Corneal Topography and the Morphology of the Palpebral Fissure
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Corneal topography and the morphology of the palpebral fissure. Scott A. Read B.App.Sc(Optom) Hons Institute of Health and Biomedical Innovation School of Optometry Queensland University of Technology Brisbane Australia Submitted as part of the requirements for the award of the degree of Doctor of Philosophy Keywords Keywords Cornea Astigmatism Aberrations Eyelids Corneal topography Videokeratoscopy Digital imaging Eyelid morphology II Abstract Abstract The notion that forces from the eyelids can alter the shape of the cornea has been proposed for many years. In recent times, there has been a marked improvement in our ability to measure and define the corneal shape, allowing subtle changes in the cornea to be measured. These improvements have led to the findings that pressure from the eyelids can cause alterations in corneal shape following everyday visual tasks such as reading. There are also theories to suggest that pressure from the eyelids may be involved in the aetiology of corneal astigmatism. In this program of research, a series of experiments were undertaken to investigate the influence of the eyelids on the shape of the cornea. In the first experiment, an investigation into the diurnal variation of corneal shape was carried out by measuring corneal topography at three different times (approximately 9 am, 1 pm and 5 pm) during the day over three days of the week (Monday, Tuesday and Friday). Highly significant diurnal changes were found to occur in the corneal topography of 15 of the 17 subjects. This change typically consisted of horizontal bands of distortion in the superior, and to a lesser extent, inferior cornea, increasing throughout the day (and returning to baseline the next morning). These changes appeared to be related to forces from the eyelids on the anterior cornea. Some changes were also found in corneal astigmatism. Corneal astigmatism power vector J0 (astigmatism 90/180°) was found to increase slightly over the course of the week. Whilst the changes in astigmatism were small in magnitude, this III Abstract result leaves open the possibility that pressure from the eyelid may cause changes in corneal astigmatism. If pressure from the eyelids is involved in the aetiology of corneal astigmatism, then one may expect associations to exist between certain characteristics of the eyelids and corneal shape. An experiment was then undertaken to explore these possible associations. We defined the average morphology of the palpebral fissure in different angles of vertical gaze for 100 young normal subjects. This was achieved through analysis of digital images that were captured in primary gaze, 20° downgaze and 40° downgaze. Parameters defining the size, position, angle and contour of the eyelids were determined. Highly significant changes were found to occur in the palpebral fissure with downward gaze. The palpebral aperture narrows in downward gaze, and the angle of the eyelids changes from being slightly upward slanted in primary gaze, to being slightly downward slanted in downward gaze. The eyelid margin contour also flattens significantly in downward gaze. The average topography of the central and peripheral cornea was also defined for this same population. A technique was used that allowed the capture and subsequent combination of topography data from both the central and the peripheral cornea. The use of this technique provided a large corneal topography map, with data extending close to the limbus for each subject. Marked flattening was found to occur in the peripheral cornea and a conic section was found to be a poor descriptor of corneal contour in the periphery (i.e. greater than 6 mm diameter). Corneal astigmatism was also IV Abstract found on average to reduce in the periphery. However a number of distinct patterns of peripheral corneal astigmatism were noted in the population. Corneal astigmatism in the peripheral cornea was either found to remain stable (59% of subjects), increase (10% of subjects) or reduce (31% of subjects) in magnitude in comparison to the amount of central corneal astigmatism. We also investigated associations between the parameters defining the palpebral fissure and parameters describing corneal shape in this population of subjects. A number of highly significant associations were found between the morphology of the palpebral fissure in primary gaze and the shape of the cornea. A general tendency was found for subjects with wider horizontal palpebral fissure widths to exhibit larger corneas and also flatter central corneal powers. There were also highly significant associations found between the angle of the eyelids