Choroidal vascular permeability in visually regulated eye growth

The choroidal thickness fluctuates both diurnally and in response to change s in visual input. The fluctuations may represent a physiologic means of al igning the retinal photoreceptors with the focal position of distant images during the emmetropization process. To evaluate the basis for choroidal th ickness changes, we studied the sources of the extravascular fluid in the c hick choroid in two visually-regulated ocular growth conditions: accelerate d ocular growth in goggle-induced form-deprivation myopia and ocular growth retardation in the recovery from myopia after goggle removal. Two week old chicks, controls, myopic and those recovering from myopia, rec eived fluorescein dextran (MW = 140 000) as a tracer. It was given by intra venous injection to identify a potential vascular pathway and by intracamer al injection to identify a potential pathway from the anterior chamber to t he suprachoroidal space. Using a microscopically positioned needle, clear f luid was aspirated from the suprachoroidal space of the enucleated chick ey e; this fluid presumably corresponds to the contents of the lacunae, promin ent lymphatic-like structures of the chick choroid. Plasma, aqueous humor a nd suprachoroidal fluid were sampled 1 hr after injection and assayed for b oth protein content and the tracer dye. Sulfated glycosaminoglycans were as sayed in the suprachoroidal fluid, choroid and sclera under each experiment al condition. In control chicks, aqueous humor and suprachoroidal fluid protein concentra tions were about 0.8 and 9 % of plasma levels respectively. Aqueous humor p rotein concentration was unaltered in myopic or recovering eyes. Suprachoro idal fluid protein concentration in myopic eyes fell dramatically to 1.5% o f plasma levels (P < 0.001). In contrast, recovery from myopia led to a mar ked increase in suprachoroidal fluid protein level to 30% of that in plasma (P < 0.001). None of the procedures affected suprachoroidal fluid protein in the contralateral control eyes. In all three groups of chicks, fluoresce in dextran distribution in the suprachoroidal fluid at 1 hr after intraveno us injection tracked protein levels, with reduced levels in myopic eyes and elevated levels in recovering eyes. After intracameral injection, supracho roidal fluid dextran levels were higher in injected eyes of control chicks (P < 0.01) and in recovering eyes (P < 0.001) but lower in myopic eyes (P < 0.01), compared to the levels in the respective contralateral non-injected eyes in each group. Sulfated glycosaminoglycan levels were at the limits o f detection in the suprachoroidal fluid under all conditions and, on a whol e choroid basis, were unaltered in the choroid in either myopia or recovery . Suprachoroidal fluid is lymph-like in nature and largely derives from plasm a. Sulfated glycosaminoglycan levels do not seem to regulate the fluid cont ent of the choroid in either myopia or recovery. Instead, the changes in pr otein and marker dye levels in myopic and recovering eyes suggest markedly altered choroidal circulatory dynamics and capillary permeability in both c onditions. (C) 2000 Academic Press.