Corneal hysteresis and axial length among Chinese secondary school children: the Xichang Pediatric Refractive Error Study (X-PRES) report no. 4

Corneal biomechanics are not exclusively compromised in high myopia

INTRODUCTION

Research assuming linearity has concluded that corneal biomechanics are compromised in high myopia. We investigated whether this assumption was appropriate and re-examined these associations across different levels of myopia.

METHODS

Myopic (spherical equivalent refraction, SER ≤ -0.50 D) eyes of 10,488 adults aged 40-69 years without any history of systemic and ocular conditions were identified in the UK Biobank. Ordinary least squares (OLS) regression was employed to test the linear association between corneal hysteresis (CH) or corneal resistance factor (CRF), separately, and SER while controlling for age, sex, corneal radius and intraocular pressure. Quantile regression (QR) was used to test the same set of associations across 49 equally spaced conditional quantiles of SER.

RESULTS

In OLS regression, each standard deviation (SD) decrease in CH and CRF was associated with 0.08 D (95% CI: 0.04-0.12; p < 0.001) and 0.10 D (95% CI: 0.04-0.15; p < 0.001) higher myopia, respectively. However, residual analysis indicated that the linearity assumption was violated. QR revealed no evidence of a significant association between CH/CRF and SER in low myopia, but a significant (p < 0.05) positive association became evident from -2.78 D (0.06 and 0.08 D higher myopia per SD decrease in CH and CRF). The magnitude of association increased exponentially with increasing myopia: in the -5.03 D quantile, every SD decrease in CH and CRF was associated with 0.17 D (95% CI: 0.08-0.25; p < 0.001) and 0.21 D (95% CI: 0.10-0.31; p < 0.001) higher myopia. In the -8.63 D quantile, this further increased to 0.54 D (95% CI: 0.33-0.76; p < 0.001) and 0.67 D (95% CI: 0.41-0.93; p < 0.001) higher myopia per SD decrease in CH and CRF.

CONCLUSIONS

Corneal biomechanics appeared compromised from around -3.00 D. These changes were observed to be exponential with increasing myopia.

Role of waveform signal parameters in the classification of children as relatively slow and fast myopia progressors

CLINICAL RELEVANCE

Identification of the baseline chracteristics for children undergoing orthokeratology with relatively fast myopia progression can allow a more accurate determination of the risk/benefit ratio.

BACKGROUND

This study aimed to investigate if baseline corneal biomechanics can classify relatively slow and fast myopia progression in children.

METHODS

Children aged six to 12 years with low myopia (0.50 to 4.00 D) and astigmatism (less than or equal to 1.25 D), were recruited. Participants were randomised to be fitted with orthokeratology contact lenses with a conventional compression factor (0.75 D, n = 29) or an increased compression factor (1.75 D, n = 33). Relatively fast progressors were defined as participants who had axial elongation of 0.34 mm or above per 2 years. A binomial logistic regression analysis and a classification and regression tree model were used in the data analysis. The corneal biomechanics were measured with a bidirectional applanation device. The axial length was measured by a masked examiner.

RESULTS

As there were no significant between-group differences in the baseline data (all p > 0.05), data were combined for analysis. The mean ± SD axial elongation for relatively slow (n = 27) and fast (n = 35) progressors were 0.18 ± 0.14 mm and 0.64 ± 0.23 mm per 2 years, respectively. The area under the curve (p2area1) was significantly higher in relatively fast progressors (p = 0.018). The binomial logistic regression and classification and regression tree model analysis showed that baseline age and p2area1 could differentiate between slow and fast progressors over 2 years.

CONCLUSIONS

Corneal biomechanics could be a potential predictor of axial elongation in orthokeratology contact lens-wearing children.

Corneal Biomechanical Properties Demonstrate Anisotropy and Correlate With Axial Length in Myopic Eyes

Purpose

The purpose of this study was to investigate the ex vivo and in vivo biomechanical characteristic of cornea in myopic eyes.

Methods

Fifty-one corneal stromal lenticules were obtained from myopic eyes during the SMILE procedure and were tested by a biaxial tensile system within 24 hours postoperatively. The material properties of the lenticules were described using stress-strain curves and were compared among axial length (AL) <26 mm and AL ≥ 26 mm group. Pre-operative stress-strain index (SSI) parameters were used to evaluate the biomechanical properties of the cornea in vivo.

Results

Compared with AL < 26 mm, the tangent modulus significantly decreased in horizontal and vertical directions when AL ≥ 26 mm (P < 0.05); SSI also significantly decreased when AL ≥ 26 mm (P < 0.05). Anisotropic parameter is positively correlated with AL (r = 0.307, P < 0.05). Compared with AL < 26 mm, anisotropic parameter significantly increased when AL ≥ 26 mm (P < 0.05). SSI was negatively correlated with AL (r = -0.380, P < 0.05) in the AL < 26 mm group but not in the AL ≥ 26 mm group (P > 0.05). Compared with 26 mm ≤ AL < 27 mm group, the tangent modulus significantly decreased in the horizontal direction (P < 0.05) but not in the vertical direction when 27 mm ≤ AL < 28 mm (P > 0.05).

Conclusions

The biomechanical properties of cornea decreased with the increase of AL. Tangent modulus significantly decreased in the horizontal direction compared with vertical direction. AL should be taken into account during calculation of corneal biomechanical parameters in order to improve validity.

Effect of eye rubbing on corneal biomechanical properties in myopia and emmetropia

Purpose: To investigate short-term changes in corneal biomechanical properties caused by eye rubbing in myopia and emmetropia and compare the different responses between the two groups. Methods: This was a prospective observational study of 57 eyes of 57 healthy subjects aged 45 years and younger. The participants were divided into myopia and emmetropia groups. All the subjects underwent eye rubbing by the same investigator using the same technique. Biomechanical parameters were recorded using the Corvis ST device before and after 1 min of eye rubbing. One week later, all the participants underwent the test again. Statistical methods were employed to compare the differences between the data from before and after the 1 min of eye rubbing and demonstrate the different responses of the two groups. Results: After 1 min of eye rubbing, smaller SP-A1 (p < 0.001), higher deformation and deflection amplitudes (p < 0.001, p = 0.012), higher peak distances (p < 0.001), earlier A1 times (p < 0.001), faster velocities (p < 0.001), and lower maximum inverse radii (p = 0.004) were observed. According to the automatic linear modeling analysis, the refractive states (B = -5.236, p = 0.010) and biomechanically corrected intraocular pressure (bIOP) (B = 0.196, p = 0.016) had influenced a decrease in the stiffness parameter at the first applanation (SP-A1). The central corneal thickness (CCT) had decreased only in the myopia group (p = 0.039). The change of SP-A1 in amplitude was larger in the myopia group than in the emmetropia group (p < 0.001). All the parameters returned to the baseline level 1 week later. Conclusion: Eye rubbing appears to alter corneal biomechanical properties temporarily and make the cornea softer, especially for myopic young patients.

Corneal Biomechanical Parameters after 60-Year-Old

Purpose

To determine the distribution of corneal biomechanical parameters in an elderly population.

Methods

This cross-sectional study was conducted in subjects above 60 years living in Tehran. The participants were selected using multi-stage cluster sampling. Corneal biomechanical parameters were measured in a randomly selected subsample of this population using the Reichert Ocular Response Analyzer (Reichert Ophthalmic Instruments, Inc., Buffalo, NY, USA).

