Central corneal thickness and corneal hysteresis associated with glaucoma damage

Corneal Hysteresis as a Marker for Patients with Secondary Glaucoma

PURPOSE

To investigate and compare the association of corneal hysteresis (CH) in patients with secondary glaucoma to control patients and patients with primary open-angle glaucoma (POAG). Additionally, to determine the consistency of CH measurements in patients with secondary glaucoma.

METHODS

A total of 84 patients (121 eyes) were prospectively included in this study. Twenty-three patients (46 eyes) were healthy controls, 24 patients (40 eyes) were diagnosed with POAG, and 27 patients (35 eyes) were diagnosed with a form of secondary glaucoma. CH and intraocular pressure (IOP) were measured using the Ocular Response Analyzer. Three measurements per eye were performed and used for the analysis and to determine fluctuations in CH data. One-way ANOVA with post-hoc Bonferroni analysis and Chi-Squared testing was done to determine differences between groups.

RESULTS

All patients were matched for age. Patients in both POAG and secondary glaucoma groups were matched for age and IOP. All groups had similar sex and racial compositions as well as similar proportions of diabetes, hypertension, and hyperlipidemia. CH was lower (p < .05) in patients with POAG (9.32 ± 1.64) and secondary glaucoma (7.89 ± 3.18) when compared to healthy controls (11.16 ± 1.60). Fluctuations in CH measurements were minimal in all groups. Further analysis of the secondary glaucoma group revealed no differences in CH between different types of secondary glaucoma (p > .05).

CONCLUSION

Patients with secondary glaucoma have lower CH when compared to POAG or control groups. The ORA exhibits precision of CH measurements for control, POAG, and secondary glaucoma groups, making it a reliable tool in management of secondary forms of glaucoma.

Biomechanical Correlations Between the Cornea and the Optic Nerve Head

Purpose

A thin cornea is a potent risk factor for glaucoma. The underlying mechanisms remain unexplained. It has been postulated that central corneal thickness (CCT) may be a surrogate for biomechanical parameters of the posterior eye. In this study, we aimed to explore correlations of biomechanical responses between the cornea and the optic nerve head (ONH) and the peripapillary sclera (PPS) to elevated intraocular pressure (IOP), the primary risk factor of glaucoma.

Methods

Inflation tests were performed in nine pairs of human donor globes. One eye of each pair was randomly assigned for cornea or posterior eye inflation. IOP was raised from 5 to 30 millimeters of mercury (mmHg) at 0.5 mmHg steps in the whole globe and the cornea or the ONH/PPS was imaged using a 50 MHz ultrasound probe. Correlation-based ultrasound speckle tracking was used to calculate tissue displacements and strains. Associations of radial, tangential, and shear strains at 30 mmHg between the cornea and the ONH or PPS were evaluated.

Results

Corneal shear strain was significantly correlated with ONH shear strain (R = 0.857, P = 0.003) and PPS shear strain (R = 0.724, P = 0.028). CCT was not correlated with any strains in the cornea, ONH, or PPS.

Conclusions

Our results suggested that an eye that experiences a larger shear strain in the cornea would likely experience a larger shear strain in its ONH and PPS at IOP elevations. The strong correlation between the cornea's and the ONH's shear response to IOP provides new insights and suggests a plausible explanation of the cornea's connection to glaucoma risk.

Polygenic risk score-based phenome-wide association for glaucoma and its impact on disease susceptibility in two large biobanks

BACKGROUND

Glaucoma is a leading cause of worldwide irreversible blindness. Considerable uncertainty remains regarding the association between a variety of phenotypes and the genetic risk of glaucoma, as well as the impact they exert on the glaucoma development.

METHODS

We investigated the associations of genetic liability for primary open angle glaucoma (POAG) with a wide range of potential risk factors and to assess its impact on the risk of incident glaucoma. The phenome-wide association study (PheWAS) approach was applied to determine the association of POAG polygenic risk score (PRS) with a wide range of phenotypes in 377, 852 participants from the UK Biobank study and 43,623 participants from the Penn Medicine Biobank study, all of European ancestry. Participants were stratified into four risk tiers: low, intermediate, high, and very high-risk. Cox proportional hazard models assessed the relationship of POAG PRS and ocular factors with new glaucoma events.

RESULTS

In both discovery and replication set in the PheWAS, a higher genetic predisposition to POAG was specifically correlated with ocular disease phenotypes. The POAG PRS exhibited correlations with low corneal hysteresis, refractive error, and ocular hypertension, demonstrating a strong association with the onset of glaucoma. Individuals carrying a high genetic burden exhibited a 9.20-fold, 11.88-fold, and 28.85-fold increase in glaucoma incidence when associated with low corneal hysteresis, high myopia, and elevated intraocular pressure, respectively.

CONCLUSION

Genetic susceptibility to POAG primarily influences ocular conditions, with limited systemic associations. Notably, the baseline polygenic risk for POAG robustly associates with new glaucoma events, revealing a large combined effect of genetic and ocular risk factors on glaucoma incidents.

Corneal biomechanics and their association with severity of lens dislocation in Marfan syndrome

PURPOSE

To investigate corneal biomechanical properties and its associations with the severity of lens dislocation in patients with Marfan syndrome.

METHODS

A total of 30 patients with Marfan syndrome and 30 age-, sex- and axial length (AL)-matched controls were recruited. Corneal biomechanical parameters of both groups were measured by CorVis ST and were compared between groups. Potential associations between corneal biomechanical parameters and severity of lens dislocation were also investigated.

RESULTS

Lower applanation 1 velocity (A1V) (0.13 ± 0.004 vs. 0.15 ± 0.003, P = 0.016), shorter applanation 2 time (A2T)(22.64 ± 0.11 vs. 22.94 ± 0.11, P = 0.013), longer peak distance (PD) (5.03 ± 0.07 vs. 4.81 ± 0.05, P = 0.008), longer radius (R) of highest concavity (7.44 ± 0.16 vs. 6.93 ± 0.14, P = 0.012), greater Ambrosio relational thickness horizontal (ARTh) (603 ± 20 vs. 498 ± 12, P < 0.001), and integrated radius (IR) (8.32 ± 0.25 vs. 8.95 ± 0.21, P = 0.033) were detected among Marfan eyes compared with controls (all P < 0.05). Marfan individuals with more severe lens dislocation tended to have increased stiffness parameter as longer A1T, slower A1V, shorter A2T, slower application 2 velocity (A2V), smaller PD and smaller Distance Amplitude (DA) (P < 0.05).

