Retinal vascular permeability determined by dual-tracer fluorescence angiography

Diabetic retinopathy is the leading cause of blindness in working age individuals in the United States. Breakdown of the blood-retinal barrier is one of the earliest events in the progression of diabetic retinopathy. Ideally, therapeutic measures would be directed at this early stage, but there are few sensitive, quantitative methods to assess the retinal vascular barrier in vivo. We present here a method that combines fluorescence microangiography and the simultaneous use of two fluorescent tracers to quantitatively assess the retinal vascular barrier. PS/F (permeability x surface area/flow) describing the retinal vasculature of Long Evans rats was found to be 0.086+/-0.031 (n=13, avg.+/-s.d.). Based on estimates of flow and surface area, we estimate the permeability of sodium fluorescein to be approximately 1.2 x 10(-5) cm/s. Infusion of a hyperosmolar solution of 1.6 M mannitol for 5 min significantly increased PS/F in individual veins and significantly increased a flow weighted PS/F from 0.073+/-0.028 to 0.16+/-0.034 (n=3). In conclusion, we have adapted indicator dilution techniques to quantitatively assess the retinal vasculature in vivo. We have found dual-tracer fluorescence angiography to be a sensitive indicator of increases in the blood-retinal barrier produced by hyperosmolar mannitol. This methodology is a promising new minimally invasive strategy which may be adapted to quantitatively track retinal vascular permeability.