance factor (CRF), and highest concavity time (HC-t) were all lower in POAG patients than in normal controls. The CH, time at the second applanation (A2t), HC-t, highest concavity radius (HC-R), and deformation amplitude at the highest concavity (HC-DA) were lower in OHT patients, while the CRF, time at the first applanation (A1t), and stiffness parameter at the first applanation (SP-A1) were greater in OHT patients than in normal controls. The subgroup analyses showed that the CH, A2t, length at the second applanation (A2L), and HC-DA were lower in HTG, and the CH, CRF, A1t, and HC-t were lower in NTG patients than in normal controls.

CONCLUSION

The corneas of NTG patients are more deformable than normal controls, whereas the corneas of HTG and OHT patients are stiffer.

Corneal Biomechanics Assessment with Ultra High Speed Scheimpflug Camera in Primary Open Angle Glaucoma Compared with Healthy Subjects: A meta-analysis of the Literature

PURPOSE

The aim of this meta-analysis of the literature is to provide a comprehensive analysis of the differences in Corvis ST dynamic corneal response (DCR) parameters between primary open-angle glaucoma (POAG) patients and healthy controls.

METHODS

A quantitative meta-analysis was conducted on articles published before September 10, 2021 identified by searching PubMed, EMBASE, and Web of Science. Prospective studies comparing DCR Corvis ST parameter in high tension POAG and healthy controls were included. The random-effects model was conducted. Assessment of heterogeneity was based on the calculation of I². Funnel plots evaluation and meta-regression were performed in case of detection of high heterogeneity.

RESULTS

The selection process resulted in the inclusion of six articles. Pooled analysis revealed that POAG corneas respond to mechanical stimulus with a smaller concavity, showing lower deformation amplitude (DA) (CI95% -0.991 to -0.578; p < .001; I² = 0%), higher highest concavity radius (HCR; confidence interval [CI]95% -0.01 to 0.34; p = .058; I² = 6.7%), and lower peak distance (PD; CI95% -1.06 to -0.024; p = .040; I² = 86.5%). They also show a slower loading phase, with lower highest concavity time (HCT; CI95% -0.39 to -0.02; p = .029; I² = 3.3%) and lower applanation velocity-1 (CI95% -0.641 to -0.127; p = .003; I² = 34.6%), and a faster restoration to the original form, shown by lower applanation time-2 (CI95% -1.123 to -0.544; p = .001; I² = 44.8%) compared to healthy subjects.

CONCLUSIONS

High tension POAG patients are characterized by stiffer corneas compared to healthy controls. These differences are valid also after removing the effect of age, corneal thickness, and intraocular pressure (IOP).

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.

Relationship between Diurnal Variation in Intraocular Pressure and Central Corneal Power

SIGNIFICANCE

Relationship between intraocular pressure (IOP) change and central corneal curvature is complicated by measurement techniques and corneal biomechanical parameters. Findings from this study indicate that it is worthwhile to observe the association between diurnal change in IOP and corneal power.

PURPOSE

This study aimed to investigate the relationship between the diurnal change in IOP and central corneal power among eyes with and without myopia.

METHODS

Sixty healthy eyes of 24 emmetropes and 36 myopes were recruited for this cross-sectional study. Both anterior and posterior central corneal powers of the steep (Ks), flat (Kf), mean meridian (Km), best-fit spheres, and central corneal thickness were recorded followed by the IOP (Goldmann-correlated IOP [IOPg] and corneal-compensated IOP) and corneal biomechanics (corneal hysteresis and corneal resistance factor). Measurements were obtained every 3 hours from 9.30 am to 6.30 pm . Linear-mixed model was used to determine the relationship between the change in IOP and the associated change in corneal measurements (adjusted for age, sex, refractive error, central corneal thickness, and biomechanics) among the myopic and nonmyopic eyes.

RESULTS

Group mean, amplitude of change, and the diurnal change in IOPg were (mean ± standard deviation) 15.14 ± 2.50, 3.33 ± 1.44, and 1.81 ± 1.25 mmHg, respectively. Overall, an IOP increase was associated with a decrease in the adjusted anterior corneal powers. Myopic eyes were associated with a decrease of 0.04 D (95% confidence interval [CI], 0.07 to 0.01 D; P = .02) in Ks and 0.03 D (95% CI, 0.06 to 0.001 D; P = .047) in Kf per mmHg increase in IOP, whereas for emmetropes, per mmHg increase in IOP only flattened the Kf by 0.03 D (95% CI, 0.06 to 0.004 D; P = .02).

CONCLUSIONS

Change in anterior corneal power was inversely related to the change in IOPg, with myopic and nonmyopic eyes reporting a significant but differential impact of IOP. Clinicians must keep in mind the impact of large IOP fluctuation on the anterior corneal power.

Repeatability of Corneal Hysteresis Measurements in Glaucoma Patients During Routine Follow Up and After Cataract Surgery

PRCIS

Corneal hysteresis (CH) measurements are relatively stable during routine follow up of glaucoma patients over an intermediate time frame. Cataract surgery does not change the CH significantly after an average follow up of 6 months.

PURPOSE

The aim was to assess the repeatability of CH measurements in glaucoma patients over time, during routine follow up and after cataract surgery.

MATERIALS AND METHODS

Retrospective analysis of patients records in a glaucoma clinic where routine measurements by the Ocular Response Analyzer were done. Patients with at least 2 CH measurements were included. Repeatability was assessed using the intraclass correlation coefficient (ICC).

RESULTS

A total of 164 eyes (87 patients) were included. Twenty-eight of them had measurements before and after cataract surgery with mean follow up time of 29.64±9.63 weeks. There was no evidence for a difference in CH between the before and after cataract surgery measurements (ICC=0.79, 95% confidence interval: 0.60-0.89). In the remaining 136 eyes, without any surgical treatment between measurements, there was moderate agreement among the repeated CH measurements (ICC=0.64, 95% confidence interval: 0.61-0.82) with mean time of 32.06±25.32 weeks between first and last measurement.

CONCLUSION

CH measurements in glaucoma patients were repeatable over a 6-month period during routine follow up or following cataract surgery. These findings suggest that the intraocular pressure reduction following cataract surgery is unlikely to be because of a change in this biomechanical property.