CONCLUSION

Marfan patients were detected to have increased corneal stiffness compared with normal subjects. Corneal biomechanical parameters were significantly associated with the severity of lens dislocation in Marfan patients.

Corneal biomechanics and diagnostics: a review

PURPOSE

Corneal biomechanics is an emerging field and the interest into physical and biological interrelations in the anterior part of the eye has significantly increased during the past years. There are many factors that determine corneal biomechanics such as hormonal fluctuations, hydration and environmental factors. Other factors that can affect the corneas are the age, the intraocular pressure and the central corneal thickness. The purpose of this review is to evaluate the factors affecting corneal biomechanics and the recent advancements in non-destructive, in vivo measurement techniques for early detection and improved management of corneal diseases.

METHODS

Until recently, corneal biomechanics could not be directly assessed in humans and were instead inferred from geometrical cornea analysis and ex vivo biomechanical testing. The current research has made strides in studying and creating non-destructive and contactless techniques to measure the biomechanical properties of the cornea in vivo.

RESULTS

Research has indicated that altered corneal biomechanics contribute to diseases such as keratoconus and glaucoma. The identification of pathological corneas through the new measurement techniques is imperative for preventing postoperative complications.

CONCLUSIONS

Identification of pathological corneas is crucial for the prevention of postoperative complications. Therefore, a better understanding of corneal biomechanics will lead to earlier diagnosis of ectatic disorders, improve current refractive surgeries and allow for a better postoperative treatment.

Characteristic Differences between Normotensive and Hypertensive Pseudoexfoliative Glaucoma

PURPOSE

To compare the differences between eyes with pseudoexfoliative glaucoma (PXG) when they are divided into two groups (hypertensive PXG and normotensive PXG) according to the intraocular pressure (IOP).

METHODS

This is a retrospective study. Data from 86 hypertensive PXG eyes and 80 normotensive PXG eyes were included. Hypertensive PXG was defined as PXG with IOP ≥ 22 mmHg, and normotensive PXG was defined as with IOP ≤ 21 mmHg). Central corneal thickness (CCT) was measured by ultrasound pachymetry. Lamina cribrosa thickness (LT) was evaluated using swept-source optical coherence tomography.

RESULTS

No significant differences were observed between hypertensive and normotensive PXG in terms of age, gender, axial length, hypertension, or diabetes. Normotensive PXG eyes had thinner CCT than hypertensive PXG eyes (p = 0.02). To compare LT, a sub-analysis was performed after matching age, VF MD and retinal nerve fiber layer thickness. The normotensive PXG group (n = 32) demonstrated significantly thinner LT compared with the hypertensive PXG group (n = 32) at similar ages and levels of glaucoma severity (p < 0.001).

CONCLUSIONS

Eyes with normotensive PXG demonstrated thinner CCT and LT compared with those with hypertensive PXG, suggesting structural vulnerability to glaucoma.

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.

Keratoconus tomographic indices in osteogenesis imperfecta

PURPOSE

Osteogenesis imperfecta (OI) is a rare inherited disease affecting collagen-rich tissues. Ocular complications have been reported such as thin corneas, low ocular rigidity, keratoconus, among others. The purpose of this study is to characterize corneal tomographic features in OI patients compared to unaffected patients, with particular focus on commonly studied keratoconus indices.

METHODS

Cross-sectional case-control study including 37 OI patients and 37 age-matched controls. Patients and controls underwent comprehensive ophthalmological examination including corneal Scheimpflug tomography with a Pentacam HR device (Oculus Optikgeräte GmbH, Wetzlar, Germany) to analyse and compare topometric, tomographic, pachymetric and Belin-Ambrósio Enhanced Ectasia Display III (BAD-D) data of both eyes of each patient.

RESULTS

Most OI patients had type I disease (n = 24; 65%) but type III-VII patients were also included. Two patients had clinically overt bilateral keratoconus. OI patients had significantly higher maximum keratometry (45.2 ± 2.1 vs. 43.7 ± 1.2; p = 0.0416), front and back elevation (3.0 ± 3.3 vs. 2.1 ± 1.3, p = 0.0201; 11.1 ± 8.2 vs. 5.0 ± 3.7, p < 0.0001), index of surface variance (25.5 ± 13 vs. 17.4 ± 8.3; p = 0.0016), index of vertical asymmetry (0.21 ± 0.14 vs. 0.15 ± 0.06; p = 0.0215), index of height asymmetry (9.2 ± 14 vs. 6.0 ± 4.5; p = 0.0421), index of height decentration (0.02 ± 0.01 vs. 0.01 ± 0.01; p < 0.0001) and average pachymetric progression (1.01 ± 0.19 vs. 0.88 ± 0.14; p < 0.0001) readings. Thinnest corneal thickness and maximum Ambrósio relational thickness were significantly lower (477 ± 52 vs. 543 ± 26; 387 ± 95 vs. 509 ± 49; p < 0.0001). Two-thirds of OI patients had corneas with a minimum thickness < 500 µm. BAD-D value was significantly higher in OI patients (2.1 ± 1.4 vs. 0.9 ± 0.2; p < 0.0001).

CONCLUSION

OI patients showed significant changes in corneal profiles compared with healthy subjects. A high proportion of patients had tomographically suspect corneas when using keratoconus diagnostic indices. Further studies are warranted to assess the true risk of corneal ectasia in OI patients.

