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Anisometropia represents a unique example of ocular development, where the two eyes of an individual, with an identical genetic background and seemingly subject to identical environmental influences, can grow asymmetrically to produce significantly different refractive errors. This review provides an overview of the research examining myopic anisometropia, the ocular characteristics underlying the condition and the potential aetiological factors involved. Various mechanical factors are discussed, including corneal structure, intraocular pressure and forces generated during near work that may contribute to development of anisomyopia. Potential visually guided mechanisms of unequal ocular growth are also explored, including the influence of astigmatism, accommodation, higher-order aberrations and the choroidal response to altered visual experience. The association between binocular vision, ocular dominance and asymmetric refraction is also considered, along with a review of the genetic contribution to the aetiology of myopic anisometropia. Despite a significant amount of research into the biomechanical, structural and optical characteristics of anisometropic eyes, there is still no unifying theory, which adequately explains how two eyes within the same visual system grow to different endpoints.

Purpose: To examine the symmetry of corneal changes following near work in the fellow eyes of non-amblyopic myopic anisometropes. Methods: Thirty-four non-amblyopic, myopic anisometropes (minimum 1 D spherical equivalent anisometropia) had corneal topography measured before and after a controlled near work task. Subjects were positioned in a headrest to minimise head movements and read continuous text on a computer monitor for 10 minutes at an angle of 25 degrees downward gaze and an accommodation demand of 2.5 D. Measures of the morphology of the palpebral aperture during primary and downward gaze were also obtained. Results: The more and less myopic eyes exhibited a high degree of interocular symmetry for measures of palpebral aperture morphology during both primary and downward gaze. Following the near work task, fellow eyes also displayed a symmetrical change in superior corneal topography (hyperopic defocus) which correlated with the position of the upper eyelid during downward gaze. Greater changes in the spherical corneal power vector (M) following reading were associated with narrower palpebral aperture during downward gaze (p = 0.07 for more myopic and p = 0.03 for less myopic eyes). A significantly greater change in J0 (an increase in against the rule astigmatism) was observed in the more myopic eyes (-0.04 ± 0.04 D) compared to the less myopic eyes (-0.02 ± 0.06 D) over a 6 mm corneal diameter (p = 0.01). Conclusions: Changes in corneal topography following near work are highly symmetrical between the fellow eyes of myopic anisometropes due to the interocular symmetry of the palpebral aperture. However, the more myopic eye exhibits changes in corneal astigmatism of greater magnitude compared to the less myopic eye.

Nearly all the eye's structures, from the retina at the back of the eye to the tear film at the front, can now be imaged and measured in vivo with outstanding resolution and precision. Imaging and measurement of the eye is fundamental to both research and clinical practice in optometry, and the technologies to do this have mushroomed exponentially in recent decades. The capabilities of these current imaging technologies are most impressive, allowing the visualization of the fine structure of the eye that was not conceivable 20 years ago. From the observation of corneal nerve fibers and epithelial cells and the three-dimensional visualization of the retina and optic nerve head to the imaging of individual photoreceptor cells, these remarkable advances in our ability to image and measure the eye have changed the way optometry is practiced clinically and have led to numerous important scientific discoveries. As these advanced imaging methods continue to develop, the potential for imaging ocular structures down to the cellular level in everyday clinical practice has become a reality, and the potential to improve patient care is truly stunning.

PURPOSE: To examine the foveal retinal thickness (RT) and subfoveal choroidal thickness (ChT) between the fellow eyes of myopic anisometropes. METHODS: Twenty-two young (mean age 23 ± 5 years), healthy myopic anisometropes (≥ 1 D spherical equivalent [SEq] anisometropia) without amblyopia or strabismus were recruited. Spectral domain optical coherence tomography (SD-OCT) was used to capture images of the retina and choroid. Customised software was used to register, align and average multiple foveal OCT B-Scan images from each subject in order to enhance image quality. Two independent masked observers then manually determined the RT and ChT at the centre of the fovea from each SD-OCT image, which were then averaged. Axial length was measured using optical low coherence biometry during relaxed accommodation. RESULTS: The mean absolute SEq anisometropia was 1.74 ± 0.95 D and the mean interocular difference in axial length was 0.58 ± 0.41 mm. There was a strong correlation between SEq anisometropia and the interocular difference in axial length (r = 0.90, p < 0.001). Measures of RT and ChT were highly correlated between the two observers (r = 0.99 and 0.97 respectively) and in close agreement (mean inter-observer difference: RT 1.3 ± 2.2 µm, ChT 1.5 ± 13.7 µm). There was no significant difference in RT between the more (218 ± 18 µm) and less myopic eyes (215 ± 18 µm) (p > 0.05). However, the mean subfoveal ChT was significantly thinner in the more myopic eye (252 ± 46 µm) compared to the fellow, less myopic eye (286 ± 58 µm) (p < 0.001). There was a moderate correlation between the interocular difference in ChT and the interocular difference in axial length (r = -0.50, p < 0.01). CONCLUSIONS: Foveal RT was similar between the fellow eyes of myopic anisometropes; however, the subfoveal choroid was significantly thinner in the more myopic (longer) eye of our anisometropic cohort. The interocular difference in ChT correlated with the magnitude of axial anisometropia.