Association Between Ocular Biomechanics Measured With Corvis ST and Glaucoma Severity in Patients With Untreated Primary Open Angle Glaucoma

Purpose

To compare the ocular biomechanical differences between normal controls and patients with untreated primary open angle glaucoma, including normal-tension glaucoma (NTG) and high-tension glaucoma (HTG), and to investigate the association between ocular biomechanics and glaucoma severity in each group.

Methods

One hundred fifty-three eyes of 153 subjects, including 51 controls, 47 NTG, and 55 HTG cases, were enrolled in this cross-sectional study. Each participant underwent biomechanical measurements by using the Corneal Visualization Scheimpflug Technology. Glaucoma severity was evaluated by mean deviation (MD), pattern standard deviation (PSD), ganglion cell complex (GCC), and retinal nerve fiber layer (RNFL) thickness.

Results

Deformation amplitude (P = 0.001) significantly increased, whereas first applanation time (P < 0.0001), highest concavity time (P = 0.001), stiffness parameter at first applanation (P = 0.009), and time of whole eye movement (WEM, P = 0.008) decreased significantly in NTG eyes compared with controls. Besides, NTG had the highest first applanation velocity than controls (P < 0.0001) and HTG (P = 0.044). Shorter time of WEM was independently correlated with worse MD (P = 0.02) and higher values of PSD (P = 0.03) in NTG. Axial length was positively related to PSD (P = 0.02) and negatively related to GCC (P < 0.0001) and RNFL (P < 0.0001) thickness in HTG.

Conclusions

NTG corneas are more deformable than healthy ones and HTG. Time of WEM, which relates to orbital compliance, is significantly associated with glaucomatous visual field defect in NTG, whereas axial length is correlated with glaucoma severity in HTG.

Translational Relevance

Ocular biomechanics may partly account for the differences of pathogenic mechanisms between NTG and HTG.

Corneal Behavior during Tonometer Measurement during the Water Drinking Test in Eyes with XEN GelStent in Comparison to Non-Implanted Eyes

Biomechanics of the cornea have significant influences on the non-contact measurement of the intraocular pressure. The corneal behaviour during tonometry is a fundamental factor in estimating its value. The aim of the study was to analyse the behaviour of the cornea during tonometric measurement with the forced change in intraocular pressure during the water drinking test. Ocular Response Analyser (Reichert) was used to the measurement. Besides four basic parameters connected with intraocular pressure (IOPg, IOPcc) and biomechanics (corneal hysteresis CH and corneal resistance factor (CRF), other parameters representing the behaviour of the cornea during a puff of air were analysed. There were 47 eyes included in the study, including 27 eyes with a XEN GelStent implanted and 20 without it. The eyes of people with monocular implementation were the reference group. The values of analysed parameters were compared before and after 10, 25, 40, and 55 min after drinking the water. The intraocular pressure increased by 2.4 mmHg (p < 0.05) for eyes with a XEN stent and 2.2 mmHg for eyes without a stent (p < 0.05) in the tenth minute after drinking of water. This change caused a decreasing of corneal hysteresis (p < 0.05) without significant changes in the corneal resistance factor (p > 0.05). Corneal hysteresis changed similarly in the reference group and the group with a XEN GelStent. The analysis of additional parameters showed a difference in the behaviour of the cornea in eyes with a XEN GelStent in comparison to the corneas of eyes without a stent. This was particularly visible in the analysis of the cornea’s behaviour during the second applanation, when the cornea returns to its baseline state after deformation caused by air puff tonometry.

Corneal Hysteresis, Intraocular Pressure, and Progression of Glaucoma: Time for a “Hyst-Oric” Change in Clinical Practice?

It is known that as people age their tissues become less compliant and the ocular structures are no different. Corneal Hysteresis (CH) is a surrogate marker for ocular compliance. Low hysteresis values are associated with optic nerve damage and visual field loss, the structural and functional components of glaucomatous optic neuropathy. Presently, a range of parameters are measured to monitor and stratify glaucoma, including intraocular pressure (IOP), central corneal thickness (CCT), optical coherence tomography (OCT) scans of the retinal nerve fibre layer (RNFL) and the ganglion cell layer (GCL), and subjective measurement such as visual fields. The purpose of this review is to summarise the current evidence that CH values area risk factor for the development of glaucoma and are a marker for its progression. The authors will explain what precisely CH is, how it can be measured, and the influence that medication and surgery can have on its value. CH is likely to play an integral role in glaucoma care and could potentially be incorporated synergistically with IOP, CCT, and visual field testing to establish risk stratification modelling and progression algorithms in glaucoma management in the future.

Corneal hysteresis: ready for prime time?

PURPOSE OF THE REVIEW

This review summarizes recent findings on corneal hysteresis, a biomechanical property of the cornea. Corneal hysteresis measurements can be easily acquired clinically and may serve as surrogate markers for biomechanical properties of tissues in the back of the eye, like the lamina cribrosa and peripapillary sclera, which may be related to the susceptibility to glaucomatous damage.

RECENT FINDINGS

Several studies have provided evidence of the associations between corneal hysteresis and clinically relevant outcomes in glaucoma. Corneal hysteresis has been shown to be predictive of glaucoma development in eyes suspected of having the disease. For eyes already diagnosed with glaucoma, lower corneal hysteresis has been associated with higher risk of progression and faster rates of visual field loss over time. Such associations appear to be stronger than those for corneal thickness, suggesting that corneal hysteresis may be a more important predictive factor. Recent evidence has also shown that cornealcorrected intraocular pressure measurements may present advantages compared to conventional Goldmann tonometry in predicting clinically relevant outcomes in glaucoma.

SUMMARY

Given the evidence supporting corneal hysteresis as an important risk factor for glaucoma development and its progression, practitioners should consider measuring corneal hysteresis in all patients at risk for glaucoma, as well as in those already diagnosed with the disease.

Effects of Corneal Biomechanical Properties on Rebound Tonometry (Icare200) and Applanation Tonometry (Perkins) Readings in Patients With Primary Congenital Glaucoma

OBJECTIVE

The aim was to assess the influence of corneal biomechanics on intraocular pressure (IOP) measurements made with the Icare200 (IC200) rebound tonometer and the Perkins handheld applanation tonometer in patients with primary congenital glaucoma (PCG).