Comparison of Goldmann and ORA tonometers in newly diagnosed, untreated, POAG and OHT eyes

BACKGROUND AND OBJECTIVE

The corneal biomechanical properties in naïve, untreated glaucoma and ocular hypertension (OHT) eyes is interesting, because it may be a source of error in intraocular pressure (IOP) measurements by Goldmann applanation tonometer (GAT) and Ocular Response Analyzer (ORA) The main objective of this study was to evaluate the IOP values obtained using GAT and the ORA, in primary open angle glaucoma (POAG) and in OHT untreated eyes.

MATERIAL AND METHODS

Observational, masked, cross sectional observational study. Newly diagnosed, untreated POAG and OHT eyes were included.

RESULTS

51 POAG and 34 OHT eyes were analyzed. We found that IOPcc (IOP corneal-compensated) was significantly higher than GAT IOP in POAG (p = 0.0002) while we did not find any significant difference between both tonometers in OHT (p = 0.1).

CONCLUSIONS

GAT seems to underestimate the real IOP in untreated POAG eyes and it seems to be quite accurate in OHT eyes.

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.

Racial and Ethnic Differences in the Roles of Myopia and Ocular Biometrics as Risk Factors for Primary Open-Angle Glaucoma

Purpose

Assess how the roles of refractive error (RE) and ocular biometrics as risk factors for primary open-angle glaucoma (POAG) differ by race and ethnicity.

Methods

Data from the Los Angeles Latino Eye Study (LALES) and the Chinese American Eye Study (CHES), two population-based epidemiological studies, were retrospectively analyzed. Multivariable logistic regression and interaction term analyses were performed to assess relationships between POAG and its risk factors, including RE and axial length (AL), and to assess effect modification by race/ethnicity.

Results

Analysis included 7601 phakic participants of LALES (47.3%) and CHES (52.7%) with age ≥ 50 years. Mean age was 60.6 ± 8.3 years; 60.9% were female. The prevalence and unadjusted risk of POAG were higher in LALES than CHES (6.0% and 4.0%, respectively; odds ratio [OR] = 1.55; P < 0.001). In the multivariable analysis, significant risk factors for POAG included Latino ethnicity (OR = 2.25; P < 0.001), refractive myopia (OR = 1.54 for mild, OR = 2.47 for moderate, OR = 3.94 for high compared to non-myopes; P ≤ 0.003), and longer AL (OR = 1.37 per mm; P < 0.001). AL (standardized regression coefficient [SRC] = 0.3) was 2.7-fold more strongly associated with POAG than high myopia status (SRC = 0.11). There was no modifying effect by race/ethnicity on the association between RE (per diopter) or AL (per millimeter) and POAG (P = 0.49).

Conclusions

Although the POAG risk conferred by myopic RE and longer AL is similar between Latino and Chinese Americans, the difference in POAG prevalence between the two groups is narrowed by higher myopia prevalence among Chinese Americans. Racial/ethnic populations with higher myopia incidence may become disproportionately affected by POAG in the context of the global myopia epidemic.

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.

In vivo corneal elastography: A topical review of challenges and opportunities

Clinical measurement of corneal biomechanics can aid in the early diagnosis, progression tracking, and treatment evaluation of ocular diseases. Over the past two decades, interdisciplinary collaborations between investigators in optical engineering, analytical biomechanical modeling, and clinical research has expanded our knowledge of corneal biomechanics. These advances have led to innovations in testing methods (ex vivo, and recently, in vivo) across multiple spatial and strain scales. However, in vivo measurement of corneal biomechanics remains a long-standing challenge and is currently an active area of research. Here, we review the existing and emerging approaches for in vivo corneal biomechanics evaluation, which include corneal applanation methods, such as ocular response analyzer (ORA) and corneal visualization Scheimpflug technology (Corvis ST), Brillouin microscopy, and elastography methods, and the emerging field of optical coherence elastography (OCE). We describe the fundamental concepts, analytical methods, and current clinical status for each of these methods. Finally, we discuss open questions for the current state of in vivo biomechanics assessment techniques and requirements for wider use that will further broaden our understanding of corneal biomechanics for the detection and management of ocular diseases, and improve the safety and efficacy of future clinical practice.

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.

Corneal Biomechanical Properties of Various Types of Glaucoma and Their Impact on Measurement of Intraocular Pressure

INTRODUCTION

Corneal biomechanical properties could affect intraocular pressure (IOP) measurement. The aim of this study was to evaluate the differences in corneal biomechanical properties of various types of glaucoma, assess their effect on IOP measurements.

METHODS

This is an observational clinical study of 486 subjects including 102 normal subjects, 104 ocular hypertension (OHT), 89 normal tension glaucoma (NTG), and 191 high tension glaucoma (HTG). Corneal biomechanical parameters were measured using an ocular response analyzer. The main parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated pressure measurement (IOPg), and corneal-compensated intraocular pressure (IOPcc). Ultrasound pachymetry was used to measure central corneal thickness (CCT). IOP was measured by a Goldmann applanation tonometer (GAT) and a noncontact tonometer (NCT). Visual field (VF) and refractive status were also recorded. Results were analyzed by one-way analysis of variance, univariate and multivariate linear regression analyses, and Bland-Altman plots.

RESULTS

Multiple comparison by analysis of variance showed significantly lower CH and CRF in NTG compared to HTG, OHT, and normal subjects (CH: 0.011, 0.015, and 0.033; CRF: 0.001, <0.001, and 0.042, respectively). CRF and CH associated with IOP were measured using either GAT, NCT and IOPcc-GAT, IOPcc-NCT, yet CCT was not. GAT correlated strongly with IOPg (r = 0.79; p < 0.001) and IOPcc (r = 0.77; p < 0.001), but limits of agreement between the measurements were poor. CH and CRF were both negatively correlated with VF change (p < 0.01).

CONCLUSION

CH and CRF affect the measurement of IOP and were related to types of glaucoma or severity of glaucoma. Pure CCT should not be used to correct IOP values or estimate the risk of disease.

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).

Influence of corneal biomechanical properties on intraocular pressure measurement in different types of Graves’ orbitopathy

BACKGROUND/AIMS

To compare the ocular biomechanical properties of inactive Grave's orbitopathy (GO) patients and healthy subjects and to evaluate the influence of severity and phenotype of GO on these parameters.