and the axis of corneal astigmatism, but not the magnitude of corneal astigmatism. The associations found between corneal astigmatism and palpebral fissure morphology is further evidence supporting the hypothesis that pressure from the eyelids is involved in the aetiology of corneal astigmatism. The results of these investigations have shown that corneal changes as a result of eyelid forces occur in the majority of young subjects tested over the course of a normal working day. The average morphology of the palpebral fissure and topography of the central and peripheral cornea has also been defined in detail for a large population of young subjects. Significant V Abstract associations were found between corneal astigmatism and the morphology of the palpebral fissure. Whilst these results support a model of corneal astigmatism development based on eyelid morphology, they do not prove causation. Further research including measurement of eyelid pressure and corneal rigidity may aid in understanding the exact aetiology of the magnitude and axis of corneal astigmatism. VI Contents Table of Contents Chapter 1: Literature review 1.1 Introduction 1 1.2 The shape of the cornea 2 1.2.1 Methods for measuring corneal shape 2 1.2.2 Corneal reference points 6 1.2.3 Describing the shape of the cornea 9 1.2.4 Mathematical descriptions the cornea 12 1.3 The normal shape of the cornea 18 1.3.1 Corneal aberrations 22 1.3.2 Influence of age on the normal cornea 24 1.3.3 The association between corneal shape and refractive error 25 1.3.3.1 The cornea in myopia 26 1.3.3.2 The cornea in hyperopia 29 1.4 Other factors influencing corneal shape 29 1.4.1 The pre-corneal tear film 30 1.4.2 External forces and corneal shape 31 1.4.2.1 Orthokeratology 32 1.4.2.2 The corneal epithelium 34 1.4.2.3 The corneal stroma 36 1.4.3 Accommodation and corneal topography 39 1.4.4 Extraocular muscles and corneal topography 42 VII Contents 1.4.4.1 Extraocular muscle surgery and the cornea 43 1.4.4.2 Corneal topography and nystagmus 44 1.4.5 Corneal topography and eyelid forces 45 1.4.5.1 Eyelid pathology and corneal topography 49 1.4.5.2 Visual tasks and corneal topography 51 1.5 Astigmatism 56 1.5.1 Prevalence and changes of astigmatism with age 59 1.5.1.1 Astigmatism in infants and children 59 1.5.1.2 Astigmatism in adults 61 1.5.2 Javal’s rule 63 Symmetry of axis of astigmatism between right and 1.5.3 left eyes 67 1.5.4 Myopia and astigmatism 68 1.5.5 Genetics and astigmatism 72 1.5.6 Astigmatism in ethnic and disease groups 75 1.5.7 Animal studies and astigmatism 79 1.5.8 Corneal astigmatism and eyelid forces 82 1.6 Rationale 85 Chapter 2: The diurnal variation of corneal topography and aberrations 2.1 Introduction 89 2.2 Methods 93 2.2.1 Subjects and procedures 93 2.2.2 Data analysis 96 VIII Contents 2.3 Results 99 2.4 Discussion 109 2.5 Conclusions 120 Chapter 3: The morphology of the palpebral fissure in different angles of vertical gaze 3.1 Introduction 123 3.2 Methods 127 3.2.1 Subjects and procedures 127 3.2.2 Data analysis 133 3.3 Results 138 3.3.1 Caucasian population 141 3.3.2 Difference associated with gender 146 3.3.3 Difference associated with ethnicity 148 3.4 Discussion 150 3.5 Conclusions 164 Chapter 4: The topography of the central and peripheral cornea 4.1 Introduction 167 4.2 Methods 170 4.2.1 Subjects and procedures 170 4.2.2 Data analysis 183 4.3 Results 190 4.4 Discussion 205 IX Contents 4.5 Conclusions 213 Chapter 5: The association between the topography of the cornea and the morphology of the palpebral fissure 5.1 Introduction 215 5.2 Methods 219 5.2.1 Subjects and procedures 219 5.2.2 Data analysis 224 5.3 Results 228 5.3.1 Corneal topography and eyelid morphology 228 5.3.2 Corneal diameter and corneal power 238 5.3.3 Eyelid morphology and refractive error 242 5.3.4 Corneal topography and refractive error 243 5.4 Discussion 244 5.5 Conclusions 254 Chapter 6: Conclusions 6.1 Summary and main findings 255 6.1.1 The diurnal variation of corneal topography 255 6.1.2 The association between corneal topography and eyelid morphology 257 6.2 A possible model of corneal shape and astigmatism development 261 6.3 Future research directions 269 X Contents References 271 Appendices Appendix 1 Ethics 323 Appendix 2 Publications arising from the thesis 327 Figures Figure 1.1 A typical Placido ring image as captured by the Keratron videokeratoscope 4 Figure 1.2 Illustration of the different reference axes in videokeratoscopy. 8 Figure 1.3 An example of a dioptric power map (tangential power) from the Keratron videokeratoscope. 10 Figure 1.4 Illustration of the type of conic section given by different values of the asphericity parameter Q. 14 Figure 1.5 Example of corneal topography map following reading.