8 resultados para Visual examination

em Aston University Research Archive


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Autism is a pervasive developmental disorder and Asperger’s syndrome is part of the spectrum of autism disorders. This thesis aims to: • Review and investigate current theories concerning visual function in individuals with Asperger’s syndrome and high functioning autism spectrum disorder and to translate the findings into clinical practice by developing a specific protocol for the eye examination of individuals of this population. • Investigate whether those with Asperger’s syndrome are more likely to suffer from Meares-Irlen syndrome and/or dyslexia. • Assess the integrity of the M-cell pathway in Asperger’s syndrome using perimetric tests available in optometric practice to investigate and also to describe the nature of any defects. • Evaluate eye movement strategies in Asperger’s whilst viewing both text and images. Also to evaluate the most appropriate methodology for investigating eye movements; namely optical digital eye tracking and electrophysiology methodologies. Findings of the investigations include • Eye examinations for individuals with Asperger’s syndrome should contain the same testing methods as for the general population, with special consideration for clear communication. • There is a depression of M-pathway visual field sensitivity in 57% (8/14) of people with Asperger’s syndrome, supporting previous evidence for an M-cell deficit in some individuals. • There is a raised prevalence of dyslexia in Asperger’s syndrome (26% of a sample of 31) but not necessarily of Meares-Irlen syndrome. • Gaze strategies are abnormal in Asperger’s syndrome, for both reading and viewing of images. With increased saccadic movement and decreased viewing of faces in comparison to background detail.

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The study developed statistical techniques to evaluate visual field progression for use with the Humphrey Field Analyzer (HFA). The long-term fluctuation (LF) was evaluated in stable glaucoma. The magnitude of both LF components showed little relationship with MD, CPSD and SF. An algorithm was proposed for determining the clinical necessity for a confirmatory follow-up examination. The between-examination variability was determined for the HFA Standard and FASTPAC algorithms in glaucoma. FASTPAC exhibited greater between-examination variability than the Standard algorithm across the range of sensitivities and with increasing eccentricity. The difference in variability between the algorithms had minimal clinical significance. The effect of repositioning the baseline in the Glaucoma Change Probability Analysis (GCPA) was evaluated. The global baseline of the GCPA limited the detection of progressive change at a single stimulus location. A new technique, pointwise univariate linear regressions (ULR), of absolute sensitivity and, of pattern deviation, against time to follow-up was developed. In each case, pointwise ULR was more sensitive to localised progressive changes in sensitivity than ULR of MD, alone. Small changes in sensitivity were more readily determined by the pointwise ULR than by the GCPA. A comparison between the outcome of pointwise ULR for all fields and for the last six fields manifested linear and curvilinear declines in the absolute sensitivity and the pattern deviation. A method for delineating progressive loss in glaucoma, based upon the error in the forecasted sensitivity of a multivariate model, was developed. Multivariate forecasting exhibited little agreement with GCPA in glaucoma but showed promise for monitoring visual field progression in OHT patients. The recovery of sensitivity in optic neuritis over time was modelled with a Cumulative Gaussian function. The rate and level of recovery was greater in the peripheral than the central field. Probability models to forecast the field of recovery were proposed.

