Ocular biometric investigation of anisometropia

The thesis aims to define further the biometric correlates in anisometropic eyes in order to provide a structural foundation for propositions concerning the development of ametropia.Biometric data are presented for 40 anisometropes and 40 isometropic controls drawn from Caucasian and Chinese populations.The principal finding was that the main structural correlate of myopia is an increase in axial rather than equatorial dimensions of the posterior globe. This finding has not been previously reported for in vivo work on humans. The computational method described in the thesis is a more accessible method for determination of eye shape than current imaging techniques such as magnetic resonance imaging or laser Doppler interferometry (LDI). Retinal contours derived from LDI and computation were shown to be closely matched. Corneal topography revealed no differences in corneal characteristics in anisometropic eyes, which supports the finding that anisometropia arises from differences in vitreous chamber depth.The corollary to axial expansion in myopia, that is retinal stretch in central regions of the posterior pole, was investigated by measurement of disc-to-fovea distances (DFD) using a scanning laser ophthalmoscope. DFD was found to increase with increased myopia, which demonstrates the primary contribution made by posterior central regions of the globe to axial expansion.The ocular pulse volume and choroidal blood flow, measured with the Ocular Blood Flow Tonograph, were found to be reduced in myopia; the reductions were found to be significantly correlated with vitreous chamber depth. The thesis includes preliminary data on whether the relationship arises from the influx of a blood bolus into eyes of different posterior volumes or represents actual differences in choroidal blood flow.The results presented in this thesis show the utility of computed retinal contour and demonstrate that the structural correlate of myopia is axial rather than equatorial expansion of the vitreous chamber. The technique is suitable for large population studies and its relative simplicity makes it feasible for longitudinal studies on the development of ametropia in, for example, children

Specializations of birds that attend army ant raids: an ecological approach to cognitive and behavioral studies.

Tropical birds forage at army ant raids on several continents. Obligate foraging at army ant raids evolved several times in the Neotropical true antbird family (Thamnophilidae), and recent evidence suggests a diversity of bird species from other families specialize to varying degrees on army ant exploitation. Army ant raids offer access to high prey densities, but the ant colonies are mobile and widely spaced. Successful army ant exploitation requires solving a complex foraging problem because army ant raids are unpredictable in space and time. Birds can counteract the challenges posed by the ants by using strategies that raise their chances of detecting army ant raids, and birds can use additional strategies to track army ant colonies they have located. Some features of army ant biology, such as their conspicuous swarms and columns, above-ground activity, and regular cycles of behavior, provide opportunities for birds to increase their effectiveness at exploiting raids. Changes in sensory, cognitive and behavioral systems may all contribute to specialized army ant exploitation in a bird population. The combination of specializations that are employed may vary independently among bird species and populations. The degree of army ant exploitation by birds varies geographically with latitude and elevation, and with historical patterns such as centers of distribution of obligate thamnophilid antbirds. We predict the set of specializations a given bird population exhibits will depend on local ecology, as well as phylogenetic history. Comparative approaches that focus on these patterns may indicate ecological and evolutionary factors that have shaped the costs and benefits of this foraging strategy. The development of army ant exploitation in individual birds is poorly understood, and individual expression of these specializations may depend on a combination of genetic adaptation with cognitive plasticity, possibly including social and experiential learning. Future studies that measure developmental changes and quantify individual differences in army ant exploitation are needed to establish the mechanisms underlying this behavior.This work was supported by National Science Foundation grant IOS-1209072 and Drexel University funds (to S.O’D.); the Gates Cambridge Scholarship and Murray Edwards College Overseas Bursary (to C.L.); and the University of Cambridge, Clare College and BBSRC (to N.S.C.).This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.beproc.2012.09.00

Effect of peripheral defocus on axial eye growth and modulation of refractive error in hyperopes