This study compared the corneal and total higher order aberrations between the fellow eyes in monocular amblyopia. Nineteen amblyopic subjects (8 refractive and 11 strabismic) (mean age 30 ± 11 years) were recruited. A range of biometric and optical measurements were collected from the amblyopic and non-amblyopic eye including; axial length, corneal topography and total higher order aberrations. For a sub-group of eleven non-presbyopic subjects (6 refractive and 5 strabismic amblyopes, mean age 29 ± 10 years) total higher order aberrations were also measured during accommodation (2.5 D stimuli). Amblyopic eyes were significantly shorter and more hyperopic compared to non-amblyopic eyes and the interocular difference in axial length correlated with both the magnitude of anisometropia and amblyopia (both p < 0.01). Significant differences in higher order aberrations were observed between fellow eyes, which varied with the type of amblyopia. Refractive amblyopes displayed higher levels of 4th order corneal aberrations C(4, 0)(spherical aberration), C(4, 2)(secondary astigmatism 90°) and C(4, −2)(secondary astigmatism along 45°) in the amblyopic eye compared to the non-amblyopic eye. Strabismic amblyopes exhibited significantly higher levels of C(3, 3)(trefoil) in the amblyopic eye for both corneal and total higher order aberrations. During accommodation, the amblyopic eye displayed a significantly greater lag of accommodation compared to the non-amblyopic eye, while the changes in higher order aberrations were similar in magnitude between fellow eyes. Asymmetric visual experience during development appears to be associated with asymmetries in higher order aberrations, in some cases proportional to the magnitude of anisometropia and dependent upon the amblyogenic factor.

PURPOSE: To examine the symmetry of corneal changes following near work in the fellow eyes of non-amblyopic myopic anisometropes. METHODS: Thirty-four non-amblyopic myopic anisometropes (minimum 1 D spherical equivalent anisometropia) were recruited. Corneal topography was measured with the Medmont E300 Videokeratoscope before and after a controlled near task. Subjects were positioned to minimise head movements and read continuous text on a computer monitor for 10 minutes at an angle of 25 degrees downward gaze and an accommodation demand of 2.5 D. Measures of palpebral aperture morphology during primary and downward gaze were also obtained using digital photography and analysed with customised software. RESULTS: Significant changes in corneal topography were observed after ten minutes of reading. Localised superior regions of corneal topographical change (a hyperopic shift in corneal power) were typically exhibited in both eyes following the near task. The mean change in the corneal sphero-cylinder was +0.02/-0.11 x 113 and +0.02/-0.06 x 68 for the more and less myopic eyes respectively for a 6 mm corneal diameter. A significantly greater change in corneal astigmatism power vector J0 (a larger increase in against the rule astigmatism) was observed in the more myopic eyes (p < 0.01 for a 6 mm diameter). The more and less myopic eyes exhibited a high degree of interocular symmetry for measures of palpebral aperture morphology during both primary and downward gaze. Changes in corneal power vectors following reading were associated with eyelid position during downward gaze. CONCLUSIONS: Changes in corneal topography observed following a controlled reading task were highly symmetrical between the fellow eyes of myopic anisometropes due to the interocular symmetry of the palpebral aperture. However, the more myopic eye did exhibit a small but significantly greater magnitude of change in corneal astigmatism compared to the less myopic eye following reading. These findings