MATERIALS AND METHODS

A total of 40 PCG patients and 40 healthy controls, age, and sex-matched, were recruited. IOP was measured with the Ocular Response Analyzer (IOPc, IOPg), IC200 and Perkins. The variables age, IOP, corneal hysteresis (CH), corneal resistance factor (CRF), central corneal thickness (CCT), best-corrected visual acuity, spherical equivalent, medications, and glaucoma surgeries were recorded for each subject. Univariate and multivariate analysis were used to detect effects of variables on IOP measurements.

RESULTS

Mean CCT was 545.65±71.88 μm in PCG versus 558.78±27.58 μm in controls (P=0.284). CH and CRF were significantly lower in PCG group than in control group: mean CH 8.11±1.69 versus 11.15±1.63 mm Hg (P<0.001), and mean CRF 9.27±2.35 versus 10.71±1.75 mm Hg (P=0.002). Mean differences between IOP IC200-Perkins were 0.79±0.53 mm Hg in PCG versus 0.80±0.23 mm Hg in controls (P<0.001) and mean differences IC200-IOPc were -0.89±5.15 mm Hg in PCG (P<0.001) versus 1.60±3.03 mm Hg in controls (all P<0.009). Through multivariate analysis, CRF showed positive association and CH negative association with IOP measured with Perkins or IC200 in both subject groups. No association was detected for CCT, age, or sex.

CONCLUSION

CH and CRF were identified as the main factors interfering with IOP measurements made with both tonometers in patients with PCG and healthy controls.

Differences and Limits of Agreement among Pentacam, Corvis-ST, and IOL-Master 700 Optical Biometric Devices regarding Central Corneal Thickness Measurements

Purpose:

To investigate the differences and limits of agreement in measuring corneal thickness using Pentacam, Corvis, and intraocular lens (IOL)-Master 700 devices.

Methods:

This study was conducted on 37 right eyes of 21 males and 16 females (n = 37) with a mean age of 52.11 ± 6.30 years. The central corneal thickness was measured using three optical biometric devices, including Pentacam, Corvis, and IOL-Master 700. The inclusion criteria were normal eyes without any ophthalmological abnormalities, history of ocular pathology, or ocular surgery. The data obtained from these three devices were compared two by two. The correlation and agreement limits among them were analyzed using statistical techniques.

Results:

The mean standard deviation differences between Pentacam and Corvis, Pentacam and IOL-Master 700, as well as Corvis and IOL-Master 700 regarding the corneal thickness measurement, were 22.13 ± 8.05, 7.91 ± 8.02, and 14.21 ± 9.85 μm, respectively, which were statistically significant (P < 0.0001). Based on the investigation of the limits of agreement according to the Bland Altman method, the corresponding values between Pentacam and Corvis, Pentacam and IOL-Master 700, and Corvis and IOL-Master 700 were -16.2 to +15.4, -15.8 to +16.3, and -20.1 to +20.0 μm, respectively. Furthermore, the correlation coefficients of the measurements obtained by Pentacam and Corvis, Pentacam and IOL-Master 700, as well as Corvis and IOL-Master 700 were determined 0.957, 0.964, and 0.948, respectively (P < 0.0001).

Conclusion:

The results from this study indicate that the interchangeable use of these three devices is not appropriate due to statistically significant differences and broad limits of agreement among the three devices, especially between Corvis and IOL-Master 700.

Efficacy of corneal shape index in the evaluation of ocular hypertension, primary open-angle glaucoma and exfoliative glaucoma

Purpose:

The aim of the present study is to calculate Concavity Shape Index (CSI) in patients with POAG and exfoliative glaucoma (XFG) and correlate CSI with the severity of glaucoma, comparing to control and ocular hypertension (OHT) patients.

Methods:

This was a cross‑sectional study with 146 eyes/146 subjects: 37 healthy eyes, 23 eyes with OHT and 86 glaucoma eyes (70 with POAG, 16 with XFG). The severity of glaucoma was scored with the Glaucoma Staging System 2 (GSS2). Corvis ST® was used to calculate CSI.

Results:

Central corneal thickness (CCT) was significantly thinner in POAG (526 ± 40.0 µm) and XFG (520 ± 38.2 µm) than control group (553 ± 28.8 µm). CSI had no significant differences between the groups. XFG had a higher mean of GSS 2 (2.42 ± 1.38) than POAG (1.87 ± 1.55) and OHT (1.87 ± 1.55). OHT had a significantly less deformable cornea than: control (higher A1 length, lower A1 velocity, higher A2 velocity), POAG (higher A1 length, lower A1 velocity, lower deflection amplitude at highest concavity), and XFG group (lower A1 velocity, lower deflection amplitude at highest concavity), which was independent of age and CCT. No significant correlation was found between GSS 2 and CSI.

Discussion:

OHT patients had stiffer corneas (less deformed by the air puff) when compared to control, POAG or XFG patients. A less deformable cornea could potentially be related to a more resistant LC/peripapillary sclera. As such, this would result in a lesser optic nerve susceptibility to IOP damage.

Clinical Outcomes of Laser Asymmetric Keratectomy to Manage Postoperative Adverse Effects–A Retrospective Clinical Trial

Background:

Laser asymmetric keratectomy reduces the regional asymmetry of corneal thickness.

Objective:

We aimed to describe the clinical outcomes of laser asymmetric keratectomy keratectomy with laser refractive surgery performed to resolve the adverse effects following ophthalmic surgeries.

Methods:

We compared the preoperative and postoperative outcomes and complaints of blurring after performing laser asymmetric keratectomy with laser refractive surgery in 24 eyes of 16 patients with a deviation sum in corneal thickness in four directions >80 µm. Laser asymmetric keratectomy with laser refractive surgery, with full integration of the Vision Up software, was used to analyze the corneal thickness deviation, employed selective laser ablation to create central symmetry on the thicker cornea to reduce regional asymmetry of corneal thickness, simultaneously correcting the refractive power and myopic shift. The pre-and postoperative clinical and topographic findings were analyzed.

Results:

The patients’ age was 37.57±22.30 (range, 23–65) years. The follow-up period was 16.56±3.23 months. The spherical equivalent (p=0.026), sphere (p=0.022), uncorrected distance visual acuity (LogMAR, p=0.045), blurring score (p=0.000), central corneal thickness (p=0.024), sum of deviations in corneal thickness in four directions (p=0.02), distance between the maximum posterior elevation and visual axis (p=0.04), and kappa angle (p=0.031), significantly decreased postoperatively. The efficiency and safety indices were 0.96±0.11 and 1.00, respectively. There was no myopic regression or blurred vision postoperatively.

