The Investigation of Autofluorescence in Myopes and Emmetropes

Abstract

Fundus autofluorescence (FAF) is a retinal imaging modality that captures autofluorescence signals from the posterior segment of the eye, particularly from the retinal pigment epithelium (RPE) and the associated lipofuscin. FAF signal can be obtained from all eyes but departures from normal intensity and distribution have been linked to dysfunction or atrophy of the RPE and photoreceptors. High myopia involves structural and functional changes in the retina and it was hypothesised that the axial elongation in high myopia would induce abnormalities in FAF. In this thesis, I set out to study the FAF signals in high myopia and compare it with emmetropia. The thesis has 3 main aims. First, to understand how the FAF signal captured from young healthy eyes using confocal scanning laser ophthalmoscope (cSLO) is affected by different image acquisition settings and to document FAF distribution in young and healthy eyes. Second, to document/characterise FAF in emmetropes and high myopes, and relate it with contrast sensitivity. Third, to study the link between retinal nerve fibre layer (RNFL) and parapapillary autofluorescence (PAF) intensity and compare PAF between emmetropes and high myopes. Preliminary experiments found that young emmetropic subjects with good fixation required as few as 25 scans for signal averaging during FAF imaging. There was a trend for standard deviation of sampled pixel intensities to decrease when 49 scans were used for averaging, especially for 55˚ images. The locations of FAF peaks were not affected by increasing the number of scans for signal averaging or when no photobleaching period was incorporated into the imaging protocol. FAF along 8 cardinal meridians of the young and normal eye was described using 55˚ FAF images. FAF peaks were identified 2100 - 6000µm from the fovea. Data from the superior-nasal and nasal meridians confirmed the close spatial correspondence between FAF intensity and published data on rod photoreceptor and RPE lipofuscin distribution. FAF intensity and contrast sensitivity at low and moderate spatial frequencies were similar between emmetropes and high myopes. No correlation was found between FAF intensity and contrast sensitivity. A method of sampling PAF was developed. Using this method, we found that RNFL significantly attenuated PAF signal. The PAF profile, adjusted for RNFL attenuation, was similar between emmetropes and high myopes. The rate of age-related increase in PAF was slower than perifoveal autofluorescence and this difference matched the lower phagocytic load of parapapillary RPE cells compared to perifoveal RPE cells. The work presented in this thesis confirms the close spatial relationship between autofluorescence and rod distribution and describes a method to analyse PAF. This may ultimately open up more avenues of research and clinical applications of autofluorescence imaging in high myopia and optic neuropathies in the parapapillary region. 

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ian Murray (Supervisor) & Niall Mcloughlin (Supervisor)