Performance of neuroretinal rim thickness measurement by Cirrus high-definition optical coherence tomography in myopic eyes.

Neuroretinal rim (NRR) measurement can aid the diagnosis of glaucoma. A few studies reported that Cirrus optical coherence tomography (OCT) had NRR segmentation errors. The current study investigated segmentation success of NRR in myopic eyes using the Cirrus built-in software and to determine the number of acquisitions required to identify NRR thinning. Right eye of 87 healthy adult myopes had an optic disc scanned using Cirrus HD-OCT for five successive acquisitions. A masked examiner evaluated 36 radial line images of each scan to screen for segmentation errors using the built-in software at the Bruch's membrane opening (BMO) and/or internal limiting membrane (ILM). Participants with three accurate NRR acquisitions had their average NRR thickness determined. This result was compared with average of the two acquisitions and the first acquisition. Among 435 OCT scans of the optic disc (87 eyes × 5 acquisitions), 129 (29.7%) scans had segmentation errors that occurred mainly at the ILM. The inferior-temporal and superior meridians had slightly more segmentation errors than other meridians, independent of axial length, amount of myopia, or presence of peripapillary atrophy. Sixty-five eyes (74.7%) had at least three accurate NRR measurements. The three acquisitions had high reliability in NRR thickness in the four quadrants (intraclass correlation coefficient > 0.990, coefficient of variation < 3.9%). NRR difference between the first acquisition and the average of three acquisitions was small (mean difference 2 ± 13 µm, 95% limits of agreement within ± 30 µm) among the four quadrants. Segmentation errors in NRR measurements appeared regardless of axial length, amount of myopia, or presence of peripapillary atrophy. Cirrus segmentation lines should be manually inspected when measuring NRR thickness.

Excessive use of electronic devices among children and adolescents is associated with musculoskeletal symptoms, visual symptoms, psychosocial health, and quality of life: a cross-sectional study.

Objective: Electronic devices have become an indispensable part of our daily lives. The frequency and duration of device use in children and adolescents have increased drastically over the years and the study of its negative musculoskeletal, visual and psychosocial health impacts is necessary. Materials and methods: This cross-sectional study aimed to evaluate the associations between electronic device use and the prevalence and severity of musculoskeletal symptoms, visual symptoms, psychosocial health, and quality of life in children and adolescents studying at primary and secondary schools. Data were collected through confidential online and paper-and-pencil questionnaires. Primary 4-5 and Secondary 1-4 students were recruited from 3 schools in Hong Kong. Demographics, frequency and duration of electronic device use, frequencies of musculoskeletal symptoms, visual symptoms, psychosocial health, and quality of life outcomes were measured. Results: 1,058 children and adolescents aged 9-17 years participated. Sixty-one percent and 78% of all students spent more than 2 h per day using electronic devices during school days and weekend/holidays, respectively. Extended electronic device use was associated with increased prevalence and severity of musculoskeletal symptoms (ρ's = 0.28-0.33, P's < 0.001), visual symptoms (ρ's = 0.33-0.35, P's < 0.001), and poorer device use-related psychosocial health (ρ's = 0.38-0.47, P's < 0.001). Secondary school students reported greater device use and severity of symptoms than primary school students. Conclusion: Excessive electronic device use was associated with increased prevalence and severity of physical and psychosocial symptoms, and such use is more prevalent in adolescents when compared to the children. The findings have important health implications for children and adolescents, suggesting that early intervention is needed to reduce the risk of developing device use-related disorders.

Effect of caffeine on superficial retinal vasculature of the macula in high myopes using optical coherence tomography angiography – A pilot study