METHODS

This was a cross-sectional study. All included inactive GO patients and healthy controls underwent complete ocular examination, including Goldman applanation tonometry (GAT), corneal biomechanical analysis using Ocular Response Analyser (ORA), and corneal epithelial thickness analysis using Optovue. Patients with inactive GO were classified based on the severity and orbital phenotype (predominantly myogenic or lipogenic). Comparison among groups was performed.

RESULTS

60 eyes from 30 inactive GO patients and 30 healthy eyes were examined. Corneal hysteresis (CH) was significantly lower in inactive GO patients (9.6 [p25 8.1; p75 11.2]) compared to controls (10.4 [9.8; 11.5]) (p = 0.012). In GO patients, cornea compensated intraocular pressure (IOPcc) was significantly higher than Goldman applanation tonometry IOP (IOP-GAT) (p = 0.001). A total of 13.3% GO patients were initially classified as having ocular hypertension (OHT; defined as IOP > 21 mmHg with no signs of glaucomatous optic neuropathy) based on IOP-GAT measurement. According to IOPcc, 27.8% of GO patients were classified as OHT. In GO patients, no differences were found in corneal bimechanical properties according to the disease severity or orbital phenotype.

CONCLUSIONS

CH is significantly lower in inactive GO patients compared to healthy subjects. ORA corrected IOP was significantly higher in GO patients compared to IOP-GAT. No differences in corneal biomechanical properties between mild and moderate-to-severe GO disease and between myogenic and lipogenic orbitopathy were found.

Diurnal Variation of Corneal Hysteresis in Patients With Untreated Primary Open Angle Glaucoma and Normal Individuals

PRCIS

Corneal hysteresis in both patients with untreated open angle glaucoma and normal individuals demonstrated significant diurnal variation independent of confounding factors and was higher in the nighttime than in the daytime.

PURPOSE

To investigate diurnal variations in corneal hysteresis (CH) in patients with untreated primary open angle glaucoma (POAG) and normal individuals by using an ocular response analyzer.

MATERIALS AND METHODS

This prospective study included 72 eyes of 53 patients with untreated POAG and 53 eyes of 47 normal individuals. Intraocular pressure (IOP) and CH were measured using Goldmann applanation tonometry (GAT) and ocular response analyzer, respectively, every 3 hours from 9:00 to 24:00. Mixed-effects models were used to determine factors associated with CH values and CH amplitude (maximum values minus minimum values) and to examine the diurnal variations in GAT IOP and CH in each group. Significant differences between time points were defined as significant variations.

RESULTS

The diurnal average GAT IOP and CH in patients with POAG were significantly higher and lower than those in normal individuals ( P =0.001, 0.002). In the multivariate analysis, the larger central corneal thickness was associated with larger CH values in POAG and normal eyes (both P <0.001). A larger amplitude of GAT IOP was significantly associated with a larger CH amplitude in POAG and normal eyes ( P =0.010, 0.013). CH, in both groups, showed similar significant diurnal variation and was higher in the nighttime than in the daytime, even after adjusting for confounding factors, while IOP showed an antiphase pattern.

CONCLUSION

CH in both untreated POAG patients and normal participants demonstrated similar diurnal variations, that is, higher at night, independent of confounding factors. These findings suggest that viscoelastic properties of the cornea may fluctuate diurnally independent of IOP.

Evaluation of corneal hysteresis after pars plana vitrectomy combined phacoemulsification and intraocular lens implantation

We evaluated the early effects of pars plana vitrectomy (PPV) on corneal biomechanics by comparing corneal hysteresis (CH) after cataract surgery (phacoemulsification and aspiration with intraocular lens implantation; PEA + IOL) alone and PPV combined with cataract surgery. This study included 20 eyes (18 patients), who underwent cataract surgery alone (PEA + IOL group), and 28 eyes (27 patients) who underwent PPV combined with cataract surgery (PPV triple group). The CH was 11.1 ± 1.1, 10.4 ± 1.1, and 11.0 ± 1.0 mmHg in the PEA + IOL group and 11.0 ± 1.4, 9.8 ± 1.4, and 10.6 ± 1.6 mmHg in the PPV triple group, preoperatively, at 2 weeks, and 3 months after surgery, respectively. The CH was not significantly different after surgery in the PEA + IOL group, but decreased significantly in the PPV triple group 2 weeks following surgery (p < 0.01). Intraocular pressure (IOP) and central corneal thickness (CCT) did not change significantly after surgery in either group. Preoperatively, there was a positive correlation between CH and CCT in the PPV triple group, but the correlation disappeared postoperatively. In PPV combined with cataract surgery, CH temporarily decreased postoperatively, independent of IOP and CCT. Removal of the vitreous may reduce the elasticity and rigidity of the entire eye.

Modeling ATP-mediated endothelial cell elongation on line patterns

Endothelial cell (EC) migration is crucial for a wide range of processes including vascular wound healing, tumor angiogenesis, and the development of viable endovascular implants. We have previously demonstrated that ECs cultured on 15-μm wide adhesive line patterns exhibit three distinct migration phenotypes: (a) “running” cells that are polarized and migrate continuously and persistently on the adhesive lines with possible spontaneous directional changes, (b) “undecided” cells that are highly elongated and exhibit periodic changes in the direction of their polarization while maintaining minimal net migration, and (c) “tumbling-like” cells that migrate persistently for a certain amount of time but then stop and round up for a few hours before spreading again and resuming migration. Importantly, the three migration patterns are associated with distinct profiles of cell length. Because of the impact of adenosine triphosphate (ATP) on cytoskeletal organization and cell polarization, we hypothesize that the observed differences in EC length among the three different migration phenotypes are driven by differences in intracellular ATP levels. In the present work, we develop a mathematical model that incorporates the interactions between cell length, cytoskeletal (F-actin) organization, and intracellular ATP concentration. An optimization procedure is used to obtain the model parameter values that best fit the experimental data on EC lengths. The results indicate that a minimalist model based on differences in intracellular ATP levels is capable of capturing the different cell length profiles observed experimentally.

