Optometry and Vision Science

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This is the collection for the University of Waterloo's School of Optometry and Vision Science.

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Browsing Optometry and Vision Science by Author "Hutchings, Natalie"

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    Assessment of ocular aberrations at scaled pupil size and reduced Shack-Hartmann spot number
    (University of Waterloo, 2019-05-07) Ommani, Abbas; Lakshminarayanan, Vasudevan; Hutchings, Natalie
    Wavefront aberrations describe the optical imperfections of the eye by measuring the complete refractive elements of the eye. However, the reliability of ocular aberration is uncertain under some challenges and issues. Ocular aberration is generally described in terms of Zernike polynomials. However, the Zernike polynomials are pupil size dependent, therefore, the aberration measured at a fixed pupil size cannot be used for another pupil size. One solution to this problem is to use pupil size scaling technique to scale up or down the aberration to a required pupil size; however, the validity of these techniques for clinical data is not available. To tackle this issue, validation of mathematical pupil size scaling formula by comparing the estimates of the Zernike coefficients with corresponding clinical measurements obtained at different pupil sizes is performed. The results show that the estimation of ocular wavefront aberration coefficients either scaling down from large to smaller pupils or scaling up from smaller to large pupils provides estimates that are not significantly different from clinically measured values. However, when scaling up to a larger pupil size, the estimates are more variable. These findings have implications for pupil scaling on an individual basis, such as in cases of refractive surgery or when using pupil scaling to examine a clinical cohort. Another challenge of an ocular aberration for clinical uses is when the spots on the Shack-Hartmann (SH) are missed due to the opacity of eye parameters or some other disease conditions. This issue is addressed by randomly deleting the number of spots from the SH images and comparing the results with the aberration of the original SH image without the missing spots. The results indicate that as high as 50 % of the SH spots can be deleted without affecting the estimation of spherical defocus within typical clinically acceptable limits of ±0.25D. The results are further examined with in vivo measurements of a human eye wearing a spectacle lens with various models of clustered missing spots to simulate loss that might occur with the disease. The findings of this study provide foundational data on measuring the ocular wavefront aberration when only a reduced number of SH spots are available.
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    Structure and Function in Early Glaucoma
    (University of Waterloo, 2017-01-23) Balian, Carmen; Flanagan, John; Hutchings, Natalie
    Glaucoma is a general term that includes an array of ocular conditions that cause a specific neuropathy of the optic nerve (Greenfield, Bagga, et al. 2003) of which abnormalities associated with this disorder are localized at the level of the retinal ganglion cell layer (Epstein 1997; Quigley & Broman 2006). This structure-function relationship is not clear as it relies on several factors such as variability from the structural and functional tests, differences in measurement scales between the two modalities (Greaney et al. 2002; Katz 1999; Drance 1985; Hood et al. 2007) and physiological variation amongst individuals (Pan & Swanson 2006). The global aim of this thesis was to relate visual function of the retinal ganglion cells to structure of the optic nerve head and retinal nerve fiber layer with respect to the following perimetry techniques: i) standard automated perimetry (SAP), ii) frequency doubling technology (FDT), iii) flicker defined form (FDF), and iv) the motion detection test (MDT), and the following imaging instruments: i) confocal scanning laser ophthalmoscopy (HRT), ii) optical coherence tomography (OCT), and iii) scanning laser polarimetry (GDx VCC). The specific purpose of this study was to i) compare the test-retest characteristics of the perimetry techniques, ii) determine which may be more sensitive for early detection, iii) evaluate the structure-function relationship between measures of retinal nerve fiber layer and visual function, and iv) perform a preliminary study to determine which techniques may be most suitable to monitor progression, in patients with early stage glaucoma. MDT showed little change in the 1-year follow-up study thus being unsuitable for monitoring change. FDT and FDF gave a similar performance and are likely optimal for the detection of early functional damage. Poor diagnostic agreement was seen between the HRT and each perimetry technique. Because no one perimetry test showed both high sensitivity and high specificity, it is recommended that a combination of FDF with either SAP, FDT or MDT be used as the functional component in the diagnosis and follow-up of patients with glaucoma. The strongest global structure-function correlations for OCT were seen with SAP, FDT and MDT; for GDx, the strongest association was seen with FDF. These results suggest that FDF and GDx used in combination are best to detect early glaucomatous changes.