Hyperopia is a known risk factor for the development of strabismus and amblyopia. In addition to visual consequences there is a growing body of evidence that uncorrected hyperopia may have a negative impact on educational attainment and visuocognitive and visuomotor skills. Currently, hyperopia receives much less attention from research than myopia even though the impact of moderate to high levels of hyperopia especially in one eye (anisohyperopia) can lead to amblyopia if not corrected fully at a young age. Hyperopia occurs as a consequence of insufficient ocular growth and a failure to emmetropise in childhood with the majority of hyperopic refractive errors resulting from an eye that is too short for its refractive power. In anisohyperopia it is unclear why one eye may remain hyperopic while the fellow eye grows towards an emmetropic state. Studies on animals have suggested that manipulating peripheral defocus through an optical means while simultaneously providing correct axial focus can either discourage or encourage axial eye growth to effectively treat myopia or hyperopia, respectively. Recent research has established that progression of myopia and axial eye growth can be significantly reduced in children and adolescents through the use of bifocal or multifocal contact lenses. These contact lenses while correcting the distance central myopia impose simultaneous myopic defocus. In recent years there has been a great deal of interest in delaying progression of myopia (short-sightedness) by slowing eye growth using a particular type of contact lens termed a centre-distance multifocal design. There have been encouraging results in this area to date. The proposed study here would explore the use of centre-near multifocal design contact lenses to encourage eye growth, thereby reducing hyperopia. There are three elements to the programme of research: 1. The natural progression of axial eye growth and refractive error will be measured in spectacle wearing hyperopic and anisohyperopic subjects aged between 5 and <19 years. In other words, the natural growth of the eye will be followed without intervention 2. As a paired eye control study anisohyperopes aged between 8 and <16 years will be fitted with a centre-near multifocal design contact lens in their more hyperopic eye and a single vision contact lens in the fellow eye if required. The progression of axial eye growth and refractive error will be measured and compared 3. Subjects aged between 8 and <16 years with similar levels of hyperopia in each eye will be fitted with centre-near multifocal design contact lenses in each eye. The progression of axial eye growth and refractive error will be measured and compared to subjects in the natural progression study. The objectives of the study are to: • Understand the natural progression of axial eye growth and refractive error in hyperopes and anisohyperopes • Establish if axial eye growth and refractive error can be modified using centre-near multifocal design contact lenses in hyperopes and anisohyperopes to improve visual outcome

Three-dimensional modeling of the human eye based on magnetic resonance imaging

PURPOSE. A methodology for noninvasively characterizing the three-dimensional (3-D) shape of the complete human eye is not currently available for research into ocular diseases that have a structural substrate, such as myopia. A novel application of a magnetic resonance imaging (MRI) acquisition and analysis technique is presented that, for the first time, allows the 3-D shape of the eye to be investigated fully. METHODS. The technique involves the acquisition of a T2-weighted MRI, which is optimized to reveal the fluid-filled chambers of the eye. Automatic segmentation and meshing algorithms generate a 3-D surface model, which can be shaded with morphologic parameters such as distance from the posterior corneal pole and deviation from sphericity. Full details of the method are illustrated with data from 14 eyes of seven individuals. The spatial accuracy of the calculated models is demonstrated by comparing the MRI-derived axial lengths with values measured in the same eyes using interferometry. RESULTS. The color-coded eye models showed substantial variation in the absolute size of the 14 eyes. Variations in the sphericity of the eyes were also evident, with some appearing approximately spherical whereas others were clearly oblate and one was slightly prolate. Nasal-temporal asymmetries were noted in some subjects. CONCLUSIONS. The MRI acquisition and analysis technique allows a novel way of examining 3-D ocular shape. The ability to stratify and analyze eye shape, ocular volume, and sphericity will further extend the understanding of which specific biometric parameters predispose emmetropic children subsequently to develop myopia. Copyright © Association for Research in Vision and Ophthalmology

