Abstract
The first part of this thesis was on the development of a new instrument for measurement of light reflected from the retina in a living human eye. The key element is an imaging spectrograph, with its slit placed conjugate to the pupil of the eye. The instrument measures both the
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spectral and the directional properties of the reflected light. Model analysis of the spectral reflection yields the optical densities of ocular absorbers, e.g., the eye lens, melanin, and the macular pigment. The directional properties of the retinal reflectance provide information on the integrity of the foveal cone photoreceptors. The applications of the instrument concentrate on the fovea: the region of the retina specialized for acute vision. Therefore, the apparatus was coined the "Foveal Reflection Analyzer".
In the second part of this thesis, analysis of the spectra obtained with the new instrument allowed us to demonstrate several new aspects of the retinal reflectance. First, we elaborated on spectra for the directionality described with parameter r; a measure for the peakedness of the distribution in the pupil plane. The scattering theory by Marcos et al. [J. Opt. Soc. Am. A 15, 2012-2022 (1998)] and Marcos and Burns [J. Opt. Soc. Am. A 16, 995-1004 (1999)] predicts that r versus wavelength can be described by the sum of the waveguide directionality rwg, independent from wavelength, and a contribution proportional to one over wavelength squared, attributed to scattering. The contribution of the latter component gives a measure for the row-to-row cone spacing. Our data for r as a function of wavelength corroborated the scattering theory. The estimations for cone spacing and the waveguide directionality were in agreement with the literature.
Second, we derived a model for the amplitude of the directional reflectance as a function of the wavelength. The directional reflectance was supposed to originate from the outer segments of the photoreceptors. Only two absorbers were considered of significance: the crystalline lens and the macular pigment. Model analysis of spectra obtained from five eyes with a plastic implant lens demonstrated that the source of directional reflectance is spectrally neutral. Data obtained on 39 normal eyes aged 18-64 years allowed us to study the absorption in the natural eye lens as function of age. Our newly proposed model for absorption in the lens comprises two components: a "young" template representing the pigment O-b-glucoside of 3-hydroxykynurenine (3-HKG), and an "aged" template representing the pigments accumulating in the lens with age. The final model was applied to spectra obtained from individual eyes. As expected, the density of the young template decreased with age. Concomitantly, the density of the aged template increased with age. The total optical density of the eye at 420 nm slightly increased with age. Precise individual estimates for the macular pigment density were obtained. Finally, we observed a decrease of the photoreceptor reflectivity with age, possibly reflecting a degradation of the photoreceptors.
In the final part of this thesis, the first application of the new instrument in a clinical setting is presented. This study included a group of patients with diabetes mellitus. The aim was to assess the integrity of the photoreceptors in the fovea, and to measure the optical density of the macular pigment and the eye lens. The results were compared with those of a group of healthy subjects. We found a strong reduction of the amplitude of the directional reflection in the diabetics group. This indicates changes in the integrity of the foveal cone-photoreceptors. The directionality of the cones was not reduced. Macular pigment was not significantly different in diabetics. Lens density increased at a substantially higher rate in diabetics, compared with the normal age-related increase in the controls. The instrument proved to be a fast and practical test on photoreceptor integrity. Macular pigment could be assessed efficiently and accurately in the patients, their reduced visual acuity presented no problem for the method. The results of this first pilot study warrant the initiation of further clinical studies on a larger scale. For this purpose, our group has recently developed a table-top version of the instrument.
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