Results

Of 470 subjects, the data of 420 participants aged over 60 years were analyzed (mean age: 69.3 ± 6.5 years and range: 61-88 years), 363 (86.4%) of whom were male. The mean and standard deviation of corneal hysteresis (CH) and corneal resistance factor (CRF) were 8.37 ± 1.55 mmHg (95% confidence interval [CI]: 8.02-8.72) and 9.06 ± 1.70 mmHg (95% CI: 8.69-9.44), respectively. The mean CH was 8.27 ± 1.54 mmHg in men and 9.25 ± 1.28 mmHg in women, and the mean CRF was 9.00 ± 1.71 mmHg in men and 9.63 ± 1.37 mmHg in women. According to the results of multiple linear logistic regression analysis, CH had a significant association with younger age (β = -0.05, P = 0.032), female sex (β = 1.83, P < 0.001), reduced maximum keratometry (β = -0.22, P = 0.06), and increased anterior chamber volume (β = 0.01, P = 0.007). CRF had a significant correlation with a younger age (β = -0.06, P = 0.02), female sex (β = 1.01, P = 0.05), central corneal thickness (β = 0.02, P < 0.001), and reduced maximum keratometry (β = -0.39, P = 0.010).

Conclusion

The mean CH and CRF values were low in this sample of the Iranian population aged over 60 years indicating the weaker elasticity of the corneal connective tissue.

Normal intraocular pressure in Egyptian children and meta-analysis

OBJECTIVE

To determine the normal intraocular pressure in a sample of Egyptian children 0-12 years of age, and compare it to that recorded in previous studies.

METHODS

This is a prospective cross-sectional study, including 345 eyes. All children underwent complete history taking and ophthalmic examination. Any glaucomatous patients or glaucoma suspects were excluded. Intraocular pressure was measured using Haag Streit Perkins M2 handheld applanation tonometer under topical or general anaesthesia. Three readings were recorded for each eye and the average was calculated. Central corneal thickness was measured, using handheld pachymeter (Pachmate 2), whenever possible.

RESULTS

The study included 345 eyes of 187 children (101 males and 86 females), aged 2 months to 12 years (mean: 5.69 ± 3.42 years). The recorded IOP was 5-20 mm Hg (mean: 11.5 ± 2.34 mm Hg). Central corneal thickness was 469-742 μm (mean: 564.8 ± 42.72 μm). The mean recorded intraocular pressure showed positive correlation with increased age (p = 0.026) and increased central corneal thickness (p = 0.037), with a difference of 1 mm Hg for every 100-μm change in central corneal thickness. The mean IOP was significantly lower than that recorded by most previous studies. A detailed comparative analysis is presented comparing our findings with other studies including grouped analysis, by country, ethnicity and tonometers used.

CONCLUSIONS

The mean IOP in a group of normal Egyptian children was 11.5 ± 2.34 mm Hg, with positive correlation to age and central corneal thickness. This mean intraocular pressure was lower than that previously reported in any other population.

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.

Intraocular Pressure, Age, and Central Corneal Thickness in a Healthy Chinese Children Population: The Handan Offspring Myopia Study

PURPOSE

To assess the distribution and factors related to intraocular pressure (IOP) in a Chinese children population in Northern China.

METHODS

1,238 offspring aged 6 to 18 years, residing in six villages, were recruited for this current study, the Handan Offspring Myopia Study. Participants underwent an interviewer-administered questionnaire and a complete ocular examination, including standardized measurement of IOP with Perkins applanation tonometry.

RESULTS

1,648 eyes of 828 children were included in the analysis. The mean IOP was 13.9 ± 1.6 mmHg. The mean IOP for the study population increased from 13.0 ± 1.5 mmHg for those 6 to 7 years of age to 14.2 ± 1.4 mmHg for those 15 years of age or older (P < .001). The mean central corneal thickness (CCT) was 548.7 ± 32.1 μm, and had no difference among different age groups and gender. In univariate regression analysis, age (0.12 mmHg per 1 year old), height (0.09 mmHg per 5 cm), weight (0.02 mmHg per kg), body mass index (0.07 mmHg per 1 m/kg2), systolic blood pressure (0.06 mmHg per 5 mmHg), CCT (0.06 mmHg per 5 μm), and SE (-0.11 mmHg per 1 D) were correlated with IOP. In multivariate regression analysis, higher IOP was only associated with older ages (p = .002) and thicker CCT (p = .001).

CONCLUSION

The mean IOP in healthy rural Chinese children aged 6-18 years is about 14 mmHg, which is lower than in adults in the same locality. The mean IOP is slightly increasing with age during childhood, which is opposite to the result among adults. Age and CCT are the major independent factors associated with IOP.

Corneal Biomechanical Properties in Varying Severities of Myopia

Purpose: To investigate corneal biomechanical response parameters in varying degrees of myopia and their correlation with corneal geometrical parameters and axial length.

Methods: In this prospective cross-sectional study, 172 eyes of 172 subjects, the severity degree of myopia was categorized into mild, moderate, severe, and extreme myopia. Cycloplegic refraction, corneal tomography using Pentacam HR, corneal biomechanical assessment using Corvis ST and Ocular Response Analyser (ORA), and ocular biometry using IOLMaster 700 were performed for all subjects. A general linear model was used to compare biomechanical parameters in various degrees of myopia, while central corneal thickness (CCT) and biomechanically corrected intraocular pressure (bIOP) were considered as covariates. Multiple linear regression was used to investigate the relationship between corneal biomechanical parameters with spherical equivalent (SE), axial length (AXL), bIOP, mean keratometry (Mean KR), and CCT.

Results: Corneal biomechanical parameters assessed by Corvis ST that showed significant differences among the groups were second applanation length (AL2, p = 0.035), highest concavity radius (HCR, p < 0.001), deformation amplitude (DA, p < 0.001), peak distance (PD, p = 0.022), integrated inverse radius (IR, p < 0.001) and DA ratio (DAR, p = 0.004), while there were no significant differences in the means of pressure-derived parameters of ORA between groups. Multiple regression analysis showed all parameters of Corvis ST have significant relationships with level of myopia (SE, AXL, Mean KR), except AL1 and AL2. Significant biomechanical parameters showed progressive reduction in corneal stiffness with increasing myopia (either with greater negative SE or greater AXL), independent of IOP and CCT. Also, corneal hysteresis (CH) or ability to dissipate energy from the ORA decreased with increasing level of myopia.

Conclusions: Dynamic corneal response assessed by Corvis ST shows evidence of biomechanical changes consistent with decreasing stiffness with increasing levels of myopia in multiple parameters. The strongest correlations were with highest concavity parameters where the sclera influence is maximal.

