Relationship among Corneal Biomechanics, Refractive Error, and Axial Length

Purpose: To evaluate the relationship between different ocular and corneal biomechanical parameters in emmetropic and ametropic healthy white children. Methods: This study included 293 eyes of 293 healthy Spanish children (135 boys and 158 girls), ranging in age from 6 to 17 years. Subjects were divided according to the refractive error: control (emmetropia, 99 children), myopia (100 children), and hyperopia (94 children) groups. In all cases, corneal hysteresis (CH) and corneal resistance factor (CRF) were evaluated with the Ocular Response Analyzer system. Axial length (AL) and mean corneal power were also measured by partial coherence interferometry (IOLMaster), and central corneal thickness (CCT) and anterior chamber depth were measured by anterior segment optical coherence tomography (Visante). Results: Mean (±SD) CH and CRF were 12.12 (±1.71) and 12.30 (±1.89) mm Hg, respectively. Mean (±SD) CCT was 542.68 (±37.20) μm and mean (±SD) spherical equivalent was +0.14 (±3.41) diopters. A positive correlation was found between CH and CRF (p < 0.001), and both correlated as well with CCT (p < 0.0001). Corneal resistance factor was found to decrease with increasing age (p = 0.01). Lower levels of CH were associated with longer AL and more myopia (p < 0.001 and p = 0.001, respectively). Higher values of CH were associated with increasing hyperopia. Significant differences in CH were found between emmetropic and myopic groups (p < 0.001) and between myopic and hyperopic groups (p = 0.011). There were also significant differences in CRF between emmetropic and myopic groups (p = 0.02). Multiple linear regression analysis showed that lower CH and CRF significantly associated with thinner CCT, longer AL, and flatter corneal curvature. Conclusions: The Ocular Response Analyzer corneal biomechanical properties seem to be compromised in myopia from an early age, especially in high myopia.

A new non-contact optical device for ocular biometry

Background: A new commercially available device (IOLMaster, Zeiss Instruments) provides high resolution non-contact measurements of axial length (using partial coherent interferometry), anterior chamber depth, and corneal radius (using image analysis). The study evaluates the validity and repeatability of these measurements and compares the findings with those obtained from instrumentation currently used in clinical practice. Method: Measurements were taken on 52 subjects (104 eyes) aged 18-40 years with a range of mean spherical refractive error from +7.0 D to -9.50 D. IOLMaster measurements of anterior chamber depth and axial length were compared with A-scan applanation ultrasonography (Storz Omega) and those for corneal radius with a Javal-Schiötz keratometer (Topcon) and an EyeSys corneal videokeratoscope. Results: Axial length: the difference between IOLMaster and ultrasound measures was insignificant (0.02 (SD 0.32) mm, p = 0.47) with no bias across the range sampled (22.40-27.99 mm). Anterior chamber depth: significantly shorter depths than ultrasound were found with the IOLMaster (-0.06 (0.25) mm, p <0.02) with no bias across the range sampled (2.85-4.40 mm). Corneal radius: IOLMaster measurements matched more closely those of the keratometer than those of the videokeratoscope (mean difference -0.03 v -0.06 mm respectively), but were more variable (95% confidence 0.13 v 0.07 mm). The repeatability of all the above IOLMaster biometric measures was found to be of a high order with no significant bias across the measurement ranges sampled. Conclusions: The validity and repeatability of measurements provided by the IOLMaster will augment future studies in ocular biometry.

Choroidal Thickness And Volume In Healthy Young Whites And Their Relationship With Axial Length, Ammetropy And Sex