Purpose: To investigate the interocular symmetry of optical, biometric and biomechanical characteristics between the fellow eyes of myopic anisometropes. Methods: Thirty-four young, healthy myopic anisometropic adults (≥ 1 D spherical equivalent difference between eyes) without amblyopia or strabismus were recruited. A range of biometric and optical parameters were measured in both eyes of each subject including; axial length, ocular aberrations, intraocular pressure (IOP), corneal topography and biomechanics. Ocular sighting dominance was also measured. Results: Mean absolute spherical equivalent anisometropia was 1.70 ± 0.74 D and there was a strong correlation between the degree of anisometropia and the interocular difference in axial length (r = 0.81, p < 0.001). The more and less myopic eyes displayed a high degree of interocular symmetry for the majority of biometric, biomechanical and optical parameters measured. When the level of anisometropia exceeded 1.75 D, the more myopic eye was more likely to be the dominant sighting eye than for lower levels of anisometropia (p=0.002). Subjects with greater levels of anisometropia (> 1.75 D) also showed high levels of correlation between the dominant and non-dominant eyes in their biometric, biomechanical and optical characteristics. Conclusions: Although significantly different in axial length, anisometropic eyes display a high degree of interocular symmetry for a range of anterior eye biometrics and optical parameters. For higher levels of anisometropia, the more myopic eye tends to be the dominant sighting eye.

PURPOSE: To investigate the interocular symmetry of ocular optical, biometric and biomechanical characteristics between the more and less ametropic eyes of myopic anisometropes. METHODS: Thirty-four young, healthy myopic anisometropic adults (≥ 1 D spherical equivalent difference between eyes) without amblyopia or strabismus were recruited. A range of biometric and optical parameters were measured in the more and less ametropic eye of each subject including; axial length, ocular aberrations, intraocular pressure and corneal topography, thickness and biomechanics. Morphology of the anterior eye in primary and downward gaze was examined using custom software analysis of high resolution digital images. Ocular sighting dominance was assessed using the hole-in-the-card test. RESULTS: Mean absolute spherical equivalent anisometropia was 1.74 ± 0.74 D. There was a strong correlation between the degree of anisometropia and the interocular difference in axial length (r = 0.81, p < 0.001). The more and less ametropic fellow eyes displayed a high degree of interocular symmetry for the majority of biometric, biomechanical and optical parameters measured. When the level of anisometropia exceeded 1.75 D (n = 10), the more myopic eye was the dominant sighting eye in nine of these ten subjects. Subjects with greater levels of anisometropia (> 1.75 D) also showed high levels of correlation between the dominant and non-dominant eyes in their biometric, biomechanical and optical characteristics. CONCLUSIONS: Although significantly different in axial length, anisometropic eyes display a high degree of interocular symmetry for a range of anterior eye biometrics and optical parameters. For higher levels of anisometropia, the more myopic eye tends to be the dominant sighting eye.

Purpose To investigate static upper eyelid pressure and contact with the ocular surface in a group of young adult subjects. Methods Static upper eyelid pressure was measured for 11 subjects using a piezoresistive pressure sensor attached to a rigid contact lens. Measures of eyelid pressure were derived from an active pressure cell (1.14 mm square) beneath the central upper eyelid margin. To investigate the contact region between the upper eyelid and ocular surface, we used pressure sensitive paper and the lissamine-green staining of Marx’s line. These measures combined with the pressure sensor readings were used to derive estimates of eyelid pressure. Results The mean contact width between the eyelids and ocular surface estimated using pressure sensitive paper was 0.60 ± 0.16 mm, while the mean width of Marx’s line was 0.09 ± 0.02 mm. The mean central upper eyelid pressure was calculated to be 3.8 ± 0.7 mmHg (assuming that the whole pressure cell was loaded), 8.0 ± 3.4 mmHg (derived using the pressure sensitive paper imprint widths) and 55 ± 26 mmHg (based on contact widths equivalent to Marx’s line). Conclusions The pressure sensitive paper measurements suggest that a band of the eyelid margin, significantly larger than the anatomical zone of the eyelid margin known as Marx’s line, has primary contact with the ocular surface. Using these measurements as the contact between the eyelid margin and ocular surface, we believe that the mean pressure of 8.0 ± 3.4 mmHg is the most reliable estimate of static upper eyelid pressure.