Corneal biomechanical responses detected using corvis st in primary open angle glaucoma and normal tension glaucoma

Structural differences have been reported between primary open angle glaucoma (POAG) and normal tension glaucoma (NTG), and biomechanical differences between POAG and NTG may account for why NTG patients are more vulnerable to lower intraocular pressure (IOP). This study compared the biomechanical properties of POAG and NTG patients using the Corvis scheimpflug technology (ST) non-contact Scheimpflug-based tonometer, and determined the factors associated with these properties.In this retrospective cross-sectional study, 46 eyes with POAG, 54 eyes with NTG, and 61 control eyes were included. A non-contact Scheimpflug-based tonometer was used to examine and compare the corneal biomechanical responses in the POAG, NTG, and normal groups. We used univariate and multivariate regression analyses to determine the factors associated with the deformation amplitude in each group.Baseline characteristics, including age, IOP, spherical equivalent, keratometry, axial length, and central corneal thickness, were similar among the 3 groups. Severity of glaucoma, as measured by mean deviation, was similar between POAG and NTG groups. Applanation 1 velocity and deformation amplitude were significantly smaller in POAG (0.13 ± 0.02 and 1.06 ± 0.14, respectively) than NTG (0.14 ± 0.01 and 1.13 ± 0.11, respectively) and normal groups (0.14 ± 0.02 and 1.13 ± 0.10, respectively). Radius of curvature was significantly larger in the POAG group compared to the normal group. In normal controls, IOP and keratometry were significant factors related to deformation amplitude. In POAG eyes, IOP was a statistically significant predictor of deformation amplitude. In NTG eyes, however, IOP , keratometry, and axial length were statistically significant predictors of deformation amplitude.POAG eyes showed less deformable corneas compared to NTG and normal controls. IOP was significantly correlated with deformation amplitude in all groups. However, axial length was positively correlated with deformation amplitude only in NTG eyes. Characterization of the differences in biomechanical properties between POAG and NTG may contribute to a better understanding of the underlying pathophysiologies associated with these diseases.

Comparison of Corneal Biomechanics Between Low and High Myopic Eyes-A Meta-analysis

PURPOSE

To compare the corneal biomechanical difference between the low myopic eyes and high myopic eyes.

DESIGN

Systematic review and meta-analysis.

METHODS

Data sources, including PubMed, Medline, EMBASE, Web of Science, and Chinese databases including Wanfang and China National Knowledge Infrastructure, were searched to find the relevant studies. Primary outcomes were corneal hysteresis (CH), corneal resistance factor (CRF), corneal-compensated intraocular pressure (IOPcc), Goldmann-correlated intraocular pressure (IOPg), and central corneal thickness (CCT) in high myopic eyes and low myopic eyes.

RESULTS

Eleven studies were enrolled in this study. CH and CRF were significantly higher in the low myopic eyes. The mean difference of CH was 0.73 mm Hg, 95% confidence interval (CI) [0.53 to 0.93], P < .001. The mean difference of CRF was 0.20 mm Hg, 95% CI [0.04 to 0.37], P = .02. The IOPcc and IOPg were significantly lower in the low myopic eyes. The mean difference of IOPcc was -2.53 mm Hg, 95% CI [-3.24, -1.83], P < .01. The mean difference of IOPg was -1.42 mm Hg, 95% CI [-2.26, -0.58], P = .0009. There was no significant difference between the 2 groups on CCT; the mean difference was -2.85 μm, 95% CI [-9.64.3.93], P = .41.

CONCLUSION

Corneal biomechanics are different in the high myopic eyes. Low CH and CRF and high IOPcc and IOPg are suggested to be associated factors for high myopia. Future studies are needed to investigate the underlying corneal structure difference that causes the low CH and CRF value in the high myopic eyes.

IMI - Clinical Myopia Control Trials and Instrumentation Report

The evidence-basis based on existing myopia control trials along with the supporting academic literature were reviewed; this informed recommendations on the outcomes suggested from clinical trials aimed at slowing myopia progression to show the effectiveness of treatments and the impact on patients. These outcomes were classified as primary (refractive error and/or axial length), secondary (patient reported outcomes and treatment compliance), and exploratory (peripheral refraction, accommodative changes, ocular alignment, pupil size, outdoor activity/lighting levels, anterior and posterior segment imaging, and tissue biomechanics). The currently available instrumentation, which the literature has shown to best achieve the primary and secondary outcomes, was reviewed and critiqued. Issues relating to study design and patient selection were also identified. These findings and consensus from the International Myopia Institute members led to final recommendations to inform future instrumentation development and to guide clinical trial protocols.

Four-year changes in corneal biomechanical properties in children

BACKGROUND

To determine four-year changes of corneal biomechanical parameters in Iranian children aged seven to eleven years and their correlation with optical components.

METHODS

In this four-year prospective cohort study, 468 children aged seven to eleven years who were initially evaluated in 2012 were re-evaluated in 2016-2017. Multi-stage stratified cluster sampling was applied. Cycloplegic refraction, biometry using LENSTAR/BioGraph, and corneal biomechanical assessment using Ocular Response Analyzer (ORA) were undertaken for each participant. The corneal biomechanical parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), areas under the peaks 1 and 2 (p1 and p2 areas) and irregularity indices (A and B indices).

RESULTS

All biomechanical parameters except A index decreased in phase 2. The mean changes of CH and CRF were 0.68 ± 0.16 mmHg (for both parameters) during four years. The mean difference in CH and CRF was 0.23 ± 0.23 and 0.24 ± 0.23 mmHg in females and 1.03 ± 0.23 and 0.96 ± 0.23 mmHg in males, respectively. Different age groups showed varying amounts of decrease in all parameters except for A index. The age group 'ten years' experienced the smallest decrease in CH (0.02 ± 0.48 mmHg) and CRF (0.20 ± 0.47 mmHg) and the age group 'eleven years' showed the greatest decrease in CH (1.41 ± 0.35 mmHg) and CRF (0.99 ± 0.34 mmHg). According to linear regression analysis, CH and CRF had a significant direct relationship with corneal power and an inverse relationship with axial length (p < 0.001).

CONCLUSION

Age and sex are influencing factors on the ORA parameters. Older age is associated with reduced biomechanical parameters and reductions are more significant in males than females. Axial elongation and corneal flattening decrease CH and CRF.

Effect of Intravitreal Bevacizumab Injection on Corneal in vivo Biomechanics: A Pilot Study

Purpose

To evaluate the effect of intravitreal bevacizumab (IVB) injection on corneal biomechanical parameters as measured by the ocular response analyzer (ORA) and Corneal Visualization Scheimpflug Technology (CorVis).

Methods

In this prospective pilot study, ORA and CorVis parameters were recorded before and after a three-month course of IVB injection therapy in 16 patients in the injected and the contralateral non-injected control eyes. The changes in the recorded parameters in each group and the differences between the two groups were evaluated and compared.

Results

None of the changes in ORA parameters were statistically significant in the injected and non-injected groups before and three months after injection, except for corneal resistance factor (CRF) in injected eyes (paired t-test, P = 0.039). The differences in corneal hysteresis (CH) and CRF were not statistically significant between the two groups (P = 0.441 and 0.236, respectively), but significant differences were noted between corneal compensated IOP (IOPcc) and Goldmann-correlated IOP (IOPg) (P = 0.045 and 0.047, respectively). None of the changes in CorVis parameters were statistically significant in the groups before and at the end of study, except for the time of first corneal applanation (TAp1 ms) in the injected group (P = 0.040, paired t-test). Differences in TAp1, length of the second corneal applanation (LAp2 mm), velocity of the second corneal applanation (VAp2 m/s), intraocular pressure (IOP), and central corneal thickness (CCT) also showed borderline significance between the two groups.

Conclusion

In this pilot study IVB injection could change CRF, IOPcc, IOPg, and TAP1 as measured by ORA and CorVis.

Role of corneal biomechanical properties in predicting of speed of myopic progression in children wearing orthokeratology lenses or single-vision spectacles

Objective

To determine the characteristics of children who were likely to progress rapidly and gain the greatest benefit from orthokeratology (ortho-k) treatment.