Conclusion:

Performing laser asymmetric keratectomy with laser refractive surgery improved corneal symmetry and visual acuity and reduced blurring.

Corneal biomechanical properties in patients with kidney transplant

PURPOSE

The purpose of the study was to evaluate the corneal biomechanical properties in patients post kidney transplant and to compare them with healthy age-matched control.

MATERIALS AND METHODS

In this cross-sectional study, 68 patients with kidney transplant (study group) and 68 healthy individuals (control group) were analyzed with ocular response analyzer measurements. Only the right eye of each participant was assessed for corneal hysteresis (CH), corneal resistance factor (CRF), corneal-compensated intraocular pressure (IOP_cc), and Goldmann-correlated intraocular pressure (IOP_g).

RESULTS

The mean CH in the study group was 9.70 ± 1.62 mmHg which was significantly lower than the control group with the mean CH of 10.60 ± 1.49 mmHg (P = 0.001). The mean IOP_cc was 18.17 ± 4.00 mmHg and 16.00 ± 3.94 mmHg in the study and control groups, respectively (P = 0.002). CRF and IOP_g were not significantly different between groups.

CONCLUSION

CH and IOP_ccare altered in kidney transplant patients.

Corneal Biomechanical Parameters and Central Corneal Thickness in Glaucoma Patients, Glaucoma Suspects, and a Healthy Population

PURPOSE

To evaluate and compare corneal hysteresis (CH), corneal resistance factor (CRF), and central corneal thickness (CCT), measurements were taken between a healthy population (controls), patients diagnosed with glaucoma (DG), and glaucoma suspect patients due to ocular hypertension (OHT), family history of glaucoma (FHG), or glaucoma-like optic discs (GLD). Additionally, Goldmann-correlated intraocular pressure (IOPg) and corneal-compensated IOP (IOPcc) were compared between the different groups of patients.

METHODS

In this prospective analytical-observational study, a total of 1065 patients (one eye of each) were recruited to undergo Ocular Response Analyzer (ORA) testing, ultrasound pachymetry, and clinical examination. Corneal biomechanical parameters (CH, CRF), CCT, IOPg, and IOPcc were measured in the control group (n = 574) and the other groups: DG (n = 147), FHG (n = 78), GLD (n = 90), and OHT (n = 176). We performed a variance analysis (ANOVA) for all the dependent variables according to the different diagnostic categories with multiple comparisons to identify the differences between the diagnostic categories, deeming p < 0.05 as statistically significant.

RESULTS

The mean CH in the DG group (9.69 mmHg) was significantly lower compared to controls (10.75 mmHg; mean difference 1.05, p < 0.001), FHG (10.70 mmHg; mean difference 1.00, p < 0.05), GLD (10.63 mmHg; mean difference 0.93, p < 0.05) and OHT (10.54 mmHg; mean difference 0.84, p < 0.05). No glaucoma suspects (FHG, GLD, OHT groups) presented significant differences between themselves and the control group (p = 1.00). No statistically significant differences were found in the mean CRF between DG (11.18 mmHg) and the control group (10.75 mmHg; mean difference 0.42, p = 0.40). The FHG and OHT groups showed significantly higher mean CRF values (12.32 and 12.41 mmHg, respectively) than the DG group (11.18 mmHg), with mean differences of 1.13 (p < 0.05) and 1.22 (p < 0.001), respectively. No statistically significant differences were found in CCT in the analysis between DG (562 μ) and the other groups (control = 556 μ, FHG = 576 μ, GLD = 569 μ, OHT = 570 μ). The means of IOPg and IOPcc values were higher in the DG patient and suspect groups than in the control group, with statistically significant differences in all groups (p < 0.001).

CONCLUSION

This study presents corneal biomechanical values (CH, CRF), CCT, IOPg, and IOPcc for diagnosed glaucoma patients, three suspected glaucoma groups, and a healthy population, using the ORA. Mean CH values were markedly lower in the DG group (diagnosed with glaucoma damage) compared to the other groups. No significant difference was found in CCT between the DG and control groups. Unexpectedly, CRF showed higher values in all groups than in the control group, but the difference was only statistically significant in the suspect groups (FHG, GLD, and OHT), not in the DG group.

Synergic effect of corneal hysteresis and central corneal thickness in the risk of early-stage primary open-angle glaucoma progression

PURPOSE

To evaluate corneal hysteresis (CH), acquired with ocular response analyzer (ORA), as a risk factor for glaucoma progression in early-stage primary open-angle glaucoma (POAG).

METHODS

In a historical cohort study, patients diagnosed in 2011 with early-stage POAG according to the Hodapp, Parrish and Anderson classification modified for Octopus perimetry and followed up until glaucomatous progression development; otherwise, observations were censored in October 2018. Cox regression was used to obtain hazard ratios (HR) to evaluate baseline variables (CH, central corneal thickness, gender, age IOP and glaucoma family history) as risk factors for perimetric glaucoma progression. A likelihood ratio test for interaction was performed in order to assess the effect of the combination of CH and CCT on the risk of progression.

RESULTS

Of the cohort of 1573 patients, 11.38% developed early-stage POAG progression during the follow-up. The mean follow-up time was 3.28 ± 1.92 years. Patients without progression had a higher CH (11.35 ± 1.43 vs 9.07 ± 1.69 mmHg; p < 0.001) and CCT (570.75 ± 17.71 vs 554.51 ± 23.20; p < 0.001). In the multivariate analysis, each 1 mmHg of lower CH was associated with an increase of 2.13 times in the HR of progression (95% CI: 1.92-2.32; p < 0.001). CH hazard ratio was modified by CCT, with higher values of CCT and CH resulting in a higher HR of early glaucoma progression (p < 0.001).

CONCLUSIONS

CH can be considered as a risk factor of progression in early-stage POAG. The risk associated with CH changed depending on CCT values, acting synergistically slowing the risk of glaucoma progression with higher values.