Purpose: To monitor effect of caffeine on vasculature of the inner retina of high myopesMethods: This was a crossover, self‐control, randomized trial. Healthy young high myopes were recruited to take 200 mg of caffeine capsule and placebo capsule, randomly assigned in two visits separated by at least one week. Superficial retinal vasculature in terms of vessel length density (VD) and perfusion area density (PD) was captured and monitored using a spectral domain optical coherence tomography angiography (OCTA) machine. After baseline measurements, blood pressure (BP), intraocular pressure (IOP), and subfoveal choroidal thickness (ChT) were also monitored at 30‐min intervals till 3 h.Results: Eighteen subjects (6 male, 24.3 ± 3.1 years) completed the study. After taking the caffeine capsule, there was a significant increase in BP (p < 0.01), and reduction in ChT (p < 0.01), with no change in IOP (p = 0.36). VD demonstrated a trend of reduction at the central 1‐mm circle, and 1‐3 mm annulus (p < 0.01) following the ETDRS grid. Reduction trend of PD appeared at the central 1‐mm circle, 1‐3 mm annulus, and the entire 3‐mm circle (p < 0.01). Compared with baseline, VD and PD reductions were significant 180 min after taking the caffeine capsule at the central 1‐mm circle, but the reduction was small (VD: by 1mm-1; PD: by 1%). Changes in other regions were not significant.Conclusions: The current study found significant reduction in VD and PD after taking 200 mg of caffeine capsule. Such a small amount of alteration may be clinically irrelevant. (AU)

COVID-19: ensuring safe clinical teaching at university optometry schools.

The COVID-19 pandemic has been spreading across the globe for several months. The nature of the virus (SARS-CoV-2) with easy person-to-person transmissions and the severe clinical course observed in some people necessitated unprecedented modifications of everyday social interactions. These included the temporary suspension of considerable elements of clinical teaching at optometry schools worldwide. This article describes the challenges optometry schools were facing in early to mid 2020. The paper highlights the experiences of six universities in five countries on four continents. Strategies to minimise the risk of virus transmission, to ensure safe clinical optometric teaching and how to overcome the challenges presented by COVID-19 are described. An outlook on opportunities to further improve optometric education is provided.

Impact of Axial Eye Size on Retinal Microvasculature Density in the Macular Region.

Mechanical tissue stresses are important contributors to the increased risk of sight-threatening pathology in larger, more myopic eyes. The contribution of altered ocular vasculature to the development of this pathology is less well defined. The current study investigated the impact of eye size on the superficial vasculature of the macula. Subjects (n = 104) aged 18-50, with no history of ocular or vascular disease, or myopia control, were recruited from university staff and student populations in Australia and Hong Kong. Refractive error, ocular size, retinal morphology and vascular morphology were quantified through open field autorefraction, ocular biometry and ocular coherence tomography angiography. Morphology of the superficial retinal capillary plexus was assessed over a 3 × 3 mm fovea-centred area. Perfusion area and vessel length densities were analysed relative to axial eye length and retinal thickness. A significant inverse association was found between axial length and vascular density measures (perfusion area density r2 = 0.186, p < 0.001; and vessel length density r2 = 0.102, p = 0.001). Perfusion area and vessel length densities were reduced by 5.8% (p = 0.001) in the longest, relative to the shortest, eyes. The aggregated ganglion cell layer inner plexiform layer thickness was also inversely associated with eye size (r2 = 0.083, p = 0.003), and reduced, by 8.1% (p < 0.001), in the longest eyes. An inverse association of eye size and superficial retinal vasculature density, that is not simply explained by retinal expansion or image magnification factors, was confirmed. These data support the hypothesis that ongoing metabolic challenges may underlie the development of myopia-related and -associated pathology in larger eyes.

Multiple scan averaging to yield accurate quantitative analysis of optical coherence tomography angiograms.

Optical coherence tomography angiography (OCTA) is widely used in ophthalmic practice. Most OCTA studies based their findings on a single OCTA measurement. We conducted an observational study of 82 eyes from 82 healthy subjects to compare variations of OCTA parameters among five successive measurements. A 3 × 3 mm Early Treatment of Diabetic Retinopathy Study grid centred at fovea was used. An average from five successive OCTA measurements (both perfusion density and vessel density) was calculated to be used as the reference standard. There was no significant difference in perfusion and vessel densities among five successive OCTA measurements, and from different levels of averaging. Perfusion density was close to the reference standard when average from three measurements was used (discrepancy within 1.5%) as compared with using just one measurement (discrepancy from 3.2% to 4.5%). Vessel density was also close to reference standard when average from three measurements was used (within 0.8 mm-1) as compared with using just one measurement (2 mm-1). Software feature that allows OCTA devices to average quantitative parameters for analysis will be useful.

Comparison of Choroidal Thickness Measurements Using Semiautomated and Manual Segmentation Methods.