Methods and analysis

The files of 113 children who participated in two myopia control studies and wore either ortho-k lenses (n=62) or single-vision spectacles (SVS) (n=51) were reviewed. Baseline cycloplegic subjective refraction, central corneal thickness, axial length, keratometry, intraocular pressure, corneal biomechanical properties and 24-month axial length data were retrieved and analysed.

Results

Multivariate analysis showed that there was significant negative correlation between axial elongation and baseline age and corneal hysteresis (p<0.05) in the SVS group. In the ortho-k group, only baseline age was significantly and negatively associated with axial elongation (p<0.01).

Conclusion

Corneal biomechanical properties and baseline age can predict the rate of axial elongation in myopic children. It may be beneficial for younger myopic children with low corneal hysteresis to commence ortho-k treatment as early as possible.

Anterior Segment Biometry and Their Correlation with Corneal Biomechanics in Caucasian Children

PURPOSE

To assess the relationship between the corneal biomechanical parameters and the anterior segment parameters in Caucasian children.

METHODS

This study included 293 eyes from 293 healthy children aged between 6 and 17 years. Corneal hysteresis (CH) and corneal resistance factor (CRF) were evaluated with the Ocular Response Analyzer, axial length (AL) with IOLMaster and the anterior segment with Pentacam. Anterior segment parameters obtained were the following: central corneal thickness (CCT), corneal volume (CV), anterior chamber depth (ACD), anterior chamber volume (ACV) and mean anterior and posterior keratometry. Two multiple linear regression models were constructed to assess the association between CH and CRF with anterior segment parameters. A value of p < 0.05 was taken as the criterion for statistical significance in all analyses.

RESULTS

The mean CH and CRF were 12.12 ± 1.71 and 12.30 ± 1.89 mmHg, respectively. Multiple linear regression revealed that CH and CRF were associated negatively with AL in both models, and positively with CCT and CV in the first and second model, respectively. Meanwhile ACD, ACV or mean keratometry did not correlated with CH and CRF. Moreover, when CCT was in the model, it explained more variability for both CH (22.1%) and CRF (30.9%) than when CV was included (16.2% for CH and 16.5% for CRF).

CONCLUSIONS

CH and CRF were correlated positively with CCT and CV, and negatively with AL in healthy Caucasian children. Moreover, corneal parameters were the most contributory variables to CH and CRF changes.

Distribution of corneal thickness and its determinants in 6-12-year-old children in an Iranian general population

Purpose

To determine the central corneal thickness (CCT), apex, and paracentral thicknesses and their determinants in children aged 6-12 years.

Methods

The present study was part of the phase 1 of Shahroud School Children Eye Cohort Study in 2015. Cluster sampling was done in urban areas while all children were invited to participate in the study in rural areas. The Pentacam HR was used for measurements. CCT was measured within the central 3 mm zone of the cornea, and corneal thickness 3 mm further from the center was considered paracentral thickness.

Results

Of 6624 students who were selected, 5620 (84.8%) participated in the study. Among 4956 students, studied in this report, 52.2% were boys, and the mean age of the study participants was 9.75 ± 1.71 years (6-12). The mean CCT and apical thickness was 556.29 ± 34.04 and 557.43 ± 34.03 μm, respectively. The mean paracentral thickness was 657.62 ± 39.11 μm in the superior, 632.65 ± 37.63 μm in the inferior, 648.64 ± 38.75 μm in the nasal, and 617.36 ± 37.19 μm in the temporal region. A multiple regression model showed that CCT decreased by 4.70 μm with every 1 diopter increase in the mean keratometry and increased by 20.06 μm with every 1 mm increase in the anterior chamber depth (ACD) (Both P-Value < 0.001). Age, sex, ethnicity and residence place were also found to be associated with CCT.

Conclusions

This study is the first to describe the distribution of corneal thickness in Iranian children with a large sample size. This study showed that corneal thickness was significantly correlated with younger age, female gender, urban residence, and a number of biometric variables.

Measurement repeatability of the dynamic Scheimpflug analyzer

PURPOSE

To evaluate the repeatability of corneal deformation parameters measured using a dynamic Scheimpflug analyzer and the impact of baseline clinical factors on the repeatability of each parameter.

STUDY DESIGN

Retrospective, cross-sectional study.

METHODS

Forty-eight eyes (48 healthy subjects; mean age, 49.0 ± 19.5 years) underwent repeated examinations with the Scheimpflug analyzer to evaluate the test-retest variability. The intraclass correlation coefficient (ICC) and repeatability coefficient as indicators of variability were computed for 35 parameters measured with the Scheimpflug analyzer. The associations between the magnitude of the test-retest variability and baseline factors, such as age, axial length (AL), intraocular pressure (IOP), and central corneal thickness (CCT), were analyzed.

RESULTS

The test-retest repeatability was excellent for 22 (62.9%) of 35 parameters (ICC ≥ 0.75), good for seven (20%), (ICC ≥ 0.6), fair for four (11.4%), (ICC ≥ 0.4), and poor for two (5.7%) parameters (ICC < 0.4). Age was associated positively with the magnitude of variability in 13 (37.1%) parameters; measurement variability was affected significantly by AL (5 parameters, 14.3%) and CCT (7 parameters, 20%) but, except for one parameter not by IOP.

CONCLUSION

Most parameters of the dynamic Scheimpflug analyzer showed favorable measurement reliability in healthy subjects. However, six parameters showed poor-to-fair repeatability. Age, AL, and CCT significantly affected the repeatability of several parameters. These results should be considered when clinicians use this device in clinical practice.

Corneal biomechanical data and biometric parameters measured with Scheimpflug-based devices on normal corneas

AIM

To analyze the correlations between ocular biomechanical and biometric data of the eye, measured by Scheimpflug-based devices on healthy subjects.

METHODS

Three consecutive measurements were carried out using the corneal visualization Scheimpflug technology (CorVis ST) device on healthy eyes and the 10 device-specific parameters were recorded. Pentacam HR-derived parameters (corneal curvature radii on the anterior and posterior surfaces; apical pachymetry; corneal volume; corneal aberration data; depth, volume and angle of the anterior chamber) and axial length (AL) from IOLMaster were correlated with the 10 specific CorVis ST parameters.

RESULTS

Measurements were conducted in 43 eyes of 43 volunteers (age 61.24±15.72y). The 10 specific CorVis ST data showed significant relationships with corneal curvature radii both on the anterior and posterior surface, pachymetric data, root mean square (RMS) data of lower-order aberrations, and posterior RMS of higher-order aberrations and spherical aberration of the posterior cornea. Anterior chamber depth showed a significant relationship, but there were no significant correlations between corneal volume, anterior chamber volume, mean chamber angle or AL and the 10 specific CorVis ST parameters.

CONCLUSIONS

CorVis ST-generated parameters are influenced by corneal curvature radii, some corneal RMS data, but corneal volume, anterior chamber volume, chamber angle and AL have no correlation with the biomechanical parameters. The parameters measured by CorVis ST seem to refer mostly to corneal properties of the eye.

