Diurnal Variation of Corneal Tangent Modulus in Normal Chinese

The Impact of Intraocular Pressure Changes on Corneal Biomechanics in Primary Open-angle Glaucoma

PURPOSE

To investigate the relationship between intraocular pressure (IOP) changes and corneal biomechanical properties, determine the quantitative relationship between IOP changes and corneal biomechanical properties in patients with glaucoma and observe the differences among different types of glaucoma when the effects of high-level IOP were excluded.

DESIGN

Prospective clinical cohort study.

METHODS

Setting: Institutional.

PATIENTS

Treatment-naive patients with primary open-angle glaucoma or ocular hypertension (OHT) were included.

OBSERVATION PROCEDURES

IOP was measured using a Goldmann applanation tonometer. Corneal biomechanics were evaluated using a corneal indentation device and corneal visualization Scheimpflug technology. Medication therapy was used for IOP reduction. Repeated measurements were taken at the baseline visit and each week thereafter within a month. Paired t tests were used to compare IOP and corneal biomechanical metrics before and after IOP-lowering therapy. One-way analysis of variance was employed to investigate potential differences across groups, with a Bonferroni post hoc correction administered for multiple intergroup comparisons.

MAIN OUTCOME MEASURES

Corneal biomechanical parameters following IOP changes.

RESULTS

Eighty-one participants (mean age, 41.63 ± 17.33 years) were included in this study. The cohort comprised 20 patients with normal-tension glaucoma (NTG), 47 with high-tension glaucoma (HTG), and 14 with OHT. The baseline corneal stiffness (88.58 ± 18.30 N/m) and corneal modulus (0.71 ± 0.16 MPa) were greater than the post-IOP reduction values (67.15 ± 9.24 N/m and 0.54 ± 0.08 MPa, respectively; P < .001). The relationships between changes in IOP and changes in corneal biomechanical parameters were Δ corneal stiffness = 2.06*ΔIOP+6.47 (P < .001) and Δ corneal modulus = 0.017*ΔIOP+0.051 (P < .001). After IOP reduction, the mean corneal stiffness at the 4th week in the NTG group was significantly lower (60.97 ± 6.36 N/m) than that in the HTG (67.25 ± 9.01 N/m) and OHT (75.62 ± 6.52 N/m, P < .001) groups. Additionally, the stiffness of HTG patients was lower than that of OHT patients (P = .003).

CONCLUSIONS

Changes in IOP have an impact on corneal biomechanical parameters. Decreases in corneal stiffness and modulus were observed after IOP reduction. When the effect of high-level IOP was excluded, corneal biomechanics varied according to the type of glaucoma. The HTG corneas were softer than the OHT corneas, and the NTG corneas were even softer.

PAX6 gene promoter methylation is correlated with myopia in Chinese adolescents: a pilot sutdy

PURPOSE

A large number of epidemiological studies have shown that myopia is a complex disease involving genetic, environmental, and behavioral factors. The purpose of this study was to explore the role of PAX6 gene methylation in myopia in Chinese adolescents.

METHODS

Eighty junior high school students were divided into four groups based on their vision test results: mild myopia, moderate myopia, severe myopia, and non-myopia control. The methylation level of PAX6 gene promoter was detected by bisulfate pyrosequencing.

RESULTS

The methylation level of PAX6 gene in myopia group (8.06% ± 1.43%) was slightly but significantly higher than that in non-myopia controls (7.26% ± 1.17%). In addition, PAX6 gene methylation levels presented a decreasing pattern along with the aggravation of myopia. Post-hoc analysis indicated significant inter-group differences for the mild myopia group and other groups (All p < .05). In the subgroup analysis by gender, the methylation level of PAX6 gene promoter in girls was higher than that in boys (p = .023). The ROC curves showed a high accuracy of PAX6 gene methylation to predict mild myopia (AUC (95% CI) = 0.828 (0.709-0.947), p < .001).