Goldmann Tonometry and Corneal Biomechanics

Glaucoma is the second cause of irreversible blindness in the world. Intraocular pressure (IOP) is a recognized major risk factor for the development and progression of glaucomatous damage. Goldmann applanation tonometry (GAT) is internationally accepted as the gold standard for the measurement of IOP. The purpose of this study was to search for correlations between Goldmann tonometry and corneal mechanical properties and thickness by means of in vitro tests. IOP was measured by the Goldmann applanation tonometer (GIOP), and by a pressure transducer inserted in the anterior chamber of the eye (TIOP), at increasing pressure levels by addition of saline solution in the anterior chamber of enucleated pig eyes (n = 49). Mechanical properties were also determined by inflation tests. The GAT underestimated the real measurements made by the pressure transducer, with most common differences in the range 15–28 mmHg. The difference between the two instruments, highlighted by the Bland–Altman test, was confirmed by ANOVA, normality tests, and Mann–Whitney’s tests, both on the data arranged for infusions and for the data organized by pressure ranges. Pearson correlation tests revealed a negative correlation between (TIOP-GIOP) and both corneal stiffness and corneal thickness. In conclusion, data obtained showed a discrepancy between GIOP and TIOP more evident for softer and thinner corneas, that is very important for glaucoma detection.

Abnormal corneal properties in osteogenesis imperfecta and glaucoma: a case series

Objective

We aimed to carry out ocular examination and genetic studies in a family in which some members are affected with osteogenesis imperfecta (OI) and primary open-angle glaucoma (POAG). We compared the corneal properties of affected and unaffected members (ie, cases and controls).

Methods

Eight family members from two generations, both affected and unaffected, were examined. Corneal hysteresis (CH), intraocular pressure (IOP) measured with Goldmann applanation tonometer, central corneal thickness (CCT) and cornea-corrected IOP (IOPcc) were recorded. Blood samples were obtained from seven family members, both affected and unaffected, and tested for a panel of genes associated with OI.

Results

Family members affected with OI (n=6) had a heterozygous splice site mutation in intron 26 of the COL1A1 gene. The family members affected with OI had reduced CCT (476.5±24.6 µm) and CH (7.9 ±1.4 mmHg) compared with the unaffected controls (CCT, 575.8±10.8 µm; CH, 12.3±0.8 mmHg). Two of the six patients affected with OI had a glaucoma diagnosis and were on topical therapy and under regular clinical review.

Conclusions

Patients affected with OI have a significant risk of developing POAG due to the effects of abnormal collagen on various ocular structures. Two of these effects which place them at risk are reduced CCT and CH. They should be screened and monitored for glaucoma from a young age, and the examination should include corneal biomechanical measurements and CCT to identify those most at risk. IOPcc may be a more accurate way to monitor IOP in the presence of abnormal corneal properties.

Corneal Biomechanical Assessment with Ultra-High-Speed Scheimpflug Imaging During Non-Contact Tonometry: A Prospective Review

BACKGROUND

In recent years, increasing interest has arisen in the application of data from corneal biomechanics in many areas of ophthalmology, particularly to assist in the detection of early corneal ectasia or ectasia susceptibility, to predict corneal response to surgical or therapeutic interventions and in glaucoma management. Technology has evolved and, recently, the Scheimpflug principle was associated with a non-contact air-puff tonometer, allowing a thorough analysis of corneal biomechanics and a biomechanically corrected intraocular pressure assessment, opening up new perspectives both in ophthalmology and in other medical areas. Data from corneal biomechanics assessment are being integrated in artificial intelligence models in order to increase its value in clinical practice.

OBJECTIVE

To review the state of the art in the field of corneal biomechanics assessment with special emphasis to the technology based on ultra-high-speed Scheimpflug imaging during non-contact tonometry.

SUMMARY

A meticulous literature review was performed until the present day. We used 136 published manuscripts as our references. Both information from healthy individuals and descriptions of possible associations with systemic diseases are described. Additionally, it exposed information regarding several fields of ocular pathology, from cornea and ocular surface through areas of refractive surgery and glaucoma until vascular and structural diseases of the chorioretinal unit.

Agreement of Corrected Intraocular Pressure Values Between Corvis ST and Pentacam in Patients With Keratoconus, Subclinical Keratoconus, and Normal Cornea

PURPOSE

To analyze the agreement of corrected intraocular pressure (IOP) values between Corvis ST (ΔIOP1) and Pentacam (ΔIOP2) in patients with keratoconus (KC), subclinical KC (sub-KC), and normal cornea.

METHODS

In total, 235 eyes were divided into KC, sub-KC, and control groups. Differences in ΔIOP1 (biomechanically corrected IOP minus uncorrected IOP) and ΔIOP2 (central corneal thickness-corrected amounts of IOP) were analyzed within and among groups. Topographical and biomechanical differences were compared among the 3 groups. Factors affecting differences between ΔIOP1 and ΔIOP2 were analyzed. Agreement analysis of ΔIOP2 and ΔIOP1 was performed by Bland-Altman plots for all 3 groups.

RESULTS

Mean ΔIOP1 was highest in the KC group (1.23 ± 0.84 mm Hg), followed by sub-KC and control groups (all P < 0.05). Deformation amplitude ratio at 2 mm (DA-2 mm), integrated radius, stiffness parameter at first applanation, and Corvis biomechanical index values significantly differed between sub-KC and control groups. The differences between ΔIOP1 and ΔIOP2 were affected by stiffness parameter at first applanation, after adjusting for central corneal thickness and age, in all 3 groups. The lowest agreement between ΔIOP2 and ΔIOP1 was observed in the KC group (mean difference: 1.90 mm Hg; 95% limit of agreement ranged from -0.2 to 3.9 mm Hg).

CONCLUSIONS

Among the 3 groups in this study, the KC group exhibited the worst consistency between ΔIOP2 and ΔIOP1. For the sub-KC and control groups, corrected IOP values derived by Pentacam were similar to Corvis ST. Ophthalmologists should carefully consider the mechanical properties of eyes with KC during IOP management.

Corneal biomechanical properties after soft contact lens wear measured on a dynamic Scheimpflug analyzer: A before and after study

OBJECTIVES

To evaluate the corneal biomechanics before and after daily use of contact lenses (CLs), measured by Scheimpflug-based devices.

METHODS

This prospective clinical study includes participants who were scheduled to use CLs daily for refractive error. The biomechanical parameters were measured by the Corneal Visualization Scheimpflug Technology (Corvis ST) before and one month after using the soft CLs.