Corneal Biomechanics Determination in Healthy Myopic Subjects

Purpose. To determine the corneal biomechanical properties by using the Ocular Response Analyzer™ and to investigate potential factors associated with the corneal biomechanics in healthy myopic subjects. Methods. 135 eyes from 135 healthy myopic subjects were included in this cross-sectional observational study. Cornea hysteresis (CH), corneal resistance factor (CRF), cornea-compensated intraocular pressure (IOPcc), and Goldmann-correlated intraocular pressure (IOPg) were determined with the Reichert Ocular Response Analyzer (ORA). Univariate and multivariate regression analyses were performed to investigate factors associated with corneal biomechanics. Results. The mean CH and CRF were 9.82 ± 1.34 mmHg and 9.64 ± 1.57 mmHg, respectively. In univariate regression analysis, CH was significantly correlated with axial length, refraction, central corneal thickness (CCT), and IOPg (r = −0.27, 0.23, 0.45, and 0.21, resp.; all with p ≤ 0.015), but not with corneal curvature or age; CRF was significantly correlated with CCT and IOPg (r = 0.52 and 0.70, resp.; all with p < 0.001), but not with axial length/refraction, corneal curvature, or age. In multivariate regression analysis, axial length, IOPcc, and CCT were found to be independently associated with CH, while CCT and IOPg were associated with CRF. Conclusions. Both CH and CRF were positively correlated with CCT. Lower CH but not CRF was associated with increasing degree of myopia. Evaluation of corneal biomechanical properties should take CCT and myopic status into consideration.

Changes in Corneal Biomechanical Properties after Long-Term Topical Prostaglandin Therapy

OBJECTIVE

To compare corneal biomechanical properties, measured by a newly developed tonometer (Corneal Visualization Scheimpflug Technology, Corvis ST), in untreated primary open angle glaucoma (POAG) patients, POAG patients with long-term topical prostaglandin analog (PGA) therapy and in normal controls. Further is to investigate the potential effects of PGA on corneal biomechanics.

METHODS

In this case-control study, 35 consecutive medication naïve eyes with POAG, 34 POAG eyes with at least 2 years treatment by PGA and 19 normal eyes were included. Intraocular pressure (IOP), central corneal thickness (CCT) and corneal biomechanical parameters, including deformation amplitude (DA), applanation time (AT1 and AT2), applanation length (AL1 and AL2), applanation velocity (AV1 and AV2), and peak distance and radius were measured using Corvis ST. Axial length and corneal curvature were measured with partial coherence interferometry (IOLMaster, Zeiss, Germany). General linear model analysis was performed to investigate the corneal biomechanical property changes among the normal controls, newly diagnosed POAG patients and POAG patients with long-term PGA treatment, and among the subgroups of different types of PGA treatment, including bimatoprost, latanoprost and travoprost. Furthermore, pairwise comparisons using Bonferroni correction for least squares means were employed.

RESULTS

AT1 (p<0.0001), AV1 (p<0.0001), AT2 (p = 0.0001), AV2 (p<0.0001) and DA (p = 0.0004) in newly diagnosed glaucoma patients were significantly different from those in normal subjects and in patients underwent at least 2 years topical PGA therapy after adjusting for age and gender. After adjusting for age, gender, IOP, CCT, axial length and corneal curvature, a significant difference was detected for DA between glaucoma patients without PGA treatment and patients with long-term PGA therapy (p = 0.0387). Furthermore, there were no statistical significant differences in all of the corneal biomechanical parameters among the 3 types of PGA therapy subgroups, namely bimatoprost, latanoprost and travoprost.

CONCLUSIONS

Significant changes in corneal deformation parameters were found among untreated POAG patients, POAG patients with long-term topical PGA therapy and normal controls. Long-term topical PGA treatment might have a direct effect on corneal biomechanical properties in addition to the indirect effect owing to the PGA-induced IOP reduction and CCT decrease on corneal dynamic properties.

Corneal Biomechanical Properties in Patients With Arcus Senilis

PURPOSE

To compare the corneal biomechanical properties and intraocular pressure (IOP) levels in patients with and without arcus senilis (AS).

METHODS

Ocular response analyzer measurements were performed on the right eyes of 37 patients with AS (group 1) and 37 control eyes (group 2). Corneal hysteresis, corneal resistance factor, Goldmann-correlated IOP, and corneal compensated IOP were recorded with Ocular response analyzer. Spherical equivalent value of the refractive errors, axial length, central corneal thickness, and IOP measured with Goldmann applanation tonometer were noted for each study eyes. Statistical analyses were performed with Student t, Kruskal-Wallis, and Pearson correlation tests.

RESULTS

Mean age was 67.6 ± 9.8 years in group 1 and 65.3 ± 8.1 years in group 2 (P = 0.308). Mean corneal hysteresis and corneal resistance factor readings were 9.8 ± 0.9 versus 10.6 ± 0.8 (P < 0.001) and 10.05 ± 1.07 versus 10.9 ± 0.9 (P < 0.001) in groups 1 and 2, respectively. Mean corneal compensated IOP and Goldmann-correlated IOP values were found as 16.1 ± 3.3 mm Hg versus 15.8 ± 2.6 mm Hg (P = 0.719) and 15.1 ± 3.3 mm Hg versus 15.0 ± 2.6 mm Hg (P = 0.912) in groups 1 and 2, respectively. There was no statistical difference in IOP measured with Goldmann applanation tonometer, central corneal thickness, spherical equivalent value of the refractive error, axial length measurements, and mean keratometry readings between the 2 groups (P = 0.983, P = 0.289, P = 0.938, P = 0.886, P = 0.07, respectively).

CONCLUSIONS

The mean corneal hysteresis and corneal resistance factor values of eyes with AS were lower when compared with the controls. This study demonstrated that AS may change the corneal biomechanical properties.

Corneal Thickness Profile and Associations in Chinese Children Aged 7 to 15 Years Old

Corneal thickness (CT) maps of the central (2-mm diameter), para-central (2 to 5-mm diameter), peripheral (5 to 6-mm diameter), and minimum (5-mm diameter) cornea were measured in normal Chinese school children aged 7 to 15 years old using Fourier-domain optical coherence tomography. Multiple regression analyses were performed to explore the effect of associated factors [age, gender, refraction, axial length and corneal curvature radius (CCR)] on CT and the relationship between central corneal thickness (CCT) and intraocular pressure (IOP). A total of 1228 eyes from 614 children were analyzed. The average CCT was 532.96 ± 28.33 μm for right eyes and 532.70 ±28.45 μm for left eyes. With a 10 μm increase in CCT, the IOP was elevated by 0.37 mm Hg, as measured by noncontact tonometry. The CT increased gradually from the center to the periphery. The superior and superior nasal regions had the thickest CTs, while the thinnest points were primarily located in the inferior temporal cornea. The CCT was associated with CCR (p = 0.008) but not with gender (p = 0.075), age (p = 0.286), axial length (p = 0.405), or refraction (p = 0.985). In the para-central region and the peripheral cornea, increased CT was associated with younger age, male gender, and a flatter cornea.

Effect of pathological myopia on biomechanical properties: a study by ocular response analyzer

AIM

To evaluate the ocular response analyzer (ORA) measurements of patients with pathological myopia in comparison with those of emmetropic control subjects, and to investigate the correlation between these ORA measurements and spherical equivalent (SE).

METHODS

Measurements of 53 eyes of 53 subjects with pathological myopia (SE>-6.00 D) were compared with those of 60 eyes of 60 emmetropic controls. Corneal hysteresis (CH), corneal resistance factor (CRF), noncontact tonometer intraocular pressure (IOPg), and corneal-compensated IOP (IOPcc) were obtained for each subject. The refractive error value was determined as SE via a cycloplegic refraction test.