CONCLUSIONS

The methylation of PAX6 gene might play a role in the onset and progression of myopia in Chinese adolescents. And this could potentially explore the potential molecular mechanisms of juvenile myopia in the future.

Reproducibility, Repeatability, and Correlation of Central Corneal Thickness Measurement with the Pentacam Scheimpflug System and Ultrasound Pachymetry

BACKGROUND/AIM

The importance of an accurate determination of central corneal thickness (CCT) relies on its diagnostic and therapeutic implications in glaucoma, corneal ectasias, corneal edema and endothelial function monitoring, and refractive surgery suitability, among others. We aimed to analyze the repeatability, reproducibility, correlation, and laterality variations of CCT measurements performed with the Pentacam HR and the standard of care ultrasound A-scan (USP).

METHODS

A cross-sectional study including CCT measurements of healthy individuals was performed by three independent examiners with the Pentacam HR and USP. Intra-observer and inter-observer variations were calculated with intra-class correlation coefficients (ICCs). Bland-Altman plots and 95% limits of agreement (95% LoA) were used to assess the agreement between devices. Linear correlation was calculated with Pearson's coefficient.

RESULTS

Thirty individuals (60 eyes), including 10 (33.3%) men and 20 (66.6%) women, with a mean age of 30.0 ± 9.1 years, were studied. No statistical differences were found in CCT measurements between Pentacam HR (range 500 - 609 µm) and USP (range 498 - 628 µm). There was a high degree of correlation in repeatability and reproducibility of each independent device (ICC > 0.90). Pearson's correlation between 1 vs. 2, 2 vs. 3, and 3 vs. 1 Pentacam HR attempts were 0.914, 0.958, and 0.925, respectively (p < 0.001). Corresponding results for USP were 0.957, 0.957, and 0.943 (p < 0.001). The Pentacam HR tended to overestimate CCT by a mean difference of 3.77 µm (95% LoA, - 24.9 - 18.4). Right eyes were also overestimated (- 3.6 ± 14.1 µm) with the Pentacam HR device, whereas left eyes were underestimated (1.3 ± 11.1 µm).

CONCLUSIONS

The Pentacam HR device provides reliable operator-independent estimates of CCT. Right eyes exhibited a tendency to overestimate with the Pentacam HR. We suspect this difference is due to USP underestimation related to patients' position while performing the study. In clinically relevant scenarios, performing a third measurement and cautiously measuring right eyes can provide higher accuracy.

A Novel Indentation Assessment to Measure Corneal Biomechanical Properties in Glaucoma and Ocular Hypertension

Purpose

To evaluate the ability of the new in vivo corneal indentation device (CID) to measure corneal biomechanical properties.

Methods and Results

In total, 186 eyes from 46 healthy subjects, 107 patients with primary open-angle glaucoma, and 33 patients with ocular hypertension were enrolled in a cross-sectional study. Measurements were performed using corneal visualization Scheimpflug technology (Corvis ST) and the CID. The deformation amplitude (DA), inward applanation time, inward applanation velocity (A1V), outward applanation time (A2T), outward applanation velocity (A2V), highest concavity time, DA ratio, max inverse radius (MIR), integrated radius, and stiffness parameter A1 were included as Corvis ST parameters, and stiffness and modulus were included as CID parameters. Associations between the Corvis ST and CID parameters and correlations between central corneal thickness and corneal biomechanical parameters were analyzed. The stiffness was significantly correlated with all the Corvis ST parameters (P < 0.05). The modulus was significantly correlated with the DA, A1V, A2T, A2V, highest concavity time, and MIR (P < 0.05). The DA, inward applanation time, A1V, A2T, A2V, DA ratio, MIR, integrated radius, and stiffness parameter A1 values and both CID-derived values were significantly correlated with central corneal thickness (P < 0.05).

Conclusions

Parameters derived from the CID and Corvis ST demonstrated agreement in the measurement of corneal biomechanical properties. The stiffness and modulus can characterize in vivo corneal biomechanical properties.