SIGNIFICANCE: This study demonstrated that a semiautomated segmentation method could help inexperienced practitioners to obtain choroidal thickness as good as experienced practitioners. PURPOSE: The purpose of this study was to compare choroidal thickness measurements obtained by semiautomated and manual segmentation methods. METHODS: Optical coherence tomography images of 37 eyes from 37 healthy young subjects acquired by a spectral-domain optical coherence tomography device were reviewed retrospectively. Two naive examiners measured choroidal thickness using manual and semiautomated methods, whereas two experienced examiners used only the semiautomated method. The semiautomated method referred to a fully automated segmentation program customized based on MATLAB and followed manual verification. After highlighting the inner and outer choroidal boundaries through automated segmentation, examiners reviewed these boundaries in each B-scan and conducted manual revisions if segmentation errors occurred. After selecting points where correct boundary was located, the software used a spline fit to blend the corrected region with the rest of the boundary. All measurements were summarized in a 6-mm Early Treatment Diabetic Retinopathy Study grid. Operation time spent to complete retinal and choroidal segmentation on each eye was recorded. Between-examiner agreements, that is, intraclass correlation coefficient and coefficient of reproducibility (CoR), were calculated among four sets of semiautomated measurements, and within-examiner agreements were comparisons between manual and semiautomated results from the same naive examiners. Eyes with thin or thick choroids were also analyzed separately. RESULTS: The between-examiner and within-examiner agreements were excellent with intraclass correlation coefficient of 0.976 or greater. Pairwise within-examiner CoRs ranged from 17.4 to 47.1 µm. Pairwise between-examiner CoRs were between 13.0 and 38.9 µm. Eyes with thin choroid had better agreements than those with thick choroids. On average, naive examiners saved 3 to 5 minutes per eye using the semiautomated method. CONCLUSIONS: With the help of a dedicated software, inexperienced practitioners could obtain choroidal thickness measurements with accuracy similar to experienced practitioners. Processing time with the semiautomated method was also reduced.

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).

Automatic quantification of superficial foveal avascular zone in optical coherence tomography angiography implemented with deep learning.

An accurate segmentation and quantification of the superficial foveal avascular zone (sFAZ) is important to facilitate the diagnosis and treatment of many retinal diseases, such as diabetic retinopathy and retinal vein occlusion. We proposed a method based on deep learning for the automatic segmentation and quantification of the sFAZ in optical coherence tomography angiography (OCTA) images with robustness to brightness and contrast (B/C) variations. A dataset of 405 OCTA images from 45 participants was acquired with Zeiss Cirrus HD-OCT 5000 and the ground truth (GT) was manually segmented subsequently. A deep learning network with an encoder-decoder architecture was created to classify each pixel into an sFAZ or non-sFAZ class. Subsequently, we applied largest-connected-region extraction and hole-filling to fine-tune the automatic segmentation results. A maximum mean dice similarity coefficient (DSC) of 0.976 ± 0.011 was obtained when the automatic segmentation results were compared against the GT. The correlation coefficient between the area calculated from the automatic segmentation results and that calculated from the GT was 0.997. In all nine parameter groups with various brightness/contrast, all the DSCs of the proposed method were higher than 0.96. The proposed method achieved better performance in the sFAZ segmentation and quantification compared to two previously reported methods. In conclusion, we proposed and successfully verified an automatic sFAZ segmentation and quantification method based on deep learning with robustness to B/C variations. For clinical applications, this is an important progress in creating an automated segmentation and quantification applicable to clinical analysis.

High myopes have lower normalised corneal tangent moduli (less 'stiff' corneas) than low myopes.

