Biomedical subjects

Michael J Collins

Publications and source records attributed to Michael J Collins.

At least 19 recordsLinked to original sources

PubMed2007

Visual performance after overnight orthokeratology.

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. CONCLUSION: Apart from the predicted improvements in unaided vision and reduction of the myopic refractive error, orthokeratology treatment was not found to significantly change any other aspects of visual acuity and contrast sensitivity. All visual performance measures exhibited stability over a 9-h period. Spherical refractive error changed significantly on Day 1 but became stable after a week of treatment. These results indicate that the corneal topography changes induced by orthokeratology do not induce changes in aberrations that are large enough to significantly diminish visual performance.

Circadian Rhythm↗

PubMed2007

A review of astigmatism and its possible genesis.

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.

Age Factors↗

PubMed2007

The influence of eyelid morphology on normal corneal shape.

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.

Adolescent↗

PubMed2007

Heart preservation using continuous ex vivo perfusion improves viability and functional recovery.

BACKGROUND: Cold static storage (CS) is a proven preservation method for heart transplantion, yet early postoperative graft dysfunction remains prevalent, so continuous perfusion (CP) during ex vivo transport may improve viability and function of heart grafts. METHODS AND RESULTS: Canine hearts underwent CP (n=9) or CS (n=9) for 6 h while intramyocardial pH was continuously monitored. Biopsies were assayed for ATP, caspase-3, malondialdehyde (MDA), and endothelin-1 (ET-1) levels at baseline, after preservation (t1), and after 1 h of blood reperfusion on a Langendorff model (t2). Functional recovery was determined at t2 by +dP/dt, -dP/dt, developed pressure, peak pressure and end-diastolic pressure. CP resulted in higher tissue pH and ATP stores and reduced caspase-3, MDA and ET-1 levels compared with CS at both t1 and t2. Post reperfusion recovery was significantly greater in CP vs CS for all myocardial functional parameters except end-diastolic pressure. Weight gain was significantly increased in CP vs CS at t1, but not at t2. CONCLUSIONS: Low-grade tissue acidosis and energy depletion occur during CS and are associated with oxidative injury and apoptosis during reperfusion. CP attenuates these biochemical and pathologic manifestations of tissue injury, together with improved myocardial recovery, despite mild, transient edema.

Adenosine Triphosphate↗

PubMed2006

electrochemical generation of a nonheme oxoiron(IV) complex.

The complex [Fe(IV)O(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) has been prepared by bulk electrolysis in aqueous CH3CN and CH2Cl2, and its redox properties characterized. Bulk chronocoulometry and spectropotentiometry experiments in CH3CN show that [Fe(II)(N4Py)(NCCH3)]2+ can be oxidized quantitatively to its iron(III) derivative at an applied potential of +0.71 V vs ferrocene and then to the oxoiron(IV) complex (in the presence of added water) at potentials above +1.3 V. The E1/2 value for the Fe(IV/III) couple has been estimated to be +0.90 V from spectropotentiometric titrations in CH3CN and cyclic voltammetric measurements in CH2Cl2.

Electrochemistry↗

PubMed2006

Computer simulation of visual outcomes of wavefront-only corneal ablation.

PURPOSE: To evaluate the effectiveness, predicted visual outcome, and limitations of a corneal ablation algorithm that uses wavefront aberration measurement alone without the need for corneal shape information. SETTING: Contact Lens and Visual Optical Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Australia. METHODS: Corneal topography and wavefront error data from 22 eyes of 11 potential refractive surgery candidates were used. A computer simulation of the corneal ablation was performed, and the predicted postoperative visual outcome was assessed by calculating the resulting wavefront root-mean-square (RMS) values and visual Strehl ratios. Additionally, the effect of ablation alignment error was examined. Finally, the visual outcomes of the wavefront-only corneal ablations were compared to those in an age-matched group of 20 emmetropic patients. RESULTS: Significant improvement in total and higher-order wavefront RMS was achieved postoperatively in both an ideal setting and in the case of ablation alignment errors. The predicted improvement in visual Strehl ratio in the potential refractive surgery candidates was significantly better than that in the untreated emmetropes. After additional simulated decentration of the pupil center by 150 microm, the result was slightly worse, but the change was found to not be significant when compared to the retinal image quality of emmetropes. CONCLUSIONS: Wavefront-only corneal ablation algorithms could potentially lead to significantly better visual outcomes than those normally encountered in untreated emmetropes, provided that the alignment error is not large. The presented methodology may be used as a screening tool to predict patients' visual outcomes before the surgery.

Adolescent↗

PubMed2006

Retinal image quality, reading and myopia.