Purpose: The cornea is known to be susceptible to forces exerted by eyelids. There have been previous attempts to quantify eyelid pressure but the reliability of the results is unclear. The purpose of this study was to develop a technique using piezoresistive pressure sensors to measure upper eyelid pressure on the cornea. Methods: The technique was based on the use of thin (0.18 mm) tactile piezoresistive pressure sensors, which generate a signal related to the applied pressure. A range of factors that influence the response of this pressure sensor were investigated along with the optimal method of placing the sensor in the eye. Results: Curvature of the pressure sensor was found to impart force, so the sensor needed to remain flat during measurements. A large rigid contact lens was designed to have a flat region to which the sensor was attached. To stabilise the contact lens during measurement, an apparatus was designed to hold and position the sensor and contact lens combination on the eye. A calibration system was designed to apply even pressure to the sensor when attached to the contact lens, so the raw digital output could be converted to actual pressure units. Conclusions: Several novel procedures were developed to use tactile sensors to measure eyelid pressure. The quantification of eyelid pressure has a number of applications including eyelid reconstructive surgery and the design of soft and rigid contact lenses.

Novel procedures were developed to use a thin (0.17 mm) tactile piezoresistive pressure sensor mounted on a rigid contact lens to measure upper eyelid pressure. A hydrostatic calibration system was constructed and the influence of conditioning (prestressing), drift (continued increasing response with a static load) and temperature variations on the response of the sensor were examined. To optimally position the sensor-contact lens combination under the upper eyelid margin, an in vivo measurement apparatus was constructed. Calibration gave a linear relationship between raw sensor output and actual pressure units, for loads between 1 and 10 mmHg (R²=0.96). Conditioning the sensor prior to use regulated the measurement response and sensor output stabilised about 10 seconds after loading. While sensor output drifts slightly over several hours, it was not significant over the measurement time of 1 minute used for eyelid pressure. The error associated with calibrating at room temperature but measuring at ocular surface temperature led to a very small overestimation of pressure. Eyelid pressure readings were observed when the upper eyelid was placed on the sensor and removed during a recording. When the eyelid pressure was increased by pulling the lids tighter against the eye, the readings from the sensor significantly increased.

Purpose: It is known that eyelid pressure can influence the corneal surface. However the distribution of eyelid pressure, the eyelid contact area and the biomechanics of the changes are unknown. While these factors are difficult to directly measure, analysis of eyelid-induced corneal topography changes and eyelid morphometry enable some inferences to be drawn. Methods: Eighteen subjects, aged between 19 and 29 years, with normal ocular health were recruited. Corneal topography changes were measured after four conditions consisting of two downward gaze angles (20° and 40°) and two types of visual tasks (reading and steady fixation). Digital photography recorded the width of Marx’s line, the assumed region of primary eyelid contact with the cornea. Results: Significantly larger corneal changes were found after the 40° downward gaze conditions compared with 20° due to the upper eyelid contact (p<0.001). For the 40° downward gaze tasks, the lower eyelid changes were greater than those due to the upper eyelid (p<0.01). The upper eyelid Marx's line width was associated with the amplitude of corneal change (R²=0.32, p<0.05). Conclusion: Analysis of the corneal topography changes gives insight into the pressure applied by the upper and lower eyelids in different situations. These include greater upper eyelid pressure with increasing downward gaze and larger lower eyelid pressure compared to the upper eyelid in 40° downward gaze. There was some evidence that supports Marx’s line as the primary site of contact between the eyelid margins and the cornea.

Purpose:To estimate the pressure of the eyelid against the ocular surface we developed a novel method using a thin (0.18 mm) tactile piezoresistive pressure sensor (Tekscan Inc., Boston, MA, USA) mounted on a rigid contact lens.------ Methods:Curvature of the sensor causes an offset and altered sensitivity, so a contact lens was designed with a flat region where the sensor could be adhered. We examined the response of the sensor in different conditions including the influence of conditioning (prestressing), drift (continued increasing response with a static load) and temperature variations. Techniques were developed to calibrate the sensor output into real pressure units (mmHg) and to hold and optimally place the sensor-contact lens combination under the eyelid margin.------ Results:Calibration gave a linear relationship between raw sensor output and actual pressure units, for loads between 1 and 10 mmHg. Conditioning the sensor prior to use regulated the measurement response. Sensor output stabilised about 10 seconds after loading. While the output drifted slightly over several hours, the drift was not significant over the measurement time required for eyelid pressure (<30 seconds). The error associated with calibrating at room temperature but measuring at ocular surface temperature led to a very small overestimation of pressure. Eyelid pressure readings could be seen when the upper eyelid was placed on the sensor and then removed three times during a recording. Also when the eyelid was pulled from the outer canthus (similar to the lid-pull technique for removing a contact lens) the readings from the sensor significantly increased.------ Conclusions:A number of novel procedures were developed to use a tactile sensor to measure eyelid pressure. These included designing a custom contact lens with a flat area to which the sensor could be attached, a hydrostatic calibration system and an in vivo measurement apparatus to hold and position the sensor-contact lens combina