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 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.

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.

Corneal Biomechanics in Glaucoma

Now that is possible to measure the biomechanical parameters of the cornea in vivo, scientific and clinical interest has increased in changes in these parameters in glaucoma. These parameters may act as biomarkers for early diagnosis, but also serve to emphasize the increased vulnerability to intraocular pressure, reduced blood flow or changing ocular perfusion pressure. Measuring and interpreting these parameters may help in achieving better and individually tailored glaucoma management.

Association Between Sex Hormones and Visual Field Progression in Women With Primary Open Angle Glaucoma: A Cross-Sectional and Prospective Cohort Study

Purpose: We evaluated the level of sex hormones in female patients with primary open angle glaucoma (POAG) to determine whether they are associated with the onset and/or progression of POAG.

Methods: The cross-sectional study enrolled 63 women with POAG and 56 healthy women as normal control subjects. Furthermore, 57 women with POAG were included and followed-up for at least 2 years in the cohort study. All subjects were evaluated for serum concentration of sex hormones [prolactin (PRL), luteinizing hormone (LH), testosterone (TESTO), follicle-stimulating hormone (FSH), progesterone (PROG), and estrogen (E2)] and underwent visual field (VF) examination. In the cross-sectional study, Spearman analysis, linear regression analysis, and logistic regression analysis were performed to assess risk factors for POAG in women. In the cohort study, Cox regression analyses and Kaplan–Meier survival analysis were performed to identify factors associated with VF progression in women with POAG.

Results: In the cross-sectional study, the level of E2 was significantly lower in the POAG group than in the normal group (p < 0.05). Multiple logistic regression showed that the decreased level of E2 was a risk factor of POAG (OR = 0.27, 95% CI = 0.09–0.78, p < 0.05), especially in premenopausal subjects. In the cohort study, there were 29 non-progression subjects and 28 progression subjects. Patients in the progression group had significantly lower levels of E2 than those in the no progression group (p < 0.01). The decreased level of E2 at baseline was associated with POAG progression (HR = 0.08, 95% CI = 0.02–0.46, p < 0.05), especially in premenopausal subjects. Patients with POAG and with lower baseline E2 levels had significantly lower VF non-progression rates than patients with higher E2 levels (log-rank test p < 0.001), especially premenopausal subjects (log-rank test p < 0.05). Additionally, logistic regression analyses, Cox regression analyses, and Kaplan–Meier survival analysis showed that PROG, LH, FSH, and TESTO were risk factors of POAG and/or significantly associated with POAG progression.

Conclusion: A decreased E2 level is a POAG risk factor and is associated with VF progression in women with POAG, especially in premenopausal subjects. Additionally, other sex hormones (PROG, LH, FSH, and TESTO) might also play a role in POAG pathogenesis.

Corneal Biomechanics for Ocular Hypertension, Primary Open-Angle Glaucoma, and Amyloidotic Glaucoma: A Comparative Study by Corvis ST

BACKGROUND

To evaluate biomechanical parameters of the cornea provided by Corvis ST in patients with ocular hypertension, primary open-angle glaucoma, and amyloidotic glaucoma and to compare with healthy controls.

METHODS

This was a cross-sectional study of patients with ocular hypertension, primary open-angle glaucoma, and amyloidotic glaucoma that underwent Corvis ST imaging. Primary outcome was the comparison of corneal biomechanical parameters between study groups after adjusting for age, gender, Goldmann intraocular pressure (GAT-IOP), and prostaglandin analogues medication. Secondary outcome was the comparison of different IOP measurements in each group.

RESULTS

One hundred and eighty-three eyes from 115 patients were included: 61 with primary open-angle glaucoma, 32 with amyloidotic glaucoma, 37 with ocular hypertension and 53 were healthy controls. Amyloidotic glaucoma group had smaller radius (p=0.025), lower deflection amplitude at highest concavity (p=0.019), and higher integrated radius (p=0.014) than controls. Ocular hypertension group had higher stiffness parameter at first applanation (p=0.043) than those with primary open-angle glaucoma, and higher stress-strain index (p=0.049) than those with amyloidotic glaucoma. Biomechanically corrected intraocular pressure was significantly lower than Goldmann intraocular pressure in group with primary open-angle glaucoma (p=0.005) and control group (p=0.013), and Goldmann intraocular pressure adjusted for pachymetry in group with primary open-angle glaucoma (p=0.01).

CONCLUSION

Eyes with amyloidotic glaucoma have more deformable corneas, while eyes with ocular hypertension have less deformable corneas. These findings may be linked to the susceptibility to glaucomatous damage and progression. There were significant differences between Goldmann applanation tonometry and biomechanically corrected intraocular ocular pressure provided by Corvis ST.

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.

Short-Time Changes of Intraocular Pressure and Biomechanics of the Anterior Segment of the Eye during Water Drinking Test in Patients with XEN GelStent

Purpose: Little is known about short-term changes in intraocular pressure (IOP) following minimally invasive glaucoma surgeries, such as post-XEN GelStent implantation. Although the importance of corneal biomechanics in glaucoma diagnostics has been reported, little work has been conducted on postoperative description of changes when the structure of the anterior segment is altered. The aim of presented study was to evaluate the changes in the biomechanical parameters of the anterior segment of the post-XEN GelStent implantation eyes. Patients and Methods: This investigator-initiated, open-label, prospective, single-center study recruited patients. Patients with primary open-angle glaucoma (POAG) after XEN GelStent implantation versus matched POAG controls (considered as control group/CG) treated pharmacologically were screened. Water loading was conducted using 10 mL of water per kilogram of body weight for ≤5 min. Goldmann applanation tonometry (GAT), corneal hysteresis (CH), and corneal resistance factor (CRF) were performed before water loading and after every 15 min up to 1 h. Results: The water drinking test (WDT) was positive in 3.7% (1 out of 27) of patients in the post-XEN group compared with 22.7% (5 out of 22) of patients in the control group (CG; p < 0.05). Mean fluctuations in GAT during the WDT were higher in the CG group (3.6 ± 2.5 mmHg vs. 2.9 ± 1.3 mmHg, p < 0.001). CRF and CH changed significantly only in the post-XEN group. The mean peak of CH and CRF occurred at 15 and 30 min of the test in the post-XEN group (p = 0.001). Conclusion: WDT is important to assess the ability of compensation mechanisms to reduce fluctuations in IOP after water upload. The relationship between biomechanics of the anterior segment and glaucoma may have substantial impact on surgical outcome evaluation.