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The study evaluated sources of within- and between-subject variability in standard white-on-white (W-W) perimetry and short-wavelength automated perimetry (SWAP). The Influence of staircase strategy on the fatigue effect in W-W perimetry was investigated for a 4 dB single step, single reversal strategy; a variable step size, single reversal dynamic strategy; and the standard 4-2 dB double reversal strategy. The fatigue effect increased as the duration of the examination Increased and was greatest in the second eye for all strategies. The fatigue effect was lowest for the 4dB strategy, which exhibited the shortest examination time and was greatest for the 4-2 dB strategy, which exhibited the longest examination time. Staircase efficiency was lowest for the 4 dB strategy and highest for the dynamic strategy which thus offers a reduced examination time and low inter-subject variability. The normal between-subject variability of SWAP was determined for the standard 4-2 dB double reversal strategy and the 3 dB single reversal FASTPAC strategy and compared to that of W-W perimetry, The decrease in sensitivity with Increase in age was greatest for SWAP. The between-subject variability of SWAP was greater than W-W perimetry. Correction for the Influence of ocular media absorption reduced the between-subject variability of SWAP, The FASTPAC strategy yielded the lowest between-subject variability In SWAP, but the greatest between-subject variability In WoW perimetry. The greater between-subject variability of SWAP has profound Implications for the delineation of visual field abnormality, The fatigue effect for the Full Threshold strategy in SWAP was evaluated with conventional opaque, and translucent occlusion of the fellow eye. SWAP exhibited a greater fatigue effect than W-W perimetry. Translucent occlusion reduced the between-subject variability of W-W perimetry but Increased the between-subject variability of SWAP. The elevation of sensitivity was greater with translucent occlusion which has implications for the statistical analysis of W-W perimetry and SWAP. The influence of age-related cataract extraction and IOL implantation upon the visual field derived by WoW perimetry and SWAP was determined. Cataract yielded a general reduction In sensitivity which was preferentially greater in SWAP, even after the correction of SWAP for the attenuation of the stimulus by the ocular media. There was no correlation between either backward or forward light scatter and the magnitude of the attenuation of W-W or SWAP sensitivity. The post-operative mean deviation in SWAP was positive and has ramifications for the statistical Interpretation of SWAP. Short-wavelength-sensitive pathway isolation was assessed as a function of stimulus eccentricity using the two-colour Increment threshold method. At least 15 dB of SWS pathway Isolation was achieved for 440 nm, 450 nm and 460 nm stimuli at a background luminance of 100 cdm-2, There was a slight decrease In SWS pathway Isolation for all stimulus wavelengths with increasing eccentricity which was not of clinical significance. Adopting a 450 nm stimulus may reduce between-subject variability In SWAP due to a reduction In ocular media absorption and macular pigment absorption.

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The study utilized the advanced technology provided by automated perimeters to investigate the hypothesis that patients with retinitis pigmentosa behave atypically over the dynamic range and to concurrently determine the influence of extraneous factors on the format of the normal perimetric sensitivity profile. The perimetric processing of some patients with retinitis pigmentosa was considered to be abnormal in either the temporal and/or the spatial domain. The standard size III stimulus saturated the central regions and was thus ineffective in detecting early depressions in sensitivity in these areas. When stimulus size was scaled in inverse proportion to the square root of ganglion cell receptive field density (M-scaled), isosensitive profiles did not result, although cortical representation was theoretically equivalent across the visual field. It was conjectured that this was due to variations in the ganglion cell characteristics with increasing peripheral angle, most notably spatial summation. It was concluded that the development of perimetric routines incorporating stimulus sizes adjusted in proportion to the coverage factor of retinal ganglion cells would enhance the diagnostic capacity of perimetry. Good general and local correspondence was found between perimetric sensitivity and the available retinal cell counts. Intraocular light scatter arising both from simulations and media opacities depressed perimetric sensitivity. Attenuation was greater centrally for the smaller LED stimuli, whereas the reverse was true for the larger projected stimuli. Prior perimetric experience and pupil size also demonstrated eccentricity-dependent effect on sensitivity. Practice improved perimetric sensitivity for projected stimuli at eccentricities greater than or equal to 30o; particularly in the superior region. Increase in pupil size for LED stimuli enhanced sensitivity at eccentricities greater than 10o. Conversely, microfluctuation in the accommodative response during perimetric examination and the correction of peripheral refractive error had no significant influence on perimetric sensitivity.

