Vitreous fluorophotometry: mathematical analysis of the effect of peripheral leakage on axial scans

Current instrumentation in vitreous fluorophotometry allows the determination of a fluorescein concentration profile along the optical axis of the eye. Based on an assumption of a uniform blood-retinal barrier permeability, several methods have been used for the determination of the permeability from an axial scan. The assumption of a uniform permeability is not realistic and it has been unknown to what extent the calculated common permeability reflects the local permeability in different areas of the retina. Using a mathematical model for a nonuniform permeability, we have investigated the effect of localized leakage on the axial concentrations and thereby on the calculated common permeability under the assumption of free diffusion in the vitreous body. It turns out that leakage outside the large temporal vessels has to be extremely strong to have a noticeable impact on 60 min axial scans. A 100-fold increase of the permeability in the region more than 30 degrees (central angle) from the optical center of the eye leads to just a 2-fold increase of the apparent common permeability. Thus, axial vitreous fluorophotometry almost exclusively measures the condition of the retina in the vicinity of the optical center.