Use of corneal hysteresis and corneal resistance factor in target intraocular pressure estimation in patients with early primary open-angle glaucoma

PURPOSE

To introduce a new method for estimation of the target intraocular pressure (TIOP) in naïve eyes with early primary open-angle glaucoma (POAG) using corneal hysteresis (CH) and corneal resistance factor (CRF).

METHODS

A prospective quasi-experimental study was conducted on naïve 90 eyes of 45 patients who were newly diagnosed with early primary open-angle glaucoma (POAG). They were compared to 72 eyes of 36 normal subjects. The TIOP was determined for each eye. The IOP Goldmann (IOPg), IOP corneal-compensated (IOPcc), CH and CRF were estimated by ocular response analyzer (ORA, Reichert) device. Measurements were taken for each patient prior to treatment and after 1, 3, 6, 9 and 12 months of receiving medications; either monotherapy or combination therapy.

RESULTS

For all patients, there was a significant negative correlation (p < 0.05) between IOP, either IOPg or IOPcc, and CH, while a significant positive relationship (p < 0.05) existed between IOP and CRF. For patients with early POAG, the CH was significantly increased (p ≤ 0.001), while CRF was significantly decreased (p ≤ 0.001) when TIOP was achieved. At IOP levels higher than TIOP, CH value was lower than CRF with a significant negative correlation between them in contrast to controls. This correlation was reversed on reaching TIOP and CH values became higher than CRF similar to controls.

CONCLUSION

CH, CRF and IOP measured by ORA can be used for TIOP estimation. This provides us with a guide for assessing the effectiveness of medications introduced to patients with POAG.

Corneal Hysteresis in Thais and Variation of Corneal Hysteresis in Glaucoma

PURPOSE

To collect the normal value data of corneal hysteresis in Thais and study the variation of corneal hysteresis in glaucomatous eyes.

METHODS

Retrospective cross-sectional study of corneal hysteresis (CH) in healthy non-glaucomatous and glaucomatous eyes. Demographic data, type and staging of glaucoma, Goldmann applanation tonometry (GAT) and ocular response analyzer parameters; CH, corneal-compensated intraocular pressure (IOPcc) and Goldmann-correlated intraocular pressure (IOPg) were collected.

RESULTS

Data from one eye of 465 normal participants were included for the normal value data of CH. Mean CH, IOPcc and IOPg were 10.18 ± 1.48, 15.01 ± 3.04 and 14.16 ± 3.06 mmHg, respectively. Average age was 57.21 ± 14.4 years. CH at the fifth percentile was 8.0 mmHg. Women had significantly higher CH than men (10.29 ± 1.46 vs 9.90 ± 1.49 mmHg, p=0.009). Moderate negative correlation was found between age and CH, r = -0.338, p<0.001. There were 695 glaucomatous eyes from 429 patients including primary-open angle glaucoma (POAG), primary close-angle glaucoma (PACG), normal tension glaucoma (NTG) and ocular hypertension (OHT). CH in each glaucoma type and severity stage (early, moderate and severe) were as follows: POAG: 8.74 ± 1.52 mmHg (9.22 ± 1.47, 8.74 ± 1.23 and 7.92 ± 1.40 mmHg, p<0.001), PACG: 9.09 ± 1.72 mmHg (9.85 ± 1.45, 9.04 ± 1.68 and 8.45 ± 1.74 mmHg, p= 0.004), NTG: 9.55 ± 1.67 mmHg (9.47 ± 1.38, 9.75 ± 2.42 and 9.77 ± 1.34 mmHg, p 0.525) and OHT: 10.10 ± 1.40 mmHg.

CONCLUSION

Compared with normal value data of corneal hysteresis, CH in glaucomatous eyes was lower. The more advanced glaucoma stage was associated with lower CH. Arising from normal value data, a low percentile of CH could be applied as the deviation value from normal and this dynamic property of CH could represent a glaucoma predictor in an effort to improve glaucoma care.

Corneal Factors Associated with the Amount of Visual Field Damage in Eyes with Newly Diagnosed, Untreated, Open-angle Glaucoma

INTRODUCTION

The aim of this study was to evaluate the correlation between several ocular parameters (intraocular pressure [IOP], corneal biomechanical properties) and the visual field (VF) mean deviation (VF MD) in eyes with open-angle glaucoma (OAG).

METHODS

We conducted a cross-sectional, observational study in which we measured the IOP with Goldmann applanation tonometry, the central corneal thickness (CCT), and the corneal parameters obtained from the Ocular Response Analyzer® (ORA®) and the Corvis® ST non-contact tonometer, in newly diagnosed and treatment-naïve eyes with OAG, to investigate whether there was any correlation between these ocular parameters and the VF MD.

RESULTS

A total of 51 eyes were analyzed. A statistically significant correlation was found only between the VF MD and corneal hysteresis (CH) (P = 0.003, r2 = 0.16) and CCT (P = 0.03, r2 = 0.08).

CONCLUSIONS

These results demonstrate that CH and CCT are associated with the amount of VF damage in treatment-naïve OAG eyes.

How to Measure Intraocular Pressure: An Updated Review of Various Tonometers

Intraocular pressure (IOP) is an important measurement that needs to be taken during ophthalmic examinations, especially in ocular hypertension subjects, glaucoma patients and in patients with risk factors for developing glaucoma. The gold standard technique in measuring IOP is still Goldmann applanation tonometry (GAT); however, this procedure requires local anesthetics, can be difficult in patients with scarce compliance, surgical patients and children, and is influenced by several corneal parameters. Numerous tonometers have been proposed in the past to address the problems related to GAT. The authors review the various devices currently in use for the measurement of intraocular pressure (IOP), highlighting the main advantages and limits of the various tools. The continuous monitoring of IOP, which is still under evaluation, will be an important step for a more complete and reliable management of patients affected by glaucoma.