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The study investigated the potential applications and the limitations of non-standard techniques of visual field investigation utilizing automated perimetry. Normal subjects exhibited a greater sensitivity to kinetic stimuli than to static stimuli of identical size. The magnitude of physiological SKD was found to be largely independent of age, stimulus size, meridian and eccentricity. The absence of a dependency on stimulus size indicated that successive lateral spatial summation could not totally account for the underlying mechanism of physiological SKD. The visual field indices MD and LV exhibited a progressive deterioration during the time course of a conventional central visual field examination both for normal subjects and for ocular hypertensive patients. The fatigue effect was more pronounced in the latter stages and for the second eye tested. The confidence limits for the definition of abnormality should reflect the greater effect of fatigue on the second eye. A 330 cdm-2 yellow background was employed for blue-on-yellow perimetry. Instrument measurement range was preserved by positioning a concave mirror behind the stimulus bulb to increase the light output by 60% . The mean magnitude of SWS pathway isolation was approximately 1.4 log units relative to a 460nm stimulus filter. The absorption spectra of the ocular media exhibited an exponential increase with increase in age, whilst that of the macular pigment showed no systematic trend. The magnitude of ocular media absorption was demonstrated to reduce with increase in wavelength. Ocular media absorption was significantly greater in diabetic patients than in normal subjects. Five diabetic patients with either normal or borderline achromatic sensitivity exhibited an abnormal blue-on-yellow sensitivity; two of these patients showed no signs of retinopathy. A greater vulnerability of the SWS pathway to the diabetic disease process was hypothesized.

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The diagnosis and monitoring of ocular disease presents considerable clinical difficulties for two main reasons i) the substantial physiological variation of anatomical structure of the visual pathway and ii) constraints due to technical limitations of diagnostic hardware. These are further confounded by difficulties in detecting early loss or change in visual function due to the masking of disease effects, for example, due to a high degree of redundancy in terms of nerve fibre number along the visual pathway. This thesis addresses these issues across three areas of study: 1. Factors influencing retinal thickness measures and their clinical interpretation As the retina is the principal anatomical site for damage associated with visual loss, objective measures of retinal thickness and retinal nerve fibre layer thickness are key to the detection of pathology. In this thesis the ability of optical coherence tomography (OCT) to provide repeatable and reproducible measures of retinal structure at the macula and optic nerve head is investigated. In addition, the normal physiological variations in retinal thickness and retinal nerve fibre layer thickness are explored. Principal findings were: • Macular retinal thickness and optic nerve head measurements are repeatable and reproducible for normal subjects and diseased eyes • Macular and retinal nerve fibre layer thickness around the optic nerve correlate negatively with axial length, suggesting that larger eyes have thinner retinae, potentially making them more susceptible to damage or disease • Foveola retinal thickness increases with age while retinal nerve fibre layer thickness around the optic nerve head decreases with age. Such findings should be considered during examination of the eye with suspect pathology or in long-term disease monitoring 2. Impact of glucose control on retinal anatomy and function in diabetes Diabetes is a major health concern in the UK and worldwide and diabetic retinopathy is a major cause of blindness in the working population. Objective, quantitative measurements of retinal thickness. particularly at the macula provide essential information regarding disease progression and the efficacy of treatment. Functional vision loss in diabetic patients is commonly observed in clinical and experimental studies and is thought to be affected by blood glucose levels. In the first study of its kind, the short term impact of fluctuations in blood glucose levels on retinal structure and function over a 12 hour period in patients with diabetes are investigated. Principal findings were: • Acute fluctuations in blood glucose levels are greater in diabetic patients than normal subjects • The fluctuations in blood glucose levels impact contrast sensitivity scores. SWAP visual fields, intraocular pressure and diastolic pressure. This effect is similar for type 1 and type 2 diabetic patients despite the differences in their physiological status. • Long-term metabolic control in the diabetic patient is a useful predictor in the fluctuation of contrast sensitivity scores. • Large fluctuations in blood glucose levels and/or visual function and structure may be indicative of an increased risk of development or progression of retinopathy 3. Structural and functional damage of the visual pathway in glaucomatous optic neuropathy The glaucomatous eye undergoes a number of well documented pathological changes including retinal nerve fibre loss and optic nerve head damage which is correlated with loss of functional vision. In experimental glaucoma there is evidence that glaucomatous damage extends from retinal ganglion cells in the eye, along the visual pathway, to vision centres in the brain. This thesis explores the effects of glaucoma on retinal nerve fibre layer thickness, ocular anterior anatomy and cortical structure, and its correlates with visual function in humans. Principal findings were: • In the retina, glaucomatous retinal nerve fibre layer loss is less marked with increasing distance from the optic nerve head, suggesting that RNFL examination at a greater distance than traditionally employed may provide invaluable early indicators of glaucomatous damage • Neuroretinal rim area and retrobulbar optic nerve diameter are strong indicators of visual field loss • Grey matter density decreases at a rate of 3.85% per decade. There was no clear evidence of a disease effect • Cortical activation as measured by fMRI was a strong indicator of functional damage in patients with significant neuroretinal rim loss despite relatively modest visual field defects These investigations have shown that the effects of senescence are evident in both the anterior and posterior visual pathway. A variety of anatomical and functional diagnostic protocols for the investigation of damage to the visual pathway in ocular disease are required to maximise understanding of the disease processes and thereby optimising patient care.