Purpose: To assess corneal refractive changes after visual tasks of 15 minutes duration and their association with eyelid morphology. Setting: Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Queensland, Australia. Methods: Eighteen young subjects with normal ocular health were recruited. Corneal topography was measured with a videokeratoscope prior to and after four conditions consisting of two downward gaze angles (20 degrees and 40 degrees) and two types of visual tasks (reading and steady fixation). Anterior eye photography in downward gaze was used to determine the eyelid angle, tilt and position with respect to the cornea. Results: There were significant changes in corneal refractive power after the 15 minute downward gaze tasks. The largest group mean corneal sphero-cylindrical change was +0.33/-0.30x84 after reading in 40 degrees downward gaze (4 mm corneal diameter). The refractive changes after the 40 degrees tasks were significantly larger than the changes after the 20 degrees tasks (p<0.001). The changes in refractive RMSE were significant for all conditions, except the 20 degrees steady fixation task, for 4 and 6 mm analysis diameters (p<0.05). Significant correlations were found between some aspects of eyelid morphometry and corneal refractive change. Conclusion: The pressure of the eyelids on the cornea in short-term downward gaze resulted in optically and clinically relevant corneal changes. Correlation between the refractive corneal changes and eyelid parameters suggests that the angle, shape and position of the eyelids influence the nature of the corneal changes. When high accuracy is required, refraction should be qualified in terms of the visual tasks undertaken prior to assessment.

Purpose:The cornea is the principal optical element of the eye so the regularity of its surface topography is critical for visual optics. However the cornea is known to be susceptible to forces exerted by the eyelids. These corneal changes are bands of 'wave-like' change that are parallel to the position of the eyelid margin. There is little known about eyelid pressure on the cornea which is dependent on eyelid force and the contact area. By analysing the depth and width of corneal topography changes after various downward gaze tasks, inferences could be drawn about upper and lower eyelid pressure. Methods:Corneal topography changes due to eyelid pressure were measured using the Medmont E300 Corneal Topographer (Medmont Pty. Ltd. Victoria, Australia), for eighteen subjects aged between 18 and 29 years. Four conditions were considered, consisting of two downward gaze angles (20° and 40°) and two visual tasks (reading and staring). The amplitude and width of the ‘wave-like’ changes were analysed for each of the four conditions and for both upper and lower eyelids. Anterior eye digital photography was used to determine the position of the eyelids in downward gaze and the width of Marx's line. Results:For each condition the average peak-to-valley amplitudes of corneal change were between 1.4 and 2.4 µm. For the upper eyelid, the downward gaze angle magnitude had a significant impact on the peak-to-valley amplitude (p<0.001), with corneal changes after the 40° tasks being 25% greater than after the 20° tasks. The topographical changes showed a characteristic 'wave-like' pattern, with an outer peak, a valley and an inner peak (closer to corneal centre). The upper eyelid produced a larger outer peak compared to the inner peak (p<0.001). The corneal changes after the 40° downward gaze tasks were greater for the lower eyelid than for the upper eyelid (p<0.01). The amplitude of corneal change produced by the upper eyelid was associated with the width of Marx's line (R2=0.32

Astigmatism is a refractive condition encountered commonly in clinical practice. This review presents an overview of research that has been carried out examining various aspects of this refractive error. We examine the components of astigmatism and the research into the prevalence and natural course of astigmatic refractive errors throughout life. The prevalence of astigmatism in various ethnic groups and diseases and syndromes is also discussed. We highlight the extensive investigations that have been conducted into the possible aetiology of astigmatism, however, no single model or theory of the development of astigmatism has been proven conclusively. Theories of the development of astigmatism based on genetics, extraocular muscle tension, visual feedback and eyelid pressure are considered. Observations and evidence from the literature supporting and contradicting these hypotheses are presented. Recent advances in technology such as wavefront sensors and videokeratoscopes have led to an increased understanding of ocular astigmatism and with continued improvements in technology, our knowledge of astigmatism and its genesis should continue to grow.