Tonometry by Ocular Response Analyzer in Keratoconic and Warpage Eyes in Comparison with Normal Eyes

Purpose

To compare intraocular pressure (IOP) values measured by ocular response analyzer (ORA) in contact lens-induced corneal warpage, normal, and keratoconic eyes.

Methods

In a prospective, observational case-control study, 94 eyes of 47 warpage-suspected cases and 46 eyes of 23 keratoconic patients were enrolled. Warpage-suspected cases were followed until a definite diagnosis was made (warpage, nonwarpage normal, or keratoconus). ORA tonometry and corneal biomechanics testing were performed for all cases in each visit. We had 2-3 measured corneal-compensated IOP (IOPcc) and Goldmann-correlated IOP (IOPg) for each patient (based on group) with at least 2-week interval.

Results

After following up of warpage-suspected patients, finally 44 eyes of 22 patients had confirmed soft contact lens-related corneal warpage. Forty-six eyes of 23 people were finally diagnosed as nonwarpage normal eyes. Forty-six eyes of 23 known keratoconus patients were also included for comparison. The demographic and refractive data were not different between the warpage and nonwarpage normal groups but were different in the keratoconus group. Both IOPcc and IOPg were statistically different with the highest value in the warpage group followed by normal and keratoconus groups; the same trend was observed in central corneal thickness (CCT). The mean of IOPg was 14.94 ± 2.65, 13.7 ± 2.33, and 10.86 ± 3 and IOPcc was 15.73 ± 2.4, 15.28 ± 2.43, and 14.08 ± 2.55 in the warpage, normal, and keratoconus groups, respectively. IOPg and IOPcc in the warpage group (based on baseline diagnosis) did not regress to become closer to IOP of normal eyes after discontinuation of contact lens in their follow-up visits (P value for IOPg and IOPcc trends in the warpage group was 0.07 and 0.09 controlling for CCT, respectively). Both IOPcc and IOPg were significantly lower in keratoconic eyes in comparison with normal eyes. After correction for the confounding effect of CCT, a lower IOPcc in keratoconus versus warpage remained significant (P = 0.02).

Conclusion

Both IOPcc and IOPg were statistically different with the highest value in the warpage group followed by normal and keratoconus groups, just like their CCT. After correction for the confounding effect of CCT, there was no statistically significant difference between the three groups in their measured IOPcc and IOPg except for IOPcc in keratoconus versus warpage (P = 0.02).

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 Changes Caused by Acute Elevation of IOP in Eyes with and without Glaucoma

SIGNIFICANCE

Although corneal biomechanical parameters are well linked with glaucoma, their clinical utility has not yet been fully elucidated. This study was designed to provide unique evidence about the dynamic nature of corneal biomechanical parameters and their potential prognostic ability for glaucoma.

PURPOSE

This study aimed to evaluate the effect of acute intraocular pressure (IOP) elevation on corneal hysteresis (CH) and corneal resistance factor (CRF) and the associations of these biomechanical parameters with glaucomatous disease.

METHODS

Subjects participating in a prospective, longitudinal glaucoma research study had CH and CRF measured before and during ophthalmodynamometry during visits in the years 2011 to 2012. All participants were diagnosed with primary open-angle glaucoma, ocular hypertension, glaucoma suspect, or normal eyes and had a minimum of 3 years of study participation with at least five reliable visual field (VF) tests. Changes in CH, CRF, and IOP induced by ophthalmodynamometry were compared between diagnostic groups and evaluated for relationships with existing and future glaucomatous VF loss.

RESULTS

In 248 eyes of 248 subjects followed up for 7.7 ± 2.3 years, ophthalmodynamometry induced a mean IOP increase from 15.1 to 29.9 mmHg, causing a mean 34 ± 28% increase in CRF and 21 ± 25% decrease in CH. Magnitude of CH change did not differ between diagnostic groups or between eyes that did (n = 20) and did not (n = 95) develop new VF loss during the study period, nor was it related to rate of future VF progression.

CONCLUSIONS

Ophthalmodynamometry-induced IOP elevation resulted in significant acute changes in CH and CRF in this study; this suggests accounting for IOP may be important in clinical interpretation of these parameters. However, because the degree of CH change was not related to glaucoma or its progression, acute changes in CH and CRF do not seem to have a prognostic value for glaucoma.

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.

Corneal hysteresis as a risk factor for optic nerve head surface depression and retinal nerve fiber layer thinning in glaucoma patients

To evaluate the role of corneal hysteresis (CH) as a risk factor for progressive ONH surface depression and RNFL thinning measured by confocal scanning laser ophthalmoscopy (CSLO) and spectral-domain optical coherence tomography (SD-OCT), respectively in glaucoma patients. Prospective study. A total of 146 eyes of 90 patients with glaucoma were recruited consecutively. The CH measurements were acquired at baseline and 4-months interval using the Ocular Response Analyzer (Reichert Instruments, Depew, NY). Eyes were imaged by CSLO (Heidelberg Retinal Tomograph [HRT]; Heidelberg Engineering, GmbH, Dossenheim, Germany) and SD-OCT (Cirrus HD-OCT; Carl Zeiss Meditec AG, Dublin, CA) at approximately 4-month intervals for measurement of ONH surface topography and RNFL thickness, respectively. Significant ONH surface depression and RNFL thinning were defined with reference to Topographic Change Analysis (TCA) with HRT and Guided Progression Analysis (GPA) with Cirrus HD-OCT, respectively. Multivariate cox proportional hazards models were used to investigate whether CH is a risk factor for ONH surface depression and RNFL progression after adjusting potential confounding factors. All patients with glaucoma were followed for an average of 6.76 years (range, 4.56–7.61 years). Sixty-five glaucomatous eyes (44.5%) of 49 patients showed ONH surface depression, 55 eyes (37.7%) of 43 patients had progressive RNFL thinning and 20 eyes (13.7%) of 17 patients had visual field progression. In the cox proportional hazards model, after adjusting baseline diastolic IOP, CCT, age, baseline disc area and baseline MD, baseline CH was significantly associated with ONH surface depression and visual field progression (HR = 0.71, P = 0.014 and HR = 0.54, P = 0.018, respectively), but not with RNFL thinning (HR = 1.03, P = 0.836). For each 1-mmHg decrease in baseline CH, the hazards for ONH surface depression increase by 29%, and the hazards for visual field progression increase by 46%. The CH measurements were significantly associated with risk of glaucoma progression. Eyes with a lower CH were significantly associated with an increased risk of ONH surface depression and visual field progression in glaucoma patients.