PURPOSE: To compare corneal tangent moduli between low and high myopes. METHODS: Corneal hysteresis (CH) and corneal resistance factor (CRF) of 32 low and 32 high myopes were obtained using an Ocular Response Analyzer, followed by a corneal indentation device that measured corneal stiffness. Corneal topography, pachymetry, Goldmann applanation tonometry intraocular pressure (GAT-IOP), and corneal compensated intraocular pressure (IOPcc) were also obtained. Corneal tangent modulus was calculated on the basis of corneal stiffness, central corneal thickness and corneal radius. Comparisons between groups and associations between corneal biomechanical and ocular parameters were performed. RESULTS: Corneal tangent moduli were positively correlated with GAT-IOP (R2 = 0.078, p = 0.025), and IOPcc (R2 = 0.12, p = 0.006). Despite similarity in corneal thickness and radius, high myopes exhibited a significantly higher IOPcc (16.4 ± 2.51 mmHg) than low myopes (13.1 ± 1.96 mmHg; t(62) = -5.57, p < 0.0001). Both groups had similar corneal stiffness (0.063 ± 0.0085 and 0.063 ± 0.0079 N mm-1 for low and high myopes, respectively) and CRF (9.6 ± 1.58 and 9.5 ± 1.90 mmHg for low and high myopes, respectively). Moreover, high myopes exhibited a significantly lower CH (9.5 ± 1.51 mmHg) than low myopes (10.6 ± 1.38 mmHg; t(62) = 2.92, p = 0.005). After normalising corneal tangent moduli to the mean intraocular pressure in normal eyes (15.5 mmHg) using IOPcc, high myopes showed a significantly lower corneal tangent moduli (0.47 ± 0.087 MPa) than low myopes (0.57 ± 0.099 MPa; t(62) = 4.17, p < 0.0001). CONCLUSIONS: High myopes had lower normalised corneal tangent moduli than low myopes, which indicated that their corneas were less stiff. This is the first in vivo study comparing elastic moduli of the cornea in different refractive groups. Further studies are warranted to understand whether a less stiff cornea is a cause for or an outcome from myopia development.

Diurnal Variation of Corneal Tangent Modulus in Normal Chinese.

PURPOSE: The aim of this study was to investigate the diurnal variation of corneal tangent modulus, measured using a novel corneal indentation device, in healthy Chinese subjects. METHODS: The central corneal thickness (CCT), mean central corneal radius (meanK), intraocular pressure (IOP), and corneal stiffness of 25 young adults aged 21 to 25 years (23.0 ± 1.0 yrs) were measured at 3-hour intervals from 09:00 to 21:00 in the course of 1 day. Corneal tangent modulus was calculated on the basis of corneal stiffness, CCT, and meanK. Repeated-measures analyses of variance were performed to compare the diurnal changes in ocular parameters over time. RESULTS: Significant diurnal variations were observed in CCT and IOP (P < 0.001 and P = 0.025, respectively). Both parameters showed a decreasing trend throughout the day. MeanK and corneal stiffness did not show any significant diurnal changes (P = 0.251 and P = 0.516, respectively). Mean corneal tangent modulus across all measurements was 0.047 ± 0.085 MPa, and its diurnal rhythm ranged from 0.469 to 0.485 MPa. The variation was nonsignificant (P = 0.526). CONCLUSIONS: The elastic properties of the cornea in healthy Chinese subjects were stable during wake time. The present study shows that the corneal indentation device obtains stable corneal biomechanics similar to other clinical devices. Future studies investigating the differences in corneal biomechanics among patients with various ocular conditions are warranted.

Repeatability of a novel corneal indentation device for corneal biomechanical measurement.

PURPOSE: The aim of the study was to evaluate the repeatability of a new device for measuring corneal biomechanics. METHODS: Twenty-nine normal subjects aged 20-28 years (23.4 ± 1.7 years) underwent measurements of corneal stiffness and tangent elastic modulus using a novel corneal indentation device. Corneal topography, axial biometry and Goldmann applanation tonometry were also performed during the visit. Subjects returned after about 1 week, at approximately the same time, and with the corneal biomechanics, corneal topography and Goldmann applanation tonometry measured again. Both the intrasession and intersession repeatability was assessed. RESULTS: Both the corneal stiffness and tangent elastic modulus demonstrated good intrasession repeatability (corneal stiffness: coefficient of variation = 7.32%, intraclass correlation coefficient = 0.75; tangent elastic modulus: coefficient of variation = 7.34%, intraclass correlation coefficient = 0.84). The mean modulus after normalised to normal intraocular pressure of 15.5 mmHg was 0.755 ± 0.159 MPa. There was no significant difference between the two visits (paired t-tests: p > 0.05). The repeatability [1.96 times the standard deviation (S.D.) of the intersession difference] of the corneal stiffness and the tangent elastic modulus was 0.0022 N mm(-1) and 0.197 MPa, respectively. CONCLUSION: The corneal indentation device has good intrasession and intersession repeatability. It has good potential to measure corneal biomechanics clinically, even at different corneal regions.

Correction on the distortion of Scheimpflug imaging for dynamic central corneal thickness.