RESULTS

Twenty-three subjects (46 eyes), including 16 female (76.2%) with a mean age of 28±7.29 years, were enrolled. There was no significant difference among biomechanical factors measured before and after contact lens wear (P>0.05). Using regression analysis of the biomechanical markers, we found a statistically significant association between second applanation length (A2 length) (P=0.001), highest concavity radius (HCR) (P=0.05), deflection amplitude ratio (DA_ratio) (P=0.05) and integrated radius (P<0.001) with age. Regarding spherical equivalent, we found a statistically significant association between central corneal thickness (CCT) (P=0.05), A2 length (P=0.03) and stiffness parameter at first applanation (SPA₁) (P=0.02).

CONCLUSIONS

We did not find a significant difference in terms of corneal biomechanical parameters between baseline and month 1; but regression analyses showed a statistically significant association between A2 length, HCR, DA_ratio, integrated radius, CCT and SPA₁ and certain subject characteristics.

The Effect of Antiglaucoma Procedures (Trabeculectomy vs. Ex-PRESS Glaucoma Drainage Implant) on the Corneal Biomechanical Properties

The aim of this study is to investigate the effect of two antiglaucoma procedures, namely trabeculectomy and Ex-PRESS mini-shunt insertion on the biomechanical properties of the cornea. This is a prospective study. Thirty patients (30 eyes) were included in the study. Nineteen eyes had an Ex-PRESS shunt inserted (Group 1) and 11 had trabeculectomy (Group 2). The examination time points for both groups were one to three weeks preoperatively and at month 1, 6, and 12 postoperatively. Corneal biomechanical properties (corneal hysteresis (CH) corneal resistance factor (CRF)) were measured with the Ocular Response Analyzer (ORA). In group 1, CH was significantly increased at 6 and 12 months compared to baseline values. Corneal hysteresis was also higher at 1 month postoperatively, but this increase did not reach statistical significance. In group 2, the CH was significantly increased at all time points compared to the preoperative values. CRF decreased at all time points postoperatively compared to the preoperative values in both groups. The difference (preoperative values to postoperative values at all time points) of the CH and CRF between the two groups was also compared and no significant differences were detected between the two surgical techniques. Trabeculectomy and the EX-PRESS mini-shunt insertion significantly alter the corneal biomechanical properties as a result of the surgical trauma and the presence of the shunt in the corneal periphery. When compared between them, they affect the corneal biomechanical properties in a similar way.

Changes in Corneal Biomechanics and Glaucomatous Visual Field Loss

PRECIS

A lower baseline corneal hysteresis and a decrease in corneal resistance factor (CRF) over time are associated with higher risk of visual field progression in glaucomatous and glaucoma suspect eyes.

PURPOSE

The aim was to investigate the longitudinal change in CRF and cornea hysteresis (CH) as risk factors for visual field progression.

MATERIALS AND METHODS

In this prospective observational cohort study, 72 eyes of 48 glaucoma or glaucoma suspect patients were followed for an average of 4.5 years. Baseline and follow-up CH and CRF measurements were performed with the Ocular Response Analyzer (Reichert Ophthalmic Instruments Inc., Depew, N.Y.). Evaluation of rates of visual field change during follow-up was performed using visual field mean deviation. Univariable and multivariable linear mixed models assessed the relationship of visual field progression with baseline CRF and CH as well as with changes in CRF and CH.

RESULTS

The mean baseline CH was 9.0 (95% confidence interval: 8.6-9.4)  mm Hg and the mean baseline CRF was 9.3 (95% confidence interval: 8.8-9.9) mm Hg. There was no statistically significant difference in average CH and CRF measurements over time. In multivariable modeling adjusting for age, race, and mean intraocular pressure during follow-up, each 1 mm Hg lower in baseline CH and 1 mm Hg decrease in CRF over time were associated with a 0.12 (P=0.042) and 0.14 dB/year (P=0.007) faster rate of visual field mean deviation loss, respectively. Similar findings were found in glaucoma eyes but not found in glaucoma suspect eyes.

CONCLUSION

Visual field progression was associated with a lower baseline CH and a decrease in CRF over time. Assessment of corneal resistance and elasticity at baseline and during follow-up examinations should be considered to identify those eyes at highest risk of visual field progression.

Corneal densitometry and biomechanical properties in patients with primary congenital glaucoma

OBJECTIVE

To describe corneal densitometry, topographic measurements, and biomechanical properties in patients with primary congenital glaucoma (PCG) and healthy patients. To examine correlations between variables and determine their glaucoma diagnostic capacity.

METHODS

This was a cross-sectional, case-control study conducted in 50 eyes of 50 patients with PCG (G1) and 40 eyes of 40 healthy patients (G2). The variables determined in each participant were: intraocular pressure, best corrected visual acuity (BCVA), corneal densitometry, topographic data using the Pentacam HR-Scheimpflug imaging system (Oculus Optikgerate GmbH, Wetzlar, Germany), and corneal hysteresis (CH) and corneal resistance factor (CRF) using the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Depew, NY).

RESULTS

Overall densitometry was significatively higher in the PCG group (G1: 17.94 ± 4.99 vs G2: 13.25 ± 1.96, p < 0.001). CH (G1: 8.02 ± 11.35 vs G2: 11.35 ± 1.42, p < 0.001) and CRF (G1: 9.48 ± 2.83 vs G2: 10.77 ± 1.34, p < 0.001) were lower in the glaucoma group. Mean posterior, central, and anterior elevation and mean keratometry were higher in G1 (p < 0.05). In the PCG group, overall densitometry showed significant correlation with CH (r = -0.321, p = 0.028) and with BCVA (r = -0.498, p = 0.002). AUCs (areas under the curve) for CH and overall densitometry were high (0.839 and 0.899 respectively) and the best overall densitometry; CH and CRF cutoffs were 14.0, 9.3 and 9.2 respectively.

CONCLUSION

Densitometry is increased, and biomechanical corneal properties are decreased in patients with PCG. Densitometry and visual acuity showed a negative and significant correlation, so this measurement could be used as an indirect parameter of BCVA in the clinical practice.

The Relationship Between Corvis ST Tonometry Parameters and Ocular Response Analyzer Corneal Hysteresis

PRECIS

Corvis ST Tonometry and Ocular Response Analyzer (ORA) measurements were conducted in primary open-angle glaucoma and normative subjects. Many parameters were significantly correlated, however, the strengths were weak to moderate.