PURPOSE. To investigate associations between the topography of the cornea and the morphology of the eyelids in a population of young adult subjects with a range of refractive errors. METHODS. Corneal topography data and digital images of the anterior eye were acquired for 100 young adult subjects. The corneal topography data and palpebral fissure images were analyzed to determine a range of parameters describing the shape of the cornea and the morphology of the palpebral fissure. Correlation analysis was carried out to investigate for significant associations between the parameters describing the topography of the cornea and the parameters describing the morphology of the palpebral fissure. RESULTS. A number of highly significant correlations were found between the best-fitting corneal spherocylinder and the eyelid morphology parameters. The corneal best-fit sphere (M) was significantly correlated with the horizontal palpebral fissure width (r = –0.428; P < 0.001). Corneal astigmatism power vector J45 was significantly correlated with the angle of the palpebral fissure (r = 0.392; P < 0.001). The axis of corneal astigmatism was also found to be significantly correlated with the angle of the palpebral fissure (r = 0.317; P = 0.005). CONCLUSIONS. A number of significant associations exist between the corneal spherocylinder and the morphology of the eyelids in a normal adult population.

PURPOSE: To investigate visual performance after overnight orthokeratology in terms of changes from baseline values, regression of the orthokeratology effect over time, and evaluation of the best-corrected vision after treatment. In particular, to evaluate any residual visual deficits over the duration of a day due to the abnormal corneal topography induced by orthokeratology treatment. METHOD: One eye of each of six subjects was fitted with custom designed BE orthokeratology lenses (Capricornia, Brisbane, Australia), with the fellow eye acting as a control. Unaided vision, subjective sphero-cylindrical refraction, high contrast high luminance visual acuity, low contrast high luminance visual acuity, high contrast low luminance visual acuity and letter contrast sensitivity were measured at baseline and after one night (Day 1) and eight nights (Day 8) of lens wear. Except for baseline, data were collected after overnight lens wear immediately after lens removal, and again 3, 6 and 9h after lens removal. At each time point throughout the day, the visual performance measures were evaluated with the initial refraction of the day (the 0h refraction) and also using the optimum subjective refraction at each measurement time. This method was used to evaluate the practical visual performance to be expected after orthokeratology treatment and the residual visual deficits arising from any induced corneal changes after correction of defocus. RESULTS: As expected, orthokeratology lens wear significantly changed unaided vision and refraction from baseline. However, it did not significantly affect visual acuity in different contrast conditions, or contrast sensitivity. The spherical component of refraction was the only parameter to exhibit regression over each day (p=0.021), with more stability demonstrated on Day 8 than Day 1 (p=0.012). There were no statistically significant changes of best-corrected acuity from baseline in the differing contrast and luminance conditions. CONCLU

Purpose. The purpose of this study is to investigate the normative morphology of the palpebral fissure by measuring a range of biometric eye dimensions for a population of young subjects through analysis of digital images in primary gaze and two typical angles of downward vertical gaze. Palpebral fissure characteristics are clinically important in areas such as contact lens practice. Methods. High-resolution digital images were taken of 76 young subjects with a range of refractive errors in primary gaze and 20[degrees] and 40[degrees] downward gaze. The digital images were analyzed to ascertain a range of biometric measures of the palpebral fissure for each subject in each angle of gaze. Repeated-measures analysis of variance was used to investigate changes occurring in the palpebral fissure dimensions with vertical angle of gaze. Results. Highly significant changes were found to occur in the horizontal and vertical palpebral fissure dimensions, the palpebral fissure angle, and the eyelid contour as a function of angle of gaze. The palpebral fissure narrows in the vertical dimension (from an average vertical palpebral aperture width for white subjects in primary gaze of 9.7 +/- 1.2 mm to an average width of 6.4 +/- 1.1 mm in 40[degrees] downgaze), shortens in the horizontal dimension (from average horizontal palpebral aperture width of 27.1 +/- 1.5 mm in primary gaze to an average of 25.6 +/- 1.8 mm in 40[degrees] downgaze), and becomes more "down-slanted" with increasing downward gaze. Conclusions. Highly significant changes to the palpebral fissure dimensions occur in downward vertical gaze. These changes are important because many visual tasks are performed in downward gaze. These findings have implications for the management of lid anomalies and for contact lens fitting and design.