The measurement of central corneal thickness (CCT) is important in ophthalmology. Most studies concerned the value at normal status, while rare ones focused on its dynamic changing. The commercial Corvis ST is the only commercial device currently available to visualize the two-dimensional image of dynamic corneal profiles during an air puff indentation. However, the directly observed CCT involves the Scheimpflug distortion, thus misleading the clinical diagnosis. This study aimed to correct the distortion for better measuring the dynamic CCTs. The optical path was first derived to consider the influence of factors on the use of Covis ST. A correction method was then proposed to estimate the CCT at any time during air puff indentation. Simulation results demonstrated the feasibility of the intuitive-feasible calibration for measuring the stationary CCT and indicated the necessity of correction when air puffed. Experiments on three contact lenses and four human corneas verified the prediction that the CCT would be underestimated when the improper calibration was conducted for air and overestimated when it was conducted on contact lenses made of polymethylmethacrylate. Using the proposed method, the CCT was finally observed to increase by 66 ± 34 µm at highest concavity in 48 normal human corneas.

The roles of cornea and axial length in corneal hysteresis among emmetropes and high myopes: a pilot study.

PURPOSE: To observe the roles of cornea and axial length (AL) in corneal hysteresis (CH) among emmetropes and high myopes. MATERIALS AND METHODS: The CH, corneal resistance factor (CRF) and corneal-compensated intraocular pressure (IOPcc) of 40 young emmetropes (spherical equivalent +/-0.50 D) and 40 age-matched high myopes (-6D or more) were measured by the Ocular Response Analyzer. AL was measured using a partial coherence interferometer. The mean keratometry reading (meanK) and corneal volume (CV) in 4 zones were measured using the Pentacam. Full stromal thickness (FST) and epithelial and Bowman's thicknesses (Epi + BT) were evaluated using confocal microscopy. Valid confocal microscopic results from the more emmetropic eyes of 30 emmetropes and the more myopic eyes of 27 high myopes were studied. RESULTS: High myopes exhibited significantly longer AL (27.34 ± 0.90 mm versus 23.71 ± 0.79 mm), lower CH (10.17 ± 1.38 mmHg versus 11.11 ± 1.25 mmHg), and higher IOPcc (16.50 ± 3.05 mmHg versus 13.91 ± 2.49 mmHg) compared with emmetropes. The 2 groups showed similar CRF, CVs, meanK, FST and Epi + BT. By combining the results of both groups, CH and CRF were significantly correlated with FST (r(2)= 0.368 and r(2 )= 0.408, respectively), CVs (r(2)= 0.251-0.391 and r(2)= 0.201-0.346, respectively), but not with Epi + BT or meanK. Furthermore, AL and IOPcc were significantly correlated with CH (r(2)= 0.112 and r(2)= 0.161, respectively) but not with CRF. The multivariate analysis indicated that FST (standardized coefficients, ß = 0.591) and CV (ß = 0.575) had stronger association than IOPcc and AL for CH. CONCLUSIONS: High myopes exhibited lower CH than emmetropes. Although CH had stronger correlation with FST than AL, it might contain the biomechanical properties of components other than the cornea.

Dynamic contour tonometry over silicone hydrogel contact lens.

PURPOSE: This study compared the measurements of intraocular pressure (IOP) and ocular pulse amplitude (OPA) using the Dynamic Contour Tonometry (DCT) over silicone hydrogel contact lenses of different modulus. Corneal biomechanics were also measured using the Ocular Response Analyzer (ORA). METHODS: Forty-seven young (mean age 22.3 years, standard deviation 1.2 years) subjects had IOP, OPA, corneal hysteresis (CH) and corneal resistance factor (CRF) measured without lens and with two brands of silicone hydrogel contact lenses. Each eye wore one brand followed by another, randomly assigned, and then the lenses switched over. Difference and agreement of IOP and OPA with and without silicone hydrogel contact lens were studied. RESULTS: The right and left eyes had similar corneal curvatures, central corneal thicknesses, IOP, OPA and corneal biomechanics at baseline. No significant difference was found in CH and CRF when they were measured over different contact lenses. IOP demonstrated a greater difference (95% limits of agreement: 2.73mmHg) compared with no lens when it was measured over high modulus silicone hydrogel lenses. Agreement improved over low lens modulus silicone hydrogel lenses (95% limits of agreement: 2.2-2.4mmHg). 95% limits of agreement were within 1.0mmHg for OPA. CONCLUSIONS: This study demonstrated the feasibility of DCT over silicone hydrogel lenses. Low lens modulus silicone hydrogel contact lens in situ has no clinical effect on DCT.