Analysis was undertaken of the retinal image characteristics of the best-spectacle corrected eyes of progressing myopes (n = 20, mean age = 22 years; mean spherical equivalent = -3.84 D) and a control group of emmetropes (n = 20, mean age = 23 years; mean spherical equivalent = 0.00 D) before and after a 2h reading task. Retinal image quality was calculated based upon wavefront measurements taken with a Hartmann-Shack sensor with fixation on both a far (5.5 m) and near (individual reading distance) target. The visual Strehl ratio based on the optical transfer function (VSOTF) was significantly worse for the myopes prior to reading for both the far (p = 0.01) and near (p = 0.03) conditions. The myopic group showed significant reductions in various aspects of retinal image quality compared with the emmetropes, involving components of the modulation transfer function, phase transfer function and point spread function, often along the vertical meridian of the eye. The depth of focus of the myopes (0.54 D) was larger (p = 0.02) than the emmetropes (0.42 D) and the distribution of refractive power (away from optimal sphero-cylinder) was greater in the myopic eyes (variance of distributions p < 0.05). We found evidence that the lead and lag of accommodation are influenced by the higher order aberrations of the eye (e.g. significant correlations between lead/lag and the peak of the visual Strehl ratio based on the MTF). This could indicate that the higher accommodation lags seen in myopes are providing optimized retinal image characteristics. The interaction between low and high order aberrations of the eye play a significant role in reducing the retinal image quality of myopic eyes compared with emmetropes.

Accommodation, Ocular↗

PubMed2006

Computer-aided detection and diagnosis of breast cancer.

The use of computer-aided detection (CAD) with film or digital mammography is now widely regarded as the standard of practice in mammography and has been shown to increase the rate of breast cancer detection. There are inherent limitations in 2D mammography, and new technologies involving 2D and 3D imaging with X-rays, ultrasound, and MRI are in use or under investigation. CAD can aid in the reduction of oversight error for these modalities and has the potential to assist the physician in unifying the interpretation across alternative modalities. We believe the result will be improved sensitivity and specificity due to both improved detection and diagnosis.

Breast Neoplasms↗

PubMed2006

Combining central and peripheral videokeratoscope maps to investigate total corneal topography.

PURPOSE: To extend the area of standard corneal topography maps, one central map is combined with six peripheral maps after correlating them in a custom written computer program. METHODS: The point corresponding to the vertex normal of the central map is found in each of the peripheral maps. Data from the peripheral maps can then be added on to the edges of the central map to create a topography map that extends from limbus to limbus horizontally and vertically. RESULTS: The average size of the combined maps from 15 subjects was 11.3 +/- 0.3 mm horizontally and 10.3 +/- 0.3 mm vertically, compared to 9.2 +/- 0.4 mm horizontally and 7.5 +/- 0.7 mm vertically for the standard single maps. These values represent an increase in surface area of approximately 70%. CONCLUSIONS: The topography of the entire cornea can be represented by combining multiple measurements from a Placido videokeratoscope. Conic fits based on central topography data are a poor representation of the total corneal shape.

Adult↗

PubMed2006

Factors influencing lid pressure on the cornea.

PURPOSE: To investigate the effects of eyelid pressure on corneal topography in different angles of gaze and under different eye movement conditions. METHODS: Ten young subjects with healthy eyes were recruited for the study. Corneal topography of the right eye was measured with a videokeratoscope before and after four 15-minute visual tasks (downward gaze of 25 degrees or 45 degrees , both with no eye movements or 1 Hz eye movements). The four tasks were conducted on four separate mornings. RESULTS.: All subjects showed significant regions of topographic change after each of the four tasks. In general, eye movements were found to cause greater topographic changes than tasks without eye movements, and the larger angle of downward gaze led to greater changes in topography. The topographic changes were located close to the lid margin position during each task. CONCLUSIONS: The pressure exerted by the eyelid is capable of altering corneal topography. These changes are enhanced when the eye looks downward and when the eye moves laterally.

Adult↗

PubMed2006

Blinking patterns and corneal staining.

PURPOSE: To investigate the blinking patterns of healthy subjects and soft contact lens wearers and determine whether these blinking characteristics were associated with corneal fluorescein staining. METHODS: Fifteen young soft contact lens wearers and 11 young, healthy subjects participated in the study. The subjects were selected to have no significant eyelid disease. High-speed filming (100 frames per second) was used to capture the natural blinking patterns of the subjects for approximately 3 minutes. Custom written software was used to measure the vertical palpebral aperture at the start and the end of the downward motion of the upper eyelid. The vertical gap between the lids at the lowest point of the upper lid movement during each blink was measured (closed palpebral aperture). Corneal fluorescein staining was quantified on a 0-to-4 scale for each subject. RESULTS: Closed palpebral apertures in the healthy and soft contact lens-wearing subjects showed a wide distribution, with 22% of blinks being incomplete (<2/3 open aperture) in both groups of subjects. In soft contact lens wearers, there was a significant correlation (r = 0.40, P<0.05) between the mean closed palpebral aperture and the grade of corneal staining (primarily located inferiorly). The healthy subjects did not show the same degree of correlation (r = 0.16, P>0.1). However, the distribution of closed palpebral apertures was significantly different between those subjects with corneal staining compared to those without corneal staining (Kolmogorov-Smirnov two-sample test, healthy subjects with P=0.002 and soft lens wearers with P<0.001). CONCLUSIONS: The distribution of closed palpebral apertures of healthy and soft contact lens-wearing subjects showed no clear distinction between complete and incomplete blinks. Both groups of subjects show evidence of an association between the mean closed palpebral aperture size (degree of incomplete blinking) and the grade of corneal fluorescein staining, but the association is stronger in soft contact lens wearers.