RESULTS

The mean age was 54.1±18.9y (ranging from 5 to 88) in the pathological myopic group and 56.2±19.0y (ranging from 6 to 89) in the control group. There were no significant differences between the groups concerning age and sex. CH and CRF were significantly lower in the pathological myopic group than in the control group (P<0.001, P=0.005, respectively). IOPcc and IOPg were significantly higher in the pathological myopic group than in the control group (P<0.001, P=0.009, respectively). There were significantly positive correlations between CH and SE (r=0.565, P<0.001) and between CRF and SE (r=0.364, P=0.007). There were significantly negative correlations between IOPcc and SE (r=-0.432, P=0.001) and between IOPg and SE (r=-0.401, P=0.003).

CONCLUSION

The present study displayed that pathological myopia affected biomechanical properties measured by ORA. The results of corneal biomechanical properties measured by ORA may need to be appreciated by taking refraction into account. Further, pathological myopia might be related with the increased IOP.

Corneal biomechanics, retinal nerve fiber layer, and optic disc in children

PURPOSE

To evaluate the possible associations between corneal biomechanical parameters, optic disc morphology, and retinal nerve fiber layer (RNFL) thickness in healthy white Spanish children.

METHODS

This cross-sectional study included 100 myopic children and 99 emmetropic children as a control group, ranging in age from 6 to 17 years. The Ocular Response Analyzer was used to measure corneal hysteresis (CH) and corneal resistance factor. The optic disc morphology and RNFL thickness were assessed using posterior segment optical coherence tomography (Cirrus HD-OCT). The axial length was measured using an IOLMaster, whereas the central corneal thickness was measured by anterior segment optical coherence tomography (Visante OCT).

RESULTS

The mean (±SD) age and spherical equivalent were 12.11 (±2.76) years and -3.32 (±2.32) diopters for the myopic group and 11.88 (±2.97) years and +0.34 (±0.41) diopters for the emmetropic group. In a multivariable mixed-model analysis in myopic children, the average RNFL thickness and rim area correlated positively with CH (p = 0.007 and p = 0.001, respectively), whereas the average cup-to-disc area ratio correlated negatively with CH (p = 0.01). We did not observe correlation between RNFL thickness and axial length (p = 0.05). Corneal resistance factor was only positively correlated with the rim area (p = 0.001). The central corneal thickness did not correlate with the optic nerve parameters or with RNFL thickness. These associations were not found in the emmetropic group (p > 0.05 for all).

CONCLUSIONS

The corneal biomechanics characterized with the Ocular Response Analyzer system are correlated with the optic disc profile and RNFL thickness in myopic children. Low CH values may indicate a reduction in the viscous dampening properties of the cornea and the sclera, especially in myopic children.

Valores de normalidad en parámetros oculares en niños caucásicos españoles

<p align="justify"><em>Objetivo</em>: cuantificar los parámetros biométricos oculares, las propiedades biomecánicas corneales, la configuración anatómica del nervio óptico y la capa de fibras nerviosas de la retina (CFNR) en un grupo de niños, mediante técnicas no invasivas. <em>Material y métodos</em>: se realizó un estudio transversal, descriptivo y no aleatorio en 293 niños sanos caucásicos con edades entre 6 y 17 años. <em>Resultados</em>: se encontró una correlación estadísticamente significativa entre la longitud axial (LA) y la histéresis corneal (CH) (p &lt; 0,001). También se encontró disminución de la CH con el incremento de la miopía y el aumento de esta con el incremento de la hipermetropía (p &lt; 0,001). En el segmento posterior, se evidenció correlación positiva entre el espesor medio de la CFNR y la CH (p = 0,01). Asimismo, se apreció disminución del espesor medio de la CFNR con el incremento de la LA (p &lt; 0,001) y de la miopía (p &lt; 0,0001). <em>Conclusiones</em>: la resistencia mecánica del segmento anterior del ojo está comprometida en miopes en edades tempranas y en miopías elevadas, independientemente de la edad. Valores más bajos de CH, además de una reducción en la capacidad de amortiguación viscoelástica corneal, indican la existencia de un nervio óptico más fácilmente deformable, acompañado de un adelgazamiento de la CFNR, sobre todo en niños miopes. Este estudio permite dar unos valores de normalidad de dichos parámetros y correlacionarlos con los errores refractivos.</p>

Comparison of ocular biomechanical response parameters in myopic and hyperopic eyes using dynamic bidirectional applanation analysis

PURPOSE

To compare differences in the ocular biomechanical response in myopic and hyperopic eyes.

SETTING

London Vision Clinic, London, United Kingdom, The Ohio State University, Columbus, OH, United States.

DESIGN

Retrospective study.

METHODS

The study population included myopic and hyperopic patients evaluated preoperatively for refractive surgery at the London Vision Clinic between June 2006 and May 2008. Biomechanical response parameters from the dynamic bidirectional applanation device (Ocular Response Analyzer) were analyzed using custom software for signal analysis, including corneal hysteresis (CH) and 10 other output parameters. Hyperopic eyes were compared with myopic eyes first matched for age and pachymetry and then matched for age, pachymetry, and corneal-compensated intraocular pressure (IOPcc). Nonpaired t tests were performed (P<.05) to compare parameters in the 2 groups.

RESULTS

Consecutive patients included 2608 eyes with 1623 myopic eyes and 787 hyperopic eyes that met enrollment criteria. A significant correlation (P<.0001) was shown between CH and age (negative), pachymetry (positive), and IOPcc (negative). The first match included 473 eyes in each group, and the second match included 260 eyes in each group. When matching for age and pachymetry only, certain parameters implied that hyperopic eyes were stiffer, while others implied that myopic eyes were stiffer, but IOPcc was significantly greater in the myopic group. The second match, which also controlled for IOPcc, showed that all biomechanical parameters implied that hyperopic eyes were stiffer.

CONCLUSIONS

Hyperopic eyes demonstrated stiffer response parameters than myopic eyes. Intraocular pressure was demonstrated to be a confounding factor when evaluating ocular biomechanical parameters.

FINANCIAL DISCLOSURES

Proprietary or commercial disclosures are listed after the references.

Corneal resistance factor and corneal hysteresis in a 6- to 18-year-old population

PURPOSE

To determine the distribution and normal range of the corneal resistance factor (CRF) and corneal hysteresis (CH) in the 6- to 18-year age range and their relationship with biometric components.

SETTING

Dezful, Iran.

DESIGN

Cross-sectional study.

METHODS

This study of Dezful school children used a multistage, stratified, cluster approach sampling. All students had examinations for biometry, noncycloplegic refraction, and corneal biomechanical properties; the examinations were performed in the same order in all cases.

RESULTS

Of the 864 selected students, 683 participated in the study. The mean CRF and CH was 11.74 mm Hg±1.77 (SD) (95% confidence interval [CI], 11.58-11.89) and 11.49±1.91 mm Hg (95% CI, 11.33-11.65), respectively. In a linear multiple regression model, the CRF significantly correlated with female sex (β coefficient=0.488, P=.013), central corneal thickness (CCT) (β coefficient=.034, P=.001), and keratometry (β-coefficient=0.157, P=.003) and CH significantly correlated with CCT (β coefficient=0.025, P<.001), axial length (β coefficient=-0.303, P=.011), and keratometry (β coefficient=0.11, P=.043). Each year increase in age was associated with a 42-unit decrease in the peak 1 area (P=.003).