Improving interobserver variation in corneal sublayer pachymetry using ConfoScan4 with z ring.

OBJECTIVES: To observe and improve the interobserver variation of corneal sublayer pachymetry using ConfoScan4 (CS4) z ring. METHODS: Right corneas of 34 normal subjects were scanned using CS4 (z ring) (Nidek Technologies, Padova, Italy). Corneal sublayer pachymetry was performed by two masked observers based on the written frame selection criteria from literature. The full stromal thickness (FST), epithelial and Bowman layer thickness (Epi+BT), endothelial thickness (EndoT), and total corneal thickness (CT) were obtained. Upon completion of the measurements (measurement 1), the frame selection criteria were reviewed between the observers by going through some corneal frames together. Eight subjects were excluded and corneal sublayer pachymetry was re-evaluated by the two observers using the revised criteria (measurement 2). RESULTS: Corneal thickness showed significant interobserver difference in measurement 1 (n=34), but no significant difference in measurement 2 (n=26). Endothelial thickness remained significant difference between observers throughout the study. Interobserver difference significantly reduced using the revised criteria in CT (1.93 ± 3.23 µm to 0.28 ± 1.75 µm) and EndoT (4.01 ± 4.82 µm to 0.86 ± 1.81µm). There was no significant difference between observers in Epi+BT and FST for the two measurements. Interobserver variation and the 95% confidence limits between observers for CT and corneal sublayers were reduced at least by half in measurement 2. CONCLUSIONS: The interobserver variation in corneal sublayer pachymetry could be improved by having the observers going through some corneal frames together rather than just following the written criteria. The use of CS4 (z ring) to measure CT, FST, and Epi+BT is suitable for monitoring corneal changes clinically.

Pentacam anterior chamber parameters in young and middle-aged Chinese.

BACKGROUND: The aim was to compare the anterior segment of two groups of Chinese using the Pentacam system. METHODS: Fifty young (median age 21 years) and 50 middle-aged (median age 46 years) subjects with equal numbers of males and females had their axial length measured with the IOL Master. Their axial anterior chamber depth, anterior chamber volume and temporal and nasal anterior chamber angles were measured using the Pentacam and comparisons were made between the two groups. RESULTS: Both groups shared similar axial lengths (unpaired t-test: p = 0.37) but young subjects had significantly deeper anterior chamber by 11 per cent (3.60 ± 0.40 mm) compared with middle-aged subjects (3.24 ± 0.34 mm) (analysis of variance: p < 0.001). The anterior chamber volume was significantly greater in young subjects by 24 per cent (200.12 ± 35.59 mm(3)) than the middle-aged subjects (161.03 ± 37.79 mm(3)). The associations of anterior chamber depth and anterior chamber volume with axial length were significant (R(2) from 0.16 to 0.50) and similar in both groups. Young subjects had wider anterior chamber angles (mean differences: 4.60° and 6.07° at temporal and nasal, respectively) than middle-aged subjects. Among middle-aged subjects, females had significantly shorter axial length but anterior chamber depths were similar between the two sexes. Females had significantly smaller anterior chamber volumes. CONCLUSIONS: Middle-aged subjects had more crowded anterior chambers than young subjects with similar axial lengths. Anterior chamber volume may be a more sensitive parameter to reveal this difference than a linear measurement of the anterior chamber depth.

Corneal deformation measurement using Scheimpflug noncontact tonometry.