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    Structure and Function in Early Glaucoma
    (University of Waterloo, 2016-08-24) Ho, Yuan-Hao; Flanagan, John; Hutchings, Natalie
    Glaucoma is a group of diseases, which exhibit a characteristic optic neuropathy and may result in progressive visual field loss. The most important risk factor is raised intraocular pressure (IOP) usually secondary to reduced aqueous outflow through the anterior chamber angle. It is the second leading cause of blindness globally. The diagnosis of glaucoma is difficult, as there is currently no widely-accepted “clinical standard” for diagnosis, although “progressive structural optic nerve and/or nerve fiber layer damage” is currently the most commonly accepted diagnostic criterion. Current treatments are to reduce the level of IOP, either by topical medication or surgery. Unfortunately, medical intervention frequently takes place after visual field loss has occurred. Consequently, much effort has been placed into the early diagnosis of glaucoma, in order to prevent damage. Visual field tests have been a popular clinical method to determine functional defects, and they are essential for managing and diagnosing glaucoma. Various methods and test strategies have been developed. Computerized threshold static perimetry involves determining the dimmest stimulus that can be seen at a number of pre-determined test point locations. An examiner can interpret the resulting pattern of defect; also, disease progress can be followed over time. Visual fields should not be interpreted in isolation but in conjunction with other clinical findings1. Standard automated perimetry (SAP) is our oldest and best documented, computerized, subjective visual function test. Threshold tests are commonly used for both detection and follow-up of glaucoma patients. Different testing strategies and different stimuli have been developed with expectations of raising the sensitivity for early detection of glaucoma-related functional change, such as short wavelength automated perimetry (SWAP), high-pass resolution perimetry (HRP), frequency doubling technology (FDT) and Flicker Defined Form (FDF). FDF is a temporally driven illusion in which background elements and stimulus elements are flickered in counterphase at a high temporal frequency, creating an illusory contour at the boundary between the background and the stimulus. It has been described to be a predominantly magnocellular-based stimulus due to its dependence on high temporal frequencies and its perceived low spatial frequency. The random flickering dots throughout the field of view and the complex nature of the stimulus, a phase-difference percept requiring higher order processing. Clinically, besides testing for deficits in function, measuring of retinal structure plays an important role in the diagnosis of early glaucoma. Damage results in characteristic signs in the retinal nerve fiber layer, the parapapillary retina and the optic nerve head, due to the oriented distribution of the nerve fiber in the retina. Scanning laser tomography (SLT; Heidelberg Retina Tomograph, Heidelberg, Germany) is a confocal scanning laser device that provides accurate and reproducible topographical information of the optic disc and peripapillary retina. Other methods such as optic disc photography, retinal nerve fiber layer photography, scanning laser polarimetry, and optical cohererence tomography are also designed to detect structural changes. By analyzing the neuroretinal rim within the optic disc, the SLT provides evidence of glaucoma related structural change, such as changes in the cup to disc ratio and notching and narrowing of the neuroretinal rim. Measurements were affected by age, but it is fairly robust to astigmatism and working distance. Studies have shown correlation between visual field test results and optic nerve head structural measurements. The correlation analysis of structure and function was performed to evaluate the spatial relationship. It has been proposed that both structural and functional diagnostic methods have unique value, but the combination of methods might provide early evidence for glaucoma diagnosis and management. The objectives of this thesis are: 1. To determine the normal sensitivity and confidence limits for FDF perimetry as a function of age; 2. To determine the test-retest repeatability of FDF perimetry for stable glaucoma; and 3. To investigate the structure function relationship in glaucoma using FDF perimetry and the HRT. Normative data for different perimeters are well established. It is critical to establish normal sensitivity for the FDF perimetry. Age related sensitivity loss throughout the visual field has been previously reported. Confidence limits for normality will be established in this thesis, as only then can we examine the ability of the new clinical test to detect early glaucoma. Measures of function and structure are both relevant and required for the early diagnosis of glaucoma. The relationship between the points tested in the visual field and corresponding positions at the optic nerve head have been previously described. Comparing the FDF perimetry results with the HRT optic nerve head results has the potential to be of significant value in the diagnosis of glaucoma.