Posterior retinal contour in adult human anisomyopia

PURPOSE. It is well documented that myopia is associated with an increase in axial length or, more specifically, in vitreous chamber depth. Whether the transverse dimensions of the eye also increase in myopia is relevant to further understanding of its development. METHODS. The posterior retinal surface was localized in two-dimensional space in both eyes of young adult white and Taiwanese-Chinese iso- and anisomyopes (N = 56), from measured keratometry, A-scan ultrasonography, and central and peripheral refraction (±35°) data, with the aid of a computer modeling program designed for this purpose. Anisomyopes had 2 D or more interocular difference in their refractive errors, with mean values in their more myopic eyes of -5.57 D and in their less myopic eyes of -3.25 D, similar to the means of the two isomyopic groups. The derived retinal contours for the more and less myopic eyes were compared by way of investigating ocular shape changes that accompany myopia, in the posterior region of the vitreous chamber. The presence and size of optic disc crescents were also investigated as an index of retinal stretching in myopia. RESULTS. Relative to the less myopic eyes of anisometropic subjects, the more myopic eyes were more elongated and also distorted into a more prolate shape in both the white and Chinese groups. However, the Chinese eyes showed a greater and more uniform relative expansion of the posterior retinal surface in their more myopic eyes, and this was associated with larger optic disc crescents. The changes in the eyes of whites displayed a nasal-temporal axial asymmetry, reflecting greater enlargement of the nasal retinal sector. CONCLUSIONS. Myopia is associated with increased axial length and a prolate shape. This prolate shape is consistent with the proposed idea that axial and transverse dimensions of the eye are regulated differently. The observations that ocular shape changes are larger but more symmetrical in Chinese eyes than in eyes of whites warrant further investigation

In vivo measures of anterior scleral resistance in humans with rebound tonometry

Purpose: To measure regional variations in anterior scleral resistance (ASR) using a ballistic rebound tonometer (RBT) and examine whether the variations are significantly affected by ethnicity and refractive error (RE). Methods: ASR was measured using a RBT (iCare TA01) following calibration against the biomechanical properties of agarose biogels. Eight scleral regions (nasal, temporal, superior, inferior, inferior-nasal, inferior-temporal, superior-nasal and superior-temporal) were measured at locations 4mm from the limbus. Subjects were 130 young adults comprising three ethnic groups whose RE distributions [MSE (D) ± S.D.] incorporated individuals categorised as without-myopia (NM; MSE ≥ −0.50) and with-myopia (WM; MSE < −0.50); British-White (BW): 26 NM + 0.52 ± 1.15D; 22 WM −3.83 ± 2.89D]; British-South-Asian (BSA): [9 NM + 0.49 ± 1.06D; 11 WM −5.07 ± 3.76D; Hong-Kong-Chinese (HKC): [11 NM + 0.39 ± 0.66D; 49 WM −4.46 ± 2.70D]. Biometric data were compiled using cycloplegic open-field autorefraction and the Zeiss IOLMaster. Two- and three-way repeated measures analysis of variances (anovas) tested regional differences for RBT values across both refractive status and ethnicity whilst stepwise forward multiple linear regression was used as an exploratory test. Results: Significant regional variations in ASR were identified for the BW, BSA and HKC (p < 0.001) individuals; superior-temporal region showed the lowest levels of resistance whilst the inferior-nasal region the highest. Compared to the BW and BSA groups, the HKC subjects displayed a significant increase in mean resistance for each respective region (p < 0.001). With the exception of the inferior region, ethnicity was found to be the chief predictor for variation in the scleral RBT values for all other regions. Mean RE group differences were insignificant. Conclusions: The novel application of RBT to the anterior sclera confirm regional variation in ASR. Greater ASR amongst the HKC group than the BW and BSA individuals suggests that ethnic differences in anterior scleral biomechanics may exist