PURPOSE

Reichert ORA parameters are derived from pressure information following the application of air-jet, whereas detailed structural observation can be made using the Corneal Visualization Scheimpflug Technology instrument (CST). The purpose of the study was to investigate the association between CST measurements and ORA measured corneal hysteresis (CH).

METHODS

Measurements of CST, ORA, axial length, average corneal curvature, central corneal thickness (CCT) and intraocular pressure with Goldmann applanation tonometry were carried out in 104 eyes of 104 patients with primary open-angle glaucoma and 35 eyes from normative subjects. The association between CST and ORA parameters was assessed using linear regression analysis, with model selection based on the second order bias corrected Akaike Information Criterion index.

RESULTS

Deformation amplitude ratio (corneal softness, R=-0.51), SP A1 (corneal stiffness, R=0.41), and Inverse Radius (integrated area under the curve of the inverse concave radius, R=-0.44) were significantly correlated with CH (P <0.05). The optimal model to explain CH using CST measurements was given by: CH=-76.3+4.6×A1 time (applanation time in the corneal inward movement)+1.9×A2 time (second applanation time in the corneal outward movement) + 3.1 × highest concavity deformation amplitude (magnitude of movement of the corneal apex from before deformation to its highest concavity) + 0.016×CCT (R=0.67; P<0.001).

CONCLUSIONS

CST parameters are significant, but weakly or moderately, related to ORA measured CH.

Association of 24-Hour Intraocular Pressure Fluctuation With Corneal Hysteresis and Axial Length in Untreated Chinese Primary Open-Angle Glaucoma Patients

PURPOSE

The purpose of this study was to evaluate the association of 24-hour intraocular pressure (IOP) fluctuation with corneal biomechanics and ocular biometric parameters in Chinese patients with primary open angle glaucoma (POAG) before initial treatment.

METHODS

Forty-nine Chinese patients with POAG (98 eyes) were recruited in this study before start of any POAG treatment. The 24-hour IOP was measured with a 2-hour interval by a noncontact tonometer. Corneal biomechanical properties and biometric parameters were measured once during 8 AM to 6 PM before 24-hour IOP measurement.

RESULTS

The 24-hour IOP fluctuation was defined as the differences between the peak and trough IOP measurement and was significantly associated with axial length (AL) in the multivariate analysis. The POAG subjects with AL ≤ 26 mm had significantly larger 24-hour IOP fluctuation but lower corneal hysteresis, compared to those with AL > 26 mm. In addition, subgroup analysis showed that high tension glaucoma subjects had larger 24-hour IOP fluctuation and higher corneal resistance factor than patients with normal tension glaucoma.

CONCLUSIONS

This study revealed the association of 24-hour IOP fluctuation with office hour corneal biomechanical properties and AL in patients with POAG. Their contributions to IOP fluctuation should be considered in the risk analysis of glaucoma development and progression.

TRANSLATIONAL RELEVANCE

Ocular biometric parameters are related with 24-hour IOP fluctuation in patients with POAG, which is potentially helpful in explaining different progression patterns in different types of patients.

Biomechanics of the keratoconic cornea: Theory, segmentation, pressure distribution, and coupled FE-optimization algorithm

Understanding of the corneal biomechanical properties is of high interest due to its potential application in the early diagnosis of keratoconus (KC). KC by itself is a non-inflammatory eye disorder causes corneal structural and/or compositional anomalies. The biomechanically weakened cornea is no longer able to preserve the normal shape of the cornea against the intraocular pressure (IOP) and gradually starts to bulge outward, invoking a conical shape and subsequent distorted vision. The most popular way to measure the in vivo corneal biomechanical properties is the CorVis-ST, which enables to analyze the dynamic response of the cornea under a temporary air puff pressure. However, the complications, such as the lack of knowledge on the accurate air-puff pressure distribution on the cornea's surface as a function of the distance from the apex of the cornea as well as the time, hinder us to have a reliable estimation of the cornea's mechanical parameters. This study aims to establish patient-specific geometries of the healthy and KC corneas and calculate the pressure distribution on the cornea's surface as a function of both the distance from the apex of the cornea and time, and thereafter, the viscoelastic mechanical properties of both the healthy and KC corneas using a coupled finite element (FE)-optimization algorithm. To do that, the dynamic deformation response of six healthy and six KC corneas were measured via CorVis-ST. The videos of the in vivo deformation of the corneas under the applied air puff pressure were segmented using our segmentation algorithm to determine the anterior and posterior curvatures of the corneas during the dynamic movement of the cornea. The FE model of the corneas were established using the segmented data and subjected to a negative (pre-stress), positive IOP, and air puff pressure while the floating boundary conditions were applied to the two ends of the corneas' FE models. The simulation results were imported into a loop of FE-optimization algorithm and analyzed until the deformation amplitude at the apex of the cornea reaches to its minimum difference compared to the clinical data by CorVis-ST. The results revealed that the pressure distributions found in the literature as a function of the distance from the apex of the cornea and time unable to provide satisfactory results. Therefore, the pressure distributions both as a function of the distance and time were optimized using our coupled FE-optimization algorithm and employed to estimate the viscoelastic properties of the healthy and KC corneas. The mean percentage error (MPE) of 8.45% and 10.79% were found for the healthy and KC corneas compared to the clinical data of CorVis-ST, respectively. The results also revealed a significantly higher short-time shear modulus for the KC (62.33 MPa) compared to the healthy (37.45 MPa) corneas while the long-time shear modulus of both the healthy and KC corneas were almost the same (4.01 vs. 3.91 MPa). The proposed algorithm is a noninvasive technique capable of accurately estimating the viscoelastic mechanical properties of the cornea, which can contribute to understand the mechanism of KC development and improve diagnosis and intervention in KC.