Risk Factors for the Structural Progression of Myopic Glaucomatous Eyes with a History of Laser Refractive Surgery

As laser refractive surgeries (LRS) have been widely performed to correct myopia, ophthalmologists easily encounter patients with glaucoma who have a history of LRS. It is well known that intraocular pressure (IOP) in eyes with glaucoma is not accurate when measured using Goldmann applanation tonometry. However, risk factors for glaucoma progression, particularly those associated with measured IOP, have rarely been studied. We analysed data for 40 patients with a history of LRS and 50 age-matched patients without a history of LRS. Structural progression was defined as significant changes in thickness in the peripapillary retinal nerve fibre layer as identified using optical coherence tomography event-based guided progression analysis. Risk factors were determined via Cox regression analysis. Disc haemorrhage (DH) was associated with glaucoma progression in both the non-LRS group and LRS group (hazard ratio (HR): 4.650, p = 0.012 and HR: 8.666, p = 0.019, respectively). However, IOP fluctuation was associated with glaucoma progression only in the LRS group (HR: 1.452, p = 0.023). Our results show that DH was a significant sign of progression in myopic glaucoma eyes. When treating patients with myopia and glaucoma, IOP fluctuation should be monitored more carefully, even if IOP seems to be well controlled.

Association between visual field damage and corneal structural parameters

The main goal of this study is to identify the association between corneal shape, elevation, and thickness parameters and visual field damage using machine learning. A total of 676 eyes from 568 patients from the Jichi Medical University in Japan were included in this study. Corneal topography, pachymetry, and elevation images were obtained using anterior segment optical coherence tomography (OCT) and visual field tests were collected using standard automated perimetry with 24-2 Swedish Interactive Threshold Algorithm. The association between corneal structural parameters and visual field damage was investigated using machine learning and evaluated through tenfold cross-validation of the area under the receiver operating characteristic curves (AUC). The average mean deviation was − 8.0 dB and the average central corneal thickness (CCT) was 513.1 µm. Using ensemble machine learning bagged trees classifiers, we detected visual field abnormality from corneal parameters with an AUC of 0.83. Using a tree-based machine learning classifier, we detected four visual field severity levels from corneal parameters with an AUC of 0.74. Although CCT and corneal hysteresis have long been accepted as predictors of glaucoma development and future visual field loss, corneal shape and elevation parameters may also predict glaucoma-induced visual functional loss.

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.

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.

Corneal biomechanics: Measurement and structural correlations

The characterization of corneal biomechanical properties has important implications for the management of ocular disease and prediction of surgical responses. Corneal refractive surgery outcomes, progression or stabilization of ectatic disease, and intraocular pressure determination are just examples of the many key clinical problems that depend highly upon corneal biomechanical characteristics. However, to date there is no gold standard measurement technique. Since the advent of a 1-dimensional (1D) air-puff based technique for measuring the corneal surface response in 2005, advances in clinical imaging technology have yielded increasingly sophisticated approaches to characterizing the biomechanical properties of the cornea. Novel analyses of 1D responses are expanding the clinical utility of commercially-available air-puff-based instruments, and other imaging modalities-including optical coherence elastography (OCE), Brillouin microscopy and phase-decorrelation ocular coherence tomography (PhD-OCT)-offer new opportunities for probing local biomechanical behavior in 3-dimensional space and drawing new inferences about the relationships between corneal structure, mechanical behavior, and corneal refractive function. These advances are likely to drive greater clinical adoption of in vivo biomechanical analysis and to support more personalized medical and surgical decision-making.

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.

The Relationship Between Corneal Hysteresis and Progression of Glaucoma After Trabeculectomy

PURPOSE

The purpose of this study was to investigate the association of corneal hysteresis (CH) measured with Ocular Response Analyzer on the progression of glaucoma after trabeculectomy.

MATERIALS AND METHODS

Twenty-four eyes of 19 patients with primary open-angle glaucoma underwent trabeculectomy. A series of visual fields (Humphery Field Analyzer 24-2 SITA-standard) were measured starting after 6 months after trabeculectomy (4.2±5.0 y, mean±SD). The mean total deviation (mTD) of the 52 test points were calculated. In addition, the mTD was divided into the following areas: central area (within central 10 degrees), superior area and inferior area: mTDcentre, mTDsuperior, and mTDinferior, respectively. The relationship between each area's progression rate of mTD and the 7 variables of baseline age, central corneal thickness, baseline mTD, mean intraocular pressure (IOP), SD of IOP divided by the mean IOP, the difference between baseline IOP obtained before the initiation of any treatment, mean IOP, and CH were analyzed using the linear mixed model, and the optimal model was selected using the model selection method with the second ordered Akaike Information Criterion.

RESULTS

In the optimal model for mTD progression rate, only CH was selected with the coefficient of 0.11. The optimal model for the mTDcentre progression rate included mean IOP with the coefficient of -0.043 and CH with the coefficient of 0.12, and that for mTDinferior included only CH with the coefficient of 0.089. There was no variable selected in the optimal model for the mTDsuperior progression rate.

CONCLUSION

CH is a useful measure in the management of glaucoma after trabeculectomy.

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.