PURPOSE: To determine the intraexaminer repeatability and intersession reproducibility of corneal deformation measurement using Scheimpflug noncontact tonometry (Corvis ST) on normal subjects. METHODS: Thirty-seven adults aged 20 to 48 years were invited to have their corneal deformation and curvature measurements taken using Corvis ST and Pentacam, respectively. Three consecutive measurements were taken for each instrument between 9:00 and 11:00 AM for intraexaminer repeatability analysis. Participants returned between 3:00 and 5:00 PM the same day for intersession reproducibility analysis. RESULTS: The most repeatable corneal parameter measured by Corvis ST was central corneal thickness ([CCT] ICC, 0.96; precision, 10.85 µm; repeatability, 15.34 µm; CV, 1.01%), followed by deformation amplitude ([DA] ICC, 0.80; precision, 0.08 mm; repeatability, 0.13 mm; CV, 4.33%), first applanation time ([1st A-time] ICC, 0.77; precision, 0.22 milliseconds; repeatability, 0.31 milliseconds; CV, 1.42), and intraocular pressure ([IOP] ICC, 0.75; precision, 1.39 mm Hg; repeatability, 1.97 mm Hg; CV, 4.98). Other parameters showed poor repeatability. The DA and 1st A-time showed good intersession reproducibility. The 95% limits of agreement were +0.13 to -0.13 mm for DA and +0.27 to -0.33 milliseconds for 1st A-time. The DA was negatively correlated with central corneal thickness (r = -0.53, p < 0.001) but not with corneal curvatures (flattest curvature, r = 0.13, p = 0.46; steepest curvature, r = 0.05, p = 0.75). CONCLUSIONS: Corneal deformation parameters DA and 1st A-time were repeatable and reproducible. A thinner cornea was associated with a higher corneal deformation. Measurement of DA serves as an indicator of corneal biomechanical properties.

Repeatability of corneal biomechanical measurements in children wearing spectacles and orthokeratology lenses.

PURPOSE: To determine the repeatability of corneal hysteresis (CH) and corneal resistance factor (CRF) measurements in children wearing spectacles and children under orthokeratology (ortho-k) therapy using the Ocular Response Analyzer (ORA). METHODS: CH and CRF were measured twice at a 10-min interval using ORA on the same day in both eyes of 25 children (mean age = 10.6 ± 1.2 years) wearing spectacles and 34 children (mean age = 10.9 ± 1.0 years) wearing ortho-k lenses. Four measurements were obtained from each eye in each set of measurements. Only data from the right eyes were analyzed. RESULTS: No significant between-measurement differences in CH and CRF were found in either group of subjects (paired t-tests, p > 0.05) and no correlations were found between the mean differences and their means (Pearson's correlations, -0.09 < r < 0.21, 0.24 < p < 0.85). The 95% limits of agreement (LA) were -1.87 mmHg to +2.59 mmHg and -1.53 mmHg to +1.41 mmHg for CH and -1.86 mmHg to +2.22 mmHg and -1.45 mmHg to +1.57 mmHg for CRF in the spectacle and ortho-k groups, respectively. A significant between-group difference in CRF was found (unpaired t-tests, p = 0.02). CONCLUSIONS: Repeatability of CH and CRF measurements using the ORA is within ± 2 mmHg in children wearing spectacles or ortho-k lenses. We suggest that the ORA can be used to monitor long-term corneal biomechanical changes during ortho-k treatment.

Influence of corneal astigmatism, corneal curvature and meridional differences on corneal hysteresis and corneal resistance factor.

BACKGROUND: This study investigated the influence of corneal astigmatism, corneal curvature and meridional differences on corneal hysteresis (CH) and the corneal resistance factor (CRF) in a group of normal Chinese persons. METHODS: Ninety-five participants were recruited and data from the eye with higher corneal astigmatism were analysed. The anterior corneal curvature was measured by corneal topography. The Goldmann-correlated intraocular pressure (IOPg), corneal-compensated intraocular pressure (IOPcc), CH and CRF at different meridians (default horizontal position, 10°, 20° and 30° along the superotemporal and inferonasal meridians) were obtained from an ocular response analyser. The corneal powers at these specific meridians also were calculated. RESULTS: At the default position, the IOPg and CRF had weak correlations with corneal astigmatism, while the IOPcc and CH were not significantly correlated with corneal astigmatism. Both the IOPg and IOPcc were measured significantly higher at the default position. The CH and CRF were lower at the default position but the difference in the CRF from obliquity could not reach statistical significance. The CH was not significantly correlated with the corneal power at all meridians. The CRF correlated with the corneal power only at 30° superotemporal. CONCLUSION: Corneal astigmatism and head tilt did not have much effect on the measurement of CH and the CRF, both of which were lowest along the horizontal meridian. Clinically, the difference was small. The influence of corneal power on CH and the CRF was minimal.