A 3-Year Randomized Clinical Trial of MiSight Lenses for Myopia Control

SIGNIFICANCE: Results of this randomized, double-masked clinical trial demonstrate the effectiveness of the MiSight soft contact lens in slowing myopia progression over multiple years. PURPOSE: The purpose of this study was to quantify the effectiveness of MiSight daily disposable soft contact lens in slowing the progression of juvenile-onset myopia. METHODS: Myopic children (spherical equivalent refraction, -0.75 to -4.00 D; astigmatism, <1.00 D) aged 8 to 12 years with no prior contact lens experience were enrolled in a 3-year, double-masked, randomized clinical trial at four investigational sites in four countries. Subjects in each group were matched for age, sex, and ethnicity and were randomized to either a MiSight 1-day contact lens (test) or Proclear 1-day (control; omafilcon A) and worn on a daily disposable basis. Primary outcome measures were the change in cycloplegic spherical equivalent refraction and axial length. RESULTS: Of the subjects enrolled, 75.5% (109/144) completed the clinical trial (53 test, 56 control). Unadjusted change in spherical equivalent refraction was -0.73 D (59%) less in the test group than in the control group (-0.51 ± 0.64 vs. -1.24 ± 0.61 D, P < .001). Mean change in axial length was 0.32 mm (52%) less in the test group than in the control group (0.30 ± 0.27 vs. 0.62 ± 0.30 mm, P < .001). Changes in spherical equivalent refraction and axial length were highly correlated (r = -0.90, P < .001). Over the course of the study, there were no cases of serious ocular adverse events reported. Four asymptomatic corneal infiltrative (one test, three control) events were observed at scheduled study visits. CONCLUSIONS: Results of this clinical trial demonstrate the effectiveness of the MiSight daily disposable soft contact lens in slowing change in spherical equivalent refraction and axial length

The effect of peripheral defocus on axial growth and modulation of refractive error in hyperopes

Purpose: To establish whether axial growth and refractive error can be modulated in hyperopic children by imposing relative peripheral hyperopic defocus using multifocal soft contact lenses. Methods: A prospective controlled study with hyperopic participants allocated to a control or test group. Control group participants were corrected with single vision spectacles and changes to axial length and refractive error were followed for 3 years. For the test group, axial growth and post-cycloplegic refractive error were observed with participants wearing single vision spectacles for the first 6 months of the trial and then corrected with centre-near multifocal soft contact lenses with a 2.00 D add for 2 years. The central ‘near’ portion of the contact lens corrected distance refractive error while the ‘distance’ portion imposed hyperopic defocus. Participants reverted to single vision spectacles for the final 6 months of the study. Results: Twenty-two participants, mean age 11.13 years (SD 1.72) (range 8.33-13.92), completed the trial. Axial length did not change during the first 6 months in either group (P = 1.000). Axial growth across the 2-year intervention period was 0.17 mm (SEM 0.04) (P < 0.0005) in the test group versus 0.06 mm (SEM 0.07) (P = 0.677) in the control group. Axial length was invariant during the final 6 months in either group (P = 1.000). Refractive error was stable during the first 6 months in both groups (P = 1.000). Refractive error change across the 2-year intervention period was -0.26 D (SEM 0.14) (P = 0.375) in the test group versus -0.01 D (SEM 0.09) (P = 1.000) in the control group. Neither the test (P=1.000) nor control (P=0.628) group demonstrated a change in refractive error during the final 6 months. Conclusions:  The rate of axial growth can be accelerated in children with hyperopia using centre-near multifocal soft contact lenses

Refractive and corneal astigmatism in white school children in Northern Ireland

To study the prevalence of and relation between refractive and corneal astigmatism in white school children in Northern Ireland and to describe the association between refractive astigmatism and refractive error

Conformation of the anterior segment in human myopia

Purpose Topography of the in vivo anterior segment is of relevance in understanding its role in myopia and in the development of ocular surgical procedures. Using 3D magnetic resonance (MR) images of the human eye, regional variations in surface area (SA) and bulbosity of four anterior segment regions were investigated in association with refractive status (Rx), axial length (AL) and total ocular volume (OV). Methods T2-weighted ocular MR images from 43 adults aged 18–40 years (mean ± SD; 28.65 ± 6.20) comprising 20 non-myopes (≥−0.50) 0.57 ± 1.38 and 23 myopes ( Results Significant differences in SA were observed between quadrants (p Conclusion Significant regional variation in SA exists across the anterior segment that is modulated by Rx and AL. It is unclear whether these structural characteristics are a precursor or consequence of myopia and may warrant investigation when developing biomechanical interventions