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Purpose: The aims of this study were to develop an algorithm to accurately quantify Vigabatrin (VGB)-induced central visual field loss and to investigate the relationship between visual field loss and maximum daily dose, cumulative dose and duration of dose. Methods: The sample comprised 31 patients (mean age 37.9 years; SD 14.4 years) diagnosed with epilepsy and exposed to VGB. Each participant underwent standard automated static visual field examination of the central visual field. Central visual field loss was determined using continuous scales quantifying severity in terms of area and depth of defect and additionally by symmetry of defect between the two eyes. A simultaneous multiple regression model was used to explore the relationship between these visual field parameters and the drug predictor variables. Results: The regression model indicated that maximum VGB dose was the only factor to be significantly correlated with individual eye severity (right eye: p = 0.020; left eye: p = 0.012) and symmetry of visual field defect (p = 0.024). Conclusions: Maximum daily dose was the single most reliable indicator of those patients likely to exhibit visual field defects due to VGB. These findings suggest that high maximum dose is more likely to result in visual field defects than high cumulative doses or those of long duration.

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Background: Vigabatrin (VGB) is an anti-epileptic medication which has been linked to peripheral constriction of the visual field. Documenting the natural history associated with continued VGB exposure is important when making decisions about the risk and benefits associated with the treatment. Due to its speed the Swedish Interactive Threshold Algorithm (SITA) has become the algorithm of choice when carrying out Full Threshold automated static perimetry. SITA uses prior distributions of normal and glaucomatous visual field behaviour to estimate threshold sensitivity. As the abnormal model is based on glaucomatous behaviour this algorithm has not been validated for VGB recipients. We aim to assess the clinical utility of the SITA algorithm for accurately mapping VGB attributed field loss. Methods: The sample comprised one randomly selected eye of 16 patients diagnosed with epilepsy, exposed to VGB therapy. A clinical diagnosis of VGB attributed visual field loss was documented in 44% of the group. The mean age was 39.3 years∈±∈14.5 years and the mean deviation was -4.76 dB ±4.34 dB. Each patient was examined with the Full Threshold, SITA Standard and SITA Fast algorithm. Results: SITA Standard was on average approximately twice as fast (7.6 minutes) and SITA Fast approximately 3 times as fast (4.7 minutes) as examinations completed using the Full Threshold algorithm (15.8 minutes). In the clinical environment, the visual field outcome with both SITA algorithms was equivalent to visual field examination using the Full Threshold algorithm in terms of visual inspection of the grey scale plots, defect area and defect severity. Conclusions: Our research shows that both SITA algorithms are able to accurately map visual field loss attributed to VGB. As patients diagnosed with epilepsy are often vulnerable to fatigue, the time saving offered by SITA Fast means that this algorithm has a significant advantage for use with VGB recipients.