CONCLUSIONS

The distribution of CRF and CH in an Iranian population was symmetrical and bell shaped. However, the CRF did not have a normal distribution. The mean CRF and CH were higher than those reported in almost all previous studies.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Corneal biomechanical properties in different ocular conditions and new measurement techniques

Several refractive and therapeutic treatments as well as several ocular or systemic diseases might induce changes in the mechanical resistance of the cornea. Furthermore, intraocular pressure measurement, one of the most used clinical tools, is also highly dependent on this characteristic. Corneal biomechanical properties can be measured now in the clinical setting with different instruments. In the present work, we review the potential role of the biomechanical properties of the cornea in different fields of ophthalmology and visual science in light of the definitions of the fundamental properties of matter and the results obtained from the different instruments available. The body of literature published so far provides an insight into how the corneal mechanical properties change in different sight-threatening ocular conditions and after different surgical procedures. The future in this field is very promising with several new technologies being applied to the analysis of the corneal biomechanical properties.

Assessment of corneal biomechanical properties and intraocular pressure in myopic spanish healthy population

Purpose. To examine biomechanical parameters of the cornea in myopic eyes and their relationship with the degree of myopia in a western healthy population. Methods. Corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann correlated intraocular pressure (IOP), and corneal compensated IOP (IOPcc) were measured using the ocular response analyzer (ORA) in 312 eyes of 177 Spanish subjects aged between 20 and 56 years. Refraction was expressed as spherical equivalent (SE), which ranged from 0 to -16.50 diopters (D) (mean: -3.88 ± 2.90 D). Subjects were divided into four groups according to their refractive status: group 1 or control group: emmetropia (-0.50 ≤ SE < 0.50); group 2: low myopia (-0.75 ≤ SE < 3.00 D); group 3: moderate myopia (-3.00 ≤ SE ≤ -6.00 D); and group 3: high myopia (SE greater than -6.00 D). We analyzed the relationship between corneal biomechanics measured with ORA and SE. Results. CH in the emmetropia, low myopia, moderate myopia, and high myopia groups was 11.13 ± 0.98, 11.49 ± 1.25, 10.52 ± 1.54, and 10.35 ± 1.33 mmHg, respectively. CH in the highly myopic group was significantly lower than that in the emmetropic group (P = 0.07) and low myopic group (P = 0.035); however, there were no differences with the moderate myopic group (P = 0.872). There were no statistically significant differences regarding IOP among the four groups (P > 0.05); nevertheless, IOPcc was significantly higher in the moderately myopic (15.47 ± 2.47 mmHg) and highly myopic (16.14 ± 2.59 mmHg) groups than in the emmetropia (15.15 ± 2.06 mmHg) and low myopia groups (14.53 ± 2.37 mmHg). No correlation between age and the measured parameters was found. CH and IOPcc were weakly but significantly correlated with SE (r = 0.171, P = 0.002 and r = -0.131, P = 0.021, resp.). Conclusions. Present study showed only a very weak, but significant, correlation between CH and refractive error, with CH being lower in both moderately and highly myopic eyes than that in the emmetropic and low myopic eyes. These changes in biomechanical properties of the cornea may have an impact on IOP measurement, increasing the risk of glaucoma.

Analysis of waveform-derived ORA parameters in early forms of keratoconus and normal corneas

PURPOSE

To evaluate the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Depew, NY) performance in differentiating grades I and II keratoconus from normal corneas using 41 parameters individually and to assess the effect of analyzing all parameters together.

METHODS

This study compared the mean value of 41 ORA parameters in grades I and II keratoconus with healthy age-matched control eyes. Only eyes with a central corneal thickness between 500 and 600 μm were included. The area under the receiver operating characteristic curve was calculated for each of the 41 parameters independently and for all of the parameters together.

RESULTS

This study included 136 eyes with normal corneas and 68 eyes with grades I and II keratoconus. When analyzed individually, four ORA parameters (p1area, p1area1, p2area, and p2area1) had an area under the curve greater than 0.900 for discriminating between both groups. The p2area was the parameter that achieved the largest area under the curve individually (0.931). The area under the curve increased to 0.978 when analyzing all parameters together.

CONCLUSION

Alternative ORA parameters are better for differentiating grades I and II keratoconus from normal corneas than the four parameters originally available for ophthalmologists (corneal hysteresis, Goldmann-correlated intraocular pressure, corneal-compensated intraocular pressure, and corneal resistance factor). Although the ORA did not achieve 100% accuracy, the discrimination between these two groups was optimized by combining all parameters.

The biomechanical properties of the cornea and anterior segment parameters

BACKGROUND

To investigate the biomechanical properties of the cornea measured with the Ocular Response Analyzer (ORA) and their association with the anterior segment parameters representing the geometric dimensions including the corneal volume and anterior chamber volume.

METHODS

A retrospective review of 1020 patients who visited the BGN Eye Clinic was done. The mean radius of the corneal curvature, corneal astigmatism, corneal volume, anterior chamber depth, and anterior chamber volume were measured with an anterior segment tomographer. The central corneal thickness (CCT) was measured with an ultrasonic pachymeter. The corneal diameter was measured with an Orbscan as White to White. Cornea hysteresis (CH), corneal resistance factor (CRF), Goldmann correlated intraocular pressure (IOPg), and cornea-compensated IOP (IOPcc) were measured with an ORA. Multiple linear regression models were constructed with CH and CRF as the dependent variables and age, gender, and the anterior segment parameters as the covariates.

RESULTS

958 eyes from 958 patients (mean age 26.7 years; male 43.4%) were included in this study after excluding some eyes according to the exclusion criteria. The mean CH and CRF were 10.1 and 9.9 mmHg, respectively. The mean IOPg and IOPcc were 14.8 and 15.8 mmHg. The multivariate analysis showed that CH was negatively associated with the mean radius of the cornea curvature (regression coefficient = -0.481, p = 0.023) and positively associated with CCT (regression coefficient = 0.015, p < 0.001) and corneal volume (regression coefficient =0.059, p = 0.014). The association between CH and the corneal diameter, anterior chamber depth, and anterior chamber volume were not statistically significant. The evaluation of CRF showed that CRF was negatively associated with the mean radius of the cornea curvature (regression coefficient = -0.540, p = 0.013), and positively associated with CCT (β = 0.026, p < 0.001). The association between CRF and the corneal diameter, corneal volume, anterior chamber depth, and anterior chamber volume were not statistically significant.

CONCLUSION

The CH was shown to be positively associated with the corneal volume and the association between CH and the anterior chamber volume were not significant. The associations of CRF with the corneal volume or anterior chamber volume were not significant.

Comparison of the new rebound tonometer with Goldmann applanation tonometer in a clinical setting

PURPOSE

To evaluate the clinical usefulness of a new rebound tonometer, Icare(®) PRO (Icare PRO), by comparison with Goldmann applanation tonometry (GAT) in a study on patients with glaucoma.

METHODS

One hundred and seventy-two eyes of 86 subjects were enrolled in this study. All of the subjects were examined with an autorefractometer, Icare PRO, slit-lamp biomicroscope, GAT, ultrasound A-scan and pachymeter. Three intraocular pressure (IOP) measurements were obtained by Icare PRO and GAT. The intraobserver reliabilities were established by calculating the intraclass correlation coefficients. The Bland-Altman plot was used to compare the Icare PRO and GAT.