The Relationship Between Corneal Hysteresis and Retinal Ganglion Cells - A Step Forward in Early Glaucoma Diagnosis

BACKGROUND Glaucoma is a major cause of irreversible visual field (VF) loss across the world. Many studies have assessed the accuracy of glaucoma diagnostic tests for a more precise diagnosis to quickly identify patients with higher risk of progression. MATERIAL AND METHODS We conducted a study that included 214 eyes divided into 3 groups: 79 eyes from patients diagnosed with primary open-angle glaucoma (POAG), 68 eyes from patients diagnosed with ocular hypertension (OH), and 67 eyes from normal individuals (normal eyes, NE). All patients included in the study received a complete checkup. RESULTS In POAG patients, means of central corneal thickness (CCT), corneal hysteresis (CH), corneal resistance factor (CRF), mean defect (MD), visual field index (VFI), peripapillary retinal nerve fiber layer (pRNFL), and ganglion cell complex (GCC) are lower than in OH patients, and in NE are higher than in both groups. Also, we found a statistically significant direct correlation between CH and GCC thickness. Further statistical analysis revealed that both pRNFL thickness and GCC thickness are significantly influenced by CH value in a precise manner. CONCLUSIONS The first cell type affected in glaucoma is the retinal ganglion cell. We found a positive correlation between GCC thickness and CH, suggesting that CH might be a parameter to consider in the evaluation of all glaucoma patients from their first examination. Moreover, both pRNFL thickness and GCC thickness are influenced by CH, suggesting the utility of monitoring the value of CH at every checkup to detect its decrease in glaucoma patients.

Corneal Stiffness and Collagen Cross-Linking Proteins in Glaucoma: Potential for Novel Therapeutic Strategy

Biomechanical properties of the cornea have recently emerged as clinically useful in risk assessment of diagnosing glaucoma and predicting disease progression. Corneal hysteresis (CH) is a dynamic tool, which measures viscoelasticity of the cornea. It represents the overall deformability of the cornea, and reduces significantly with age. Low CH has also been associated with optic nerve damage and progression of visual field loss in glaucoma. The extracellular matrix (ECM) constituents of the cornea, trabecular meshwork (TM), sclera, and lamina cribrosa (LC) are similar, as they are predominantly made of fibrillar collagen. This suggests that biomechanical changes in the cornea may also reflect optic nerve compliance in glaucomatous optic neuropathy, and in the known increase of TM tissue stiffness in glaucoma. Increased collagen cross-linking contributes to tissue stiffening throughout the body, which is observed in normal aging and occurs at an accelerated rate in systemic conditions such as fibrotic and cardiovascular diseases, cancer, and glaucoma. We reviewed 3 ECM cross-linking proteins that may have a potential role in the disease process of increased tissue stiffness in glaucoma, including lysyl oxidase (LOX)/lysyl oxidase-like 1 (LOXL1), tissue transglutaminase (TG2), and advanced glycation end products. We also report elevated messenger RNA (mRNA) levels of LOX and TG2 in glaucoma LC cells to support our proposed theory that increased levels of cross-linking proteins in glaucoma play a role in LC tissue stiffness. We highlight areas of research that are needed to better understand the role of cross-linking in glaucoma pathogenesis, leading potentially to a novel therapeutic strategy.

Comparison of Biomechanically Corrected Intraocular Pressure Obtained by Corvis ST and Goldmann Applanation Tonometry in Patients With Open-angle Glaucoma and Ocular Hypertension

PRECIS

Biomechanically corrected intraocular pressure (bIOP) measurements provided by the new Corvis ST (CST) were higher than measurements obtained with Goldman applanation tonometry (GAT) in eyes with ocular hypertension (OHT) or open-angle glaucoma (OAG).

PURPOSE

To compare bIOP obtained with a new version of CST with GAT measurements in patients with OAG and OHT, and to identify factors affecting IOP measurement differences between these methods.

METHODS

A total of 122 eyes with OAG or OHT were enrolled. Sixty eyes were treated with prostaglandin analogs (PGA) and 62 eyes with selective laser trabeculoplasty (SLT). IOP was measured with CST, followed by GAT. Central corneal thickness (CCT) was measured with ultrasound pachymetry. Measurements obtained with these 2 approaches were compared.

RESULTS

The overall mean IOP was 15.2±3.0 mm Hg and 14.1±3.2 mm Hg with the CST and GAT, respectively (r=0.74; P<0.0001). There was a 1.17 mm Hg bias between tonometers, with 95% limits of agreement of -2.66 to 5.01 mm Hg. According to multivariate regression analysis, differences between bIOP and GAT-IOP were associated with CCT (P=0.001) and age (P=0.007) in the PGA group, but only with CCT in the SLT group (P=0.002). bIOP and GAT-IOP values were influenced by age (PGA: P=0.014; SLT: P=0.006) and CCT (PGA: P=0.007; SLT: P=0.032), respectively.

CONCLUSIONS

BIOP values were higher and less affected by CCT than GAT-IOP values in eyes with OAG and OHT. However, these measurements may not be interchangeable in the clinic.

Intraocular Pressure Measurement Using Ocular Response Analyzer, Dynamic Contour Tonometer, and Scheimpflug Analyzer Corvis ST

Purpose

To compare intraocular pressure (IOP) measurements with Goldmann applanation tonometry (GAT), ocular response analyzer (ORA), dynamic contour tonometer (DCT), and Corvis ST (CST) in healthy subjects.

Methods

In a prospective, observational study, IOP measurements with GAT (GAT-IOPc), ORA (IOPcc), DCT (DCT-IOP), and CST (bIOP) were performed and analyzed in 94 healthy subjects.

Results

Mean age of the participants was 45.6 ± 17.2 years (range 18 to 81 years). Mean GAT-IOPc was 12.9 ± 2.4 mmHg, mean DCT-IOP was 16.1 ± 2.6 mmHg, and mean IOPcc was 15.6 ± 3.3 mmHg. DCT-IOP and IOPcc were significantly higher than GAT-IOPc (P < 0.001). Mean bIOP was 13.5 ± 2.4 mmHg that was slightly higher but not significantly different from GAT-IOPc (P=0.146). Correlation analysis of IOP values and central corneal thickness (CCT) revealed a negative correlation between GAT-IOPc and CCT (r = −0.347; P=0.001). However, IOPcc, DCT-IOP, and bIOP showed no significant correlation to CCT. Only bIOP revealed a weak but significant age dependency (r = 0.321, P=0.002).

Conclusion

All tonometry devices showed a good agreement of biomechanical corrected IOP values with GAT-IOPc. As no influence of CCT on IOPcc, DCT-IOP, and bIOP was detectable, the used correction algorithms appear to be appropriate in these tonometers in the clinical setting. The highest agreement was found between GAT-IOPc and bIOP. However, bIOP weakly correlated with participants' age. Further studies are needed to elucidate the role of bIOP for IOP measurement.