PURPOSE. To investigate the topography of the central and peripheral cornea in a group of young adult subjects with a range of normal refractive errors. METHODS. Corneal topography data were acquired for 100 young adult subjects by a method that allows central and peripheral maps to be combined to produce one large, extended corneal topography map. This computer-based method involves matching the common topographical features in the overlapping maps. Corneal height, axial radius of curvature, and axial power data were analyzed. The corneal height data were also fit with Zernike polynomials. RESULTS. Conic fitting to the corneal height data revealed the average apical radius (Ro) was 7.77 ± 0.2-mm and asphericity (Q) was –0.19 ± 0.1 for a 6-mm corneal diameter. The conic fit parameters were both found to change significantly for increasing corneal diameters. For a 10-mm corneal diameter, Ro was 7.72 ± 0.2 mm and Q was –0.36 ± 0.1. A slight but significant meridional variation was found in Q, with the steepest principal corneal meridian found to flatten at a slightly greater rate than the flattest meridian. The RMS fit error for the conic section was found to increase markedly for larger corneal diameters. Higher-order polynomial fits were needed to fit the peripheral corneal data adequately. Analysis of the axial power data revealed highly significant changes occurring in the corneal best-fit spherocylinder with increasing distance from the corneal center. The peripheral cornea was found to become significantly flatter and to decrease slightly in its toricity. Individual subjects exhibited a range of different patterns of central and peripheral corneal topography. Several of the higher order corneal surface Zernike coefficients were found to change significantly with increasing corneal diameter. CONCLUSIONS. Highly significant changes occur in the shape of the cornea in the periphery. On average, the peripheral cornea becomes significantly flatter and slightly less a

Aim: To investigate the normative values of a range of biometric measures of the anterior eye in three different angles of vertical gaze in a population of young subjects. Methods: High resolution digital images of the right eye were taken of 100 young subjects in primary gaze and 20 degree and 40 degree downward gaze. The images were analysed using customwritten software. Results: Highly significant changes were found in the horizontal and vertical palpebral fissure dimensions, the palpebral fissure angle and the eyelid contour as a function of angle of gaze (p < 0.01 for all dimensions). The palpebral fissure narrows in the vertical dimension (from an average vertical palpebral aperture width for Caucasian subjects in primary gaze of 9.7 ± 1.2 mm to an average of 6.4 ± 1.1 mm in 40 degree down-gaze), shortens in the horizontal dimension (from average horizontal palpebral aperture width of 27.1 ± 1.5 mm in primary gaze to an average of 25.6 ± 1.8 mm in 40 degree down-gaze) and becomes more ‘downslanted’ with increasing downward gaze. Significant changes also occur to the upper and lower eyelid contour in down-gaze. Some dimensions of the anterior eye were found to differ significantly as a function of gender and ethnicity. Conclusion: The dimensions of the anterior eye undergo highly significant changes in downward vertical gaze. These changes highlight the dynamic nature of the anterior ocular structures that were measured. As many visual tasks are performed in various angles of downward gaze, these findings have implications for ocular surgery and the design and fitting of contact lenses.

There is an increasing emphasis in health-care delivery on the need for evidence-based practice. While the principle of basing clinical decisions on the available evidence is long-standing, applying the appropriate and tested evidence to specific problems has now become a formalised and recognised discipline. Resources and methods to achieve this are numerous and wide-spread. In the most recent edition of his book Studying a Study and Testing a Test, Reigelman1 points out that 'efficiently reading the research evidence is the key to successfully putting progress into practice'. He provides a very useful and effective framework to follow in evaluating health-care evidence presented in the scientific literature and identifying flaws in that evidence, and establishing whether those flaws are 'fatal'….

