Diabetic macular edema: passive and active transport of fluorescein through the blood-retina barrier

PURPOSE: To investigate the passive bidirectional and active outward transport of fluorescein through the blood-retina barrier (BRB) in diabetic patients with clinically significant macular edema and in healthy controls.

METHODS: The passive and active transport of fluorescein through the BRB was quantitated by vitreous fluorometry. A previously developed method was used to model passive transport. A new simulation model was developed and evaluated for estimation of active transport. The study included 10 eyes of 5 healthy controls and 31 eyes of 20 diabetic patients with clinically significant diabetic macular edema (CSME) in at least one eye, totalling 25 eyes with CSME.

RESULTS: Passive permeability of fluorescein was increased by a factor of 12 in eyes with edema compared to healthy controls (edema, 23.7 nm/sec; healthy subjects, 1.9 nm/sec, P < 0.01), whereas the active transport was doubled (edema, 84.1 nm/sec; healthy subjects, 43.5 nm/sec, P < 0.01). Unlike active transport, passive permeability was related to the degree of retinopathy, in that eyes with severe non-proliferative diabetic retinopathy had a passive permeability that was significantly increased compared to moderate retinopathy (32.1 nm/sec and 14.6 nm/sec, respectively, P: < 0.05). The passive movement quantitated with vitreous fluorometry was larger for diffuse and mixed leakage compared to focal (P = 0.07).

CONCLUSIONS: Insofar as the movement of fluorescein can be taken as a probe for the movement of electrolytes and water, the pathogenesis of diabetic macular edema seems to involve a disruption of the BRB, presumably its inner component. The active resorptive functions of the blood-retina barrier appear to be compensatorily increased to counteract edema formation, although the increase is too small to prevent edema in the face of severe leakage through the blood-retina barrier.