Purpose: To evaluate choroidal thickness in young subjects using Enhanced Depth Imaging Spectral Domain Optical Coherence Tomography (EDI SD-OCT) describing volume differences between all the defined areas of the Early Treatment Diabetic Retinopathy Study (ETDRS). Design: Prospective, clinical study. Methods: Seventy-nine eyes of 95 healthy, young (23.8±3.2years), adult volunteers were prospectively enrolled. Manual choroidal segmentation on a 25-raster horizontal scan protocol was performed. The measurements of the nine subfields defined by the ETDRS were evaluated. Results: Mean subfoveal choroidal thickness was 345.67±81.80μm and mean total choroidal volume was 8.99±1.88mm3. Choroidal thickness and volume were higher at the superior and temporal areas compared to inferior and nasal sectors of the same diameter respectively. Strong correlations between subfoveal choroidal thickness and axial length (AL) and myopic refractive error were obtained, r = -0.649, p<0.001 and r = 0.473, p<0.001 respectively. Emmetropic eyes tended to have thicker subfoveal choroidal thickness (381.94±79.88μm versus 307.04±64.91μm) and higher total choroidal volume than myopic eyes (9.80± 1.87mm3 versus 8.14±1.48mm3). The estimation of the variation of the subfoveal choroidal thickness with the AL was-43.84μm/mm. In the myopic group, the variation of the subfoveal choroidal thickness with the myopic refractive error was -10.45μm/D. Conclusions: This study establishes for the first time a normal database for choroidal thickness and volume in young adults. Axial length, and myopic ammetropy are highly associated with choroidal parameters in healthy subjects. EDI SD-OCT exhibited a high degree of intraobserver and interobserver repeatability.

Scleral thinning after repeated intravitreal injections of antivascular endothelial growth factor agents in the same quadrant

PURPOSE We assessed the effects of intravitreal anti-vascular endothelial growth factor (anti-VEGF) therapy on scleral architecture using spectral domain anterior segment optical coherence tomography (OCT). METHODS A total of 35 eyes of 35 patients treated with at least 30 intravitreal injections in one eye in the inferotemporal quadrant with ranibizumab or aflibercept and 10 or less intravitreal injections in the fellow eye attending the intravitreal injection clinic were included. Enhanced depth imaging anterior segment OCT was used to measure scleral thickness. For each eye the sclera was measured in four quadrants at 3 mm from the limbus. In addition axial eye length was measured in all subjects using partial coherence interferometry. RESULTS The mean number of intravitreal injections was 42 (range, 30-73) and 1.6 (range, 0-9) in the fellow eyes. In the study eyes with more than 30 injections the average scleral thickness in the inferotemporal quadrant was 568.4 μm (SD ± 66 μm) and 590.6 μm (SD ± 75 μm) in the fellow eyes with 10 or less injections (P = 0.003). The mean average scleral thickness in the other three quadrants (inferonasal, superotemporal, and superonasal) was 536.6 μm in the study eyes (SD ± 100 μm) and 545.2 μm (SD ± 109 μm) in the fellow eyes (P = 0.22). There was a borderline association of the total number of injections with scleral thickness change in the inferotemporal quadrant (r = 0.3, P = 0.052). CONCLUSIONS Intravitreal injections may lead to scleral changes when applied repeatedly in the same quadrant. Thus, alternating the injection site should be considered in patients requiring multiple intravitreal injections.

Off-axis low coherence interferometry contouring

In this article we present a method to achieve tri-dimensional contouring of macroscopic objects. A modified reference wave speckle interferometer is used in conjunction with a source of reduced coherence. The depth signal is given by the envelope of the interference signal, directly determined by the coherence length of the source. Fringes are detected in the interferogram obtained by a single shot and are detected by means of adequate filtering. With the approach based on off-axis configuration, a contour line can be extracted from a single acquisition, thus allowing to use the system in harsh environment. (C) 2009 Elsevier B.V. All rights reserved.