RESULTS

There was a good correlation between the IOP measurement by GAT and that by Icare PRO (r = 0.6995, p < 0.001). The intraclass correlation coefficients of Icare PRO and GAT were 0.778 and 0.955, respectively. The IOP differences between Icare PRO and GAT (mean: 1.92 mmHg; SD: 3.29 mmHg; 95% limit of agreement: -4.52 to 8.37 mmHg) did not vary over the wide range of central corneal thickness (p = 0.498), age (p = 0.248), axial length (p = 0.277) or spherical equivalent (p = 0.075).

CONCLUSIONS

Although IOP with Icare PRO was higher than that with GAT, especially at lower GAT IOP value, Icare PRO was found to be a reliable method and showed a good correlation with GAT. The IOP difference between Icare PRO and GAT was not affected by the central corneal thickness, age, axial length or spherical equivalent. Icare PRO can be expected not only to be a good screening tool but also to be a good substitute for GAT.

Combining ocular response analyzer metrics for corneal biomechanical diagnosis

PURPOSE

To evaluate corneal biomechanical properties in non-keratoconic myopic eyes and to identify descriptors for improving the specificity of the Ocular Response Analyzer (ORA) (Reichert Ophthalmic Instruments, Depew, NY) testing for subclinical keratoconus detection.

METHODS

Observational case series of 52 non-keratoconic non-myopic eyes and 97 non-keratoconic myopic eyes (spherical equivalent < -5 diopters [D]) in dataset 1 and 87 non-keratoconic eyes and 73 eyes with subclinical keratoconus in dataset 2. Examination included corneal topography, tomography, and biomechanical testing with the ORA. Receiver operating characteristic curves and logistic regression were used to identify optimal combinations of biomechanical indices for keratoconus detection. Main outcome measures were corneal thickness-corrected hysteresis (DifCH) and resistance factor (DifCRF), the difference between these two (CH-CRF), and the diagnostic performance of their combinations.

RESULTS

Compared to non-keratoconic non-myopic eyes, non-keratoconic myopic eyes with flat corneas (average corneal power < 44.0 D) had reduced DifCH (mean ± standard deviation, 0.11 ± 1.27 vs -0.79 ± 1.50, P < .01) and DifCRF (0.24 ± 1.16 vs -0.70 ± 1.59, P < .01) values, whereas non-keratoconic myopic eyes with steep corneas showed no difference. Keratoconic eyes exhibited lower DifCH and DifCRF values than non-keratoconic myopic eyes. Combinations of DifCH, DifCRF, and CH-CRF had increased specificity (> 80%) for subclinical keratoconus compared to the DifCRF index alone (71%).

CONCLUSIONS

In biomechanical keratoconus screening, some non-keratoconic myopic eyes show altered ocular biomechanical properties and are identified as false-positive cases. The low specificity of DifCRF when dealing with these non-keratoconic eyes could be improved by considering additional biomechanical descriptors such as DifCH and CH-CRF, which seem to be indicative of the aforementioned biomechanical profile.

Intraocular pressure and central corneal thickness in the COMET cohort

PURPOSE

To describe intraocular pressure (IOP) and central corneal thickness (CCT) in ethnically diverse, myopic young adults enrolled in COMET (the Correction of Myopia Evaluation Trial) and their association with ocular and demographic factors.

METHODS

IOP (Goldmann tonometry), CCT (handheld pachymetry), refractive error (cycloplegic autorefraction), and ocular components (A-scan ultrasonography) were measured in 385 of the original 469 subjects (mean age = 20.3 ± 1.3 years). Summary statistics for descriptive analysis, Pearson correlation coefficients, and linear regression models to formally test the association of IOP and CCT with other covariates were used.

RESULTS

Mean IOP was 15.1 ± 0.1 mm Hg and differed by ethnicity and CCT but did not vary by gender, magnitude of myopia, or vitreous chamber depth (VCD). Adjusting for CCT, IOP in black participants was 1.8 mm Hg higher than in Hispanics (p = 0.0001) and 0.8 mm Hg higher than in whites (p = 0.03). Mean CCT was 562.4 ± 1.8 μm and differed by ethnicity, VCD, and IOP after adjusting for covariates. Blacks had thinner corneas than Asians, whites, and Hispanics, with adjusted differences of 15.4, 11.8, and 15.3 μm (p = 0.03, < 0.01 and < 0.01), respectively. Eyes with shorter VCD (<17.8 mm) had 8.0-μm thinner CCT (p = 0.03). CCT did not vary by gender or magnitude of myopia. Overall, a modest positive correlation (r = 0.25, P < 0.0001) was found between IOP and CCT, which varied by ethnicity in Asians (r = 0.47; p = 0.008), blacks (r = 0.29; p = 0.002), and whites (r = 0.24; p = 0.002).

CONCLUSIONS

Myopic, black young adults had higher IOP and thinner corneas relative to other ethnic groups, suggesting that evaluation of these parameters during routine examination of these individuals should begin at a young age. Their thinner CCT should also be considered in evaluations for refractive surgery.

Relative importance of factors affecting corneal hysteresis measurement

PURPOSE

To evaluate the relative influences of several demographic, ocular, and systemic parameters on corneal hysteresis (CH).

METHODS

This is a prospective, observational, cross-sectional study using subjects recruited from consecutive Albuquerque VAMC eye clinic patients. We classified eligible subjects as primary open-angle glaucoma (POAG), ocular hypertension, glaucoma suspect, or normal. We used the Ocular Response Analyzer, Pascal Dynamic Contour Tonometer, and Goldmann applanation tonometer to obtain intraocular pressure (IOP), CH, corneal resistance factor, and ocular pulse amplitude values. We also obtained corneal curvature, central corneal thickness (CCT), axial length, retinal nerve fiber layer thickness, clinical cup/disc ratio (CDR) estimates, and standard automated perimetry metrics (mean defect, pattern standard deviation). We gathered glycosylated hemoglobin (A1C) data through chart review. Multivariate regression analyses were used to determine independent relationships between CH and the other parameters.

RESULTS

Three hundred seventeen eyes in 317 subjects were studied (116 POAG, 87 ocular hypertension, 47 glaucoma suspect, and 67 normal). In univariate regression analysis, CH varied directly with CCT (β = 0.39, p < 0.001), corneal curvature (β = 0.16, p = 0.01), corneal resistance factor (β = 0.57, p < 0.001), A1C (β = 0.15, p = 0.01), mean defect (β = 0.29, p < 0.001), and retinal nerve fiber layer (β = 0.31, p < 0.001). Factors inversely related to CH were age (β = -0.22, p < 0.001), IOP (β = -0.29, p < 0.001), ocular pulse amplitude (β = -0.11, p = 0.04), CDR (β = -0.34, p < 0.001), and pattern standard deviation (β = -0.29, p < 0.001). CH was lower in POAG compared with the other diagnostic groups. In multivariate analysis, CH was independently associated with age, IOP, CCT, A1C, glaucoma diagnosis, and CDR. Of these factors, CCT and IOP demonstrated twice as much influence on CH compared with the other four factors.

CONCLUSIONS

Although this study identified six separate variables that independently influence CH values, the overall r value indicates that these variables together only explain 40% of CH variability. These results suggest that other significant sources of variability exist and deserve investigation.