Ocular fluorometry methodological improvements and clinical studies--with special reference to the blood-retina barrier permeability to fluorescein and fluorescein glucuronide.

Abstract

The measurement of fluorescence in the human eye can be made using relatively simple instruments. Fluorescence is evoked when illumination is absorbed by intrinsic fluorophores in the eye or by artificially introduced extrinsic fluorophores. Intrinsic fluorescence is evidence of important molecular characteristics of the ocular tissues, whereas the extrinsic fluorophores are used primarily in the study of the barriers between the anatomical and physiological compartments of the eye. Blood-retina barrier leakage of fluorescein can be examined after the intravenous injection of fluorescein by quantitative determination of fluorescence in plasma and in the vitreous. From these measurements of the distribution of fluorescein, the permeability of a hypothetical spherical interface between the blood and the retina can be estimated using a mathematical model of the barrier. The use of fluorescein as a tracer is problematic because of its rapid metabolic conversion to fluorescein glucuronide. This metabolite disturbs ocular fluorescence measurements because it fluoresces over the same part of the spectrum as the parent compound. Additionally, the glucuronide occurs in markedly different concentrations depending upon the patient's renal function. With the previously used fluorometry techniques it has been impossible to determine the contribution of fluorescein glucuronide to the vitreous fluorescence. The primary objective of the studies described in this thesis was to develop a method for the determination of fluorescein and fluorescein glucuronide in the human eye and in plasma, and to calculate the blood-retina barrier permeabilities of the two substances. The necessary methodological improvements included a detailed description of the geometrical optics of the eye and the optical filter properties of the lens. A new method was developed for the determination of the spatial locations of ocular fluorescence measurements and the intrinsic lens fluorescence was used to estimate lens transmittance. The new techniques were applied to clinical studies in patients with diabetic retinopathy. It was shown that in insulin-dependent diabetes mellitus, the apparent rate whereby fluorophores are accumulated in the lens is increased in inverse proportion to the quality of metabolic control, i.e. patients who have had consistently poor control have higher fluorescence than patients who have been in good control. An increase in lens fluorescence was also found in the presence of diabetic nephropathy. The results support the assumption that lens fluorometry can provide a rough estimate of cumulative glycaemia and that glucose is involved in certain age-related changes in the lens.(ABSTRACT TRUNCATED AT 400 WORDS)