Epidemiology of myopia in a United Kingdom urban child population

The Aston Eye Study (AES) was instigated in October 2005 to determine the distribution of refractive error and associated ocular biometry in a sample of UK urban school children. The AES is the first study to compare outcome measures separately in White, South Asian and Black children. Children were selected from two age groups (Year 2 children aged 6/7 years, Year8 children aged 12/13 years of age) using random cluster sampling of schools in Birmingham, West Midlands UK. To date, the AES has examined 598 children (302 Year 2,296 Year 8). Using open-field cycloplegic autorefraction, the overall prevalence of myopia (=-0.50D SER in either eye) determined was 19.6%, with a higher prevalence in older (29.4%) compared to younger (9.9%) children (p<0.001). Using multiple logistic regression models, the risk of myopia was higher in Year 8 South Asian compared to White children and higher in children attending grammar schools relative to comprehensive schools. In addition, the prevalence of uncorrected ametropia was found to be high (Year 8: 12.84%, Year 2: 15.23%), which will be of concern to bodies responsible for the implementation of school vision screening strategies. Biometric data using non-contact partial coherence interferometry revealed a contributory effect of axial length (AL) and central corneal radius (CR) on myopic refraction, resulting in a strong coefficient of determination of the AL/CR ratio on refractive error. Ocular biometric measures did not vary significantly as a function of ethnicity, suggesting a greater miscorrelation of components in susceptible ethnic groups to account for their higher myopia prevalence. Corneal radius was found to be steeper in myopes in both age groups, but was found to flatten with increasing axial length. Due to the inextricable link between myopia and axial elongation, the paradoxical finding of the cornea demands further longitudinal investigation, particularly in relation to myopia onset. Questionnaire analysis revealed a history of myopia in parents and siblings to be significantly associated with myopia in Year 8 children, with a dose-dependent rise in the odds ratio of myopia evident with increasing number of myopic parents. By classifying socioeconomic status (SES) using Index of Multiple Deprivation values, it was found that Year 8 children from moderately deprived backgrounds were more at risk of myopia compared with children located at both extremities of the deprivation spectrum. However, the main effect of SES weakened following multivariate analysis, with South Asian ethnicity and grammar schooling remaining associated with Year 8 myopia after adjustment.

Comparing different methods to measure biofilm thickness: the techniques are: optical coherence tomography, confocal laser scanning microscopy and low load compression test

Dissertação de mestrado integrado em Engenharia Biomédica (área de especialização em Engenharia Clínica)

Off-axis low-coherence interferometry for surface topology measurement.

In this communication we introduce a low or reduced coherence interferometry technique that can be used to retrieve surface topology on samples with high roughness. Moreover, we will show that the approach enables surface topology measurement also at the interface of so-called turbid media, where multiple scattering inside tissues can be a major issue, preventing accurate measurements.

Graphical modelling for brain connectivity via partial coherence

Spectral and coherence methodologies are ubiquitous for the analysis of multiple time series. Partial coherence analysis may be used to try to determine graphical models for brain functional connectivity. The outcome of such an analysis may be considerably influenced by factors such as the degree of spectral smoothing, line and interference removal, matrix inversion stabilization and the suppression of effects caused by side-lobe leakage, the combination of results from different epochs and people, and multiple hypothesis testing. This paper examines each of these steps in turn and provides a possible path which produces relatively ‘clean’ connectivity plots. In particular we show how spectral matrix diagonal up-weighting can simultaneously stabilize spectral matrix inversion and reduce effects caused by side-lobe leakage, and use the stepdown multiple hypothesis test procedure to help formulate an interaction strength.

Fibre Bragg grating sensor interrogation using an acousto-optic tunable filter and low-coherence interferometry

We describe how an acousto-optic tunable filter can be used to both demultiplex the signals from multiple fibre Bragg grating sensors and simultaneously provide wide bandwidth signal demodulation in a system using interferometric wavelength shift detection. In an experimental demonstration, the approach provided a noise limited strain resolution of 24.9 n epsilon Hz(-1/ 2) at 15 Hz.

Transversal and longitudinal images from the retina of the living eye using low coherence interferometry

An optical coherence tomography (OCT) system to produce both longitudinal and transversal images of the in vivo human eye is presented. For the first time, OCT transversal images collected from the living eye at 50-µm depth steps show details unobtainable with the state-of-the-art scanning laser ophthalmoscope. Images of up to 3×3?mm are produced from the retina in less than a second. For images larger than 1.6×1.6?mm, a path modulation is introduced by the galvanometric scanning mirror and is used as an effective phase modulation method.

Fibre Bragg grating sensor interrogation using an acousto-optic tunable filter and low-coherence interferometry

We describe how an acousto-optic tunable filter can be used to both demultiplex the signals from multiple fibre Bragg grating sensors and simultaneously provide wide bandwidth signal demodulation in a system using interferometric wavelength shift detection. In an experimental demonstration, the approach provided a noise limited strain resolution of 24.9 nε Hz -1/2 at 15 Hz. © 2007 IOP Publishing Ltd.