Purpose Few studies have investigated the peripheral cornea outside of the central 6mm. We examined the topography of the peripheral cornea in a large group of young adult subjects and report normative data for this population. Methods Corneal topography data were acquired for 100 young adult subjects using a method that allows the combination of central and peripheral maps to produce one large, extended corneal topography map. This computer-based method involves matching the common topographical features in the overlapping maps. Results The average apical radius Ro was found to be 7.78 ± 0.2 mm and the asphericity value (Q) was -0.18 ± 0.09 for the central 6mm corneal diameter. Both Ro and Q were found to change significantly with increasing corneal diameter (p <0.001). For a 10 mm corneal diameter, Ro was found to be on average 7.72 ± 0.2 mm and Q -0.36 ± 0.09. The corneal height data was also averaged along the steepest and flattest corneal meridian for each subject. The average Ro was found to be 7.68 ± 0.26 mm and Q -0.32 ± 0.1 along the steepest meridian and Ro 7.81 ± 0.24 mm and Q -0.30 ± 0.1 along the flattest meridian for a 9mm corneal diameter. The average Ro and Q values were found to differ significantly as a function of corneal meridian (both p <0.01) indicating that the steepest corneal meridian flattens in the periphery at a slightly more rapid rate. The RMS fit errors for the conic section (Q) increase markedly for larger corneal diameters. A better fit to the data was attained for the bigger corneal diameters by using higher order polynomial fits (9th order). The average corneal power vectors “M” “J0” and “J45” were all found to reduce significantly in magnitude with increasing corneal diameter (p <0.01), indicating a significant flattening and reduction in toricity for the peripheral cornea. Conclusions The peripheral cornea is poorly estimated with a simple conic section. The cornea flattens and becomes slightly less astigmatic in the periphery.

Purpose: To investigate if diurnal variation occurs in corneal topography. Methods: Seventeen young subjects had corneal topography measured 3 times a day (around 9 AM, 1 PM, and 5 PM) on 3 days of the week (Monday, Tuesday, and Friday). Corneal tangential and refractive power were analyzed. To investigate diurnal variation in corneal aberrations, the corneal height data were converted into corneal wavefront error and expressed as Zernike wavefront coefficients. Results: A significant diurnal change in corneal tangential power maps was found in 15 of the 17 subjects. This change typically consisted of a horizontal band of distortion in the superior and, to a lesser extent, inferior cornea, increasing throughout the day. Corneal refractive power showed small but significant diurnal changes occurring in the best fit sphere. Corneal wavefront error analysis revealed significant diurnal changes occurring in astigmatism 90/180 degrees and astigmatism 45/135 degrees. Two higher-order aberrations, primary vertical coma and trefoil along 30 degrees, also showed significant diurnal change. The combination of these 2 aberrations represents a "wavelike" distortion increasing throughout the day and returning to baseline the next morning. Conclusion: We have shown that significant diurnal change occurs in certain corneal aberrations. The changes appear to be related to forces from the eyelids on the anterior cornea. These findings may have important implications for customized refractive corrections and refractive error development.

Purpose To study whether corneal wavefront aberrations significantly differ between progressing myopes and stable emmetropes after reading. Methods Twenty young progressing myopic subjects and twenty young emmetropic subjects participated in the experiment. Corneal topography of the left eye was measured with a videokeratoscope prior to reading and then again after 1 hour and 2 hours of reading. The natural position of the eyelids was photographed in primary gaze and during the reading task. Results Both emmetropic and myopic groups showed corneal shape and wavefront changes following 1 hour and 2 hours of reading. The major difference between the two groups was the location and magnitude of the corneal aberration changes, which could be attributed to differences in palpebral aperture in the reading gaze position. A significantly smaller palpebral aperture was found for the myopic group in the reading gaze position (emmetropic group = 8.09 mm ±1.56; myopic group = 7.12 mm ±1.00; p = 0.025). Significant changes were found for myopes and emmetropes in trefoil along 30º and vertical coma following reading. A significant shift in direction of against–the rule astigmatism was found for the myopic group (Cyl. –0.14 X 91°, p = 0.01) but not for the emmetropic group (Cyl. –0.04 X 81°, p = 0.09). Corneal higher order wavefront RMS after reading was significantly increased in myopes compared with emmetropes (pre–reading RMS: emmetropes = 0.22 microns, myopes = 0.25 microns, p = 0.23; post–1 hour reading RMS: emmetropes = 0.23 microns, myopes = 0.31 microns, p = 0.02; post–2 hours reading RMS: emmetropes = 0.25 microns, myopes = 0.33 microns, p = 0.06). Conclusions Prolonged narrowing of the palpebral aperture during reading may compromise retinal image quality and thereby provide a cue for eye growth. Corneal wavefront changes following reading tended to be larger and occur closer to the central cornea in myopes compared with emmetropes.