Translational Relevance

Agreeing with the Corvis ST regarding the assessment of corneal biomechanical properties, the CID can be a novel clinical tool for biomechanical evaluation of the cornea.

Diurnal variation of corneal elasticity in healthy young human using air-puff optical coherence elastography

Due to the disruption of intraocular pressure (IOP) and central corneal thickness (CCT), diurnal variation in normal young human corneal elasticity is not clear. Using the custom-built air-puff optical coherence elastography, one eye of 21 normal subjects is enrolled randomly to measure the central corneal elasticity, IOP, and CCT in different time points within a day. Based on the multi-level model, the corneal elastic modulus is found to have a linear positive relation with IOP (P < .01) but not CCT (P = .175) and time point (P = .174-.686). A new indicator, corneal elasticity change rate, is proposed to present the magnitude of corneal elasticity change caused by 1 mmHg IOP, which can correct the interference effect of IOP. The results show that the corneal elasticity in the normal young human does not have the characteristics of diurnal variation under IOP control. Furthermore, IOP plays an important role in the corneal elasticity, and corneal elasticity change rate can increase the comparability of results between individuals.

Association between long-term orthokeratology responses and corneal biomechanics

Myopia is very prevalent worldwide, especially among Asian populations. Orthokeratology is a proven intervention to reduce myopia progression. The current study investigated association between baseline corneal biomechanics and orthokeratology responses, and changes of corneal biomechanics from long-term orthokeratology. We fitted 59 adult subjects having myopia between −4.00D to −5.00D with overnight orthokeratology. Corneal biomechanics was measured through dynamic bidirectional corneal applanation (in terms of corneal hysteresis, CH and corneal resistance factor, CRF) and corneal indentation (in terms of corneal stiffness, S and tangent modulus, E). Subjects with poor orthokeratology responses had lower E (mean 0.474 MPa) than subjects with good orthokeratology responses (mean 0.536 MPa). Successful orthokeratology for 6 months resulted in reducing CH (reduced by 5.8%) and CRF (reduced by 8.7%). Corneal stiffness was stable, but E showed an increasing trend. Among subjects with successful orthokeratology, a higher baseline S resulted in greater myopia reduction (Pearson correlation coefficient, r = 0.381, p = 0.02).

Corneal biomechanics after laser refractive surgery: Unmasking differences between techniques

The hypothesis that small-incision lenticule extraction provides better preservation of corneal biomechanics than previous laser refractive techniques has led to a growth in the interest in clinical and experimental research in this field. This hypothesis is based on the fact that corneal layers with greater stiffness are preserved with this new technique. However, this hypothesis is controversial because clinical research has shown a great disparity in the outcomes. In this review, we performed an in-depth analysis of the factors that might affect corneal biomechanics in laser refractive surgery procedures from a macrostructural to a microstructural viewpoint. New advances in algorithms with current devices or the introduction of new devices might help unmask the possible advantages of small-incision lenticule extraction in corneal biomechanics.

In vivo measurement of regional corneal tangent modulus

Currently available clinical devices are unable to measure corneal biomechanics other than at the central region. Corneal stiffness (S), thickness, and radius of curvature was measured at the central cornea (primary fixation) and 3 mm from the temporal limbus (primary and nasal fixations). The corneal tangent modulus (E) of 25 healthy subjects was calculated from these data. After confirming normality, repeated measures analysis of variance (RMANOVA) revealed significant difference in S (F(2, 48) = 21.36, p < 0.001) at different corneal regions and direction of fixations. E also varied significantly at different corneal regions and direction of fixations (RMANOVA: F(2, 48) = 23.06, p < 0.001). A higher S and a lower E were observed at the temporal region compared with the corneal centre. Nasal fixation further increased S and E values compared with primary fixation. Due to the specific arrangement of corneal collagen fibrils, heterogeneity of corneal biomechanical properties is expected. In future clinical practice, localized corneal biomechanical alternation and measurement might assist corneal disease detection and post-surgery management. In addition, practitioners should be aware of the fixation effect on corneal biomechanical measurement.