Adult↗

PubMed2006

The morphology of the palpebral fissure in different directions of vertical gaze.

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.

Adolescent↗

PubMed2006

The contribution of accommodation and the ocular surface to the microfluctuations of wavefront aberrations of the eye.

We have used videokeratoscopy and wavefront sensing to investigate the contribution of the ocular surface and the effect of stimulus vergence on the microfluctuations of the wavefront aberrations of the eye. The fluctuations of the wavefront aberrations were quantified by their variations around the mean and by using power spectrum analysis. Integrated power was determined in two regions: 0.1-0.7 Hz (low frequencies) and 0.8-1.8 Hz (high frequencies). Changes in the ocular surface topography were measured using high-speed videokeratoscopy and variations in the ocular wavefront aberrations were measured with a wavefront sensor. The microfluctuations of wavefront aberrations of the ocular surface were found to be considerably smaller than the microfluctuations of the wavefront aberrations of the total eye. The fluctuations in defocus while viewing a closer target at 2 or 4 D were found to be significantly greater than fluctuations in defocus when viewing a far target. This increase in defocus fluctuations (p < or = 0.001) occurred in both the low- and high-frequency regions of the power spectra.

Accommodation, Ocular↗

PubMed2006

Corneal optics after reading, microscopy and computer work.

PURPOSE: To compare lid-induced changes in corneal optics following reading, microscopy and computer work. METHODS: Nine subjects with normal ocular health were recruited for the study. Five subjects were myopic, two were emmetropic, one was astigmatic and one was hyperopic. Corneal topography was measured before and after 60 mins of reading a novel, performing a blood cell counting task on a microscope and Internet searching. Corneal topography data were used to derive the corneal wavefront Zernike coefficients up to the fourth order. A meridian analysis of instantaneous corneal power along the upper 90-degree semi-meridian was performed to examine local changes caused by eyelid pressure. Digital photography was used to capture body posture and eyelid position during the tasks. RESULTS: Each of the three tasks showed systematically different effects on both the characteristics and location of corneal topography changes. Reading and microscopy generally exhibited larger and more centrally located changes compared with the computer task. Differences in wavefront aberration characteristics between the three tasks were apparent in both lower and higher order aberrations. The location of corneal distortions differed significantly between microscopy and computer work, with microscopy causing distortions to occur closer to the videokeratoscope measurement axis compared with computer work (p = 0.015). CONCLUSIONS: Reading, microscopy and computer work have different effects on corneal aberrations. The results are in agreement with the hypothesis that lid-induced corneal aberrations may play a role in myopia development.

Adult↗

PubMed2006

The topography of the central and peripheral cornea.

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 astigmatic than the central cornea. A conic section is a poor estimator of the peripheral cornea.

Adolescent↗

PubMed2005

Accommodation stimulus-response function and retinal image quality.

Accommodation stimulus-response function (ASRF) and its relationship to retinal image quality were investigated using a modified wavefront sensor. Ten subjects were presented with six vergence stimuli between 0.17 D and 5 D. For each vergence distance, ocular wavefronts and subjective visual acuity were measured. Wavefronts were analysed for a fixed 3-mm pupil diameter and for natural pupil sizes. Visual Strehl ratio computed in the frequency domain (VSOTF) and retinal images were calculated for each condition tested. Subjective visual acuity was significantly improved at intermediate vergence distances (1D and 2D; p < 0.01), and only decreased significantly at 5 D compared with 0.17 D (p < 0.05). VSOTF magnitude was associated with subjective visual acuity and VSOTF peak location correlated with accommodation error. Apparent accommodation errors due to spherical aberration were highly correlated with accommodation lead and lag for natural pupils (R(2) = 0.80) but not for fixed 3-mm pupils (R(2) < 0.00). The combination of higher-order aberrations and accommodation errors improved retinal image quality compared with accommodation errors or higher order aberrations alone. Pupil size and higher order aberrations play an important role in the ASRF.

Accommodation, Ocular↗

PubMed2005

The biomechanics of rigid contact lens removal.

The removal of rigid contact lenses from the eye, using the eyelids, is a relatively simple procedure. However, there is a sequence of biomechanical events underlying this procedure which are not well understood. By using high-speed videokeratoscopy (50 Hz), we have shown that during the lid-pull procedure the cornea typically shows a significant increase in with-the-rule astigmatism by an average of 2.19 D (axial power). The average increase in steep K power was 1.22 D (S.D. +/-1.05 D) and the average decrease in flat K power was 0.97 D (S.D. +/-1.05 D). This change in corneal topography increases the edge lift of the lens at 12 and 6 O'clock locations of the lens, enhancing the removal process. When the subject blinks, as the lid margins reach the lens edge, the lens flexes and the eye retracts by an average of 0.66 mm (S.D. +/-0.27 mm). This retraction again enhances the act of lens removal.

Adult↗

PubMed2005

The diurnal variation of corneal topography and aberrations.

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.

Adult↗