Watershed Zone in the Human Peripheral Choroid

Purpose: To evaluate the angioarchitecture and the hemodynamics in the peripheral choroid in humans. Methods: Choroidal circulation was examined in 23 normal eyes and in 7 with equatorial lattice degeneration by wide-angle indocyanine green (ICG) angiography using a scanning laser ophthalmoscope. A hand-held 30-diopter lens was placed in front of the eye to expand the observation angle from the original 40° to 70°. The peripheral temporal Choroid and ora serrata thus could be seen in one frame. Results: An arterial watershed zone was identified in the temporal peripheral choroid in 21 of 30 eyes. This watershed zone was located in a sectorial region posterior to the ora serrata. It did not form a circular belt along the equator. The area anterior to the watershed zone was supplied by peripheral retrograde choroidal arteries. Inflow of dye into the peripheral retrograde choroidal arteries was consistently later than into the more posterior choroid via short posterior ciliary arteries. The terminal branches of peripheral and short posterior ciliary arteries did not form functional anastomoses. Peripheral retrograde choroidal arteries showed wide variations in number and distribution. These arteries originated from recurrent branches of long posterior ciliary arteries in 18 of 21 eyes and from anterior ciliary arteries in 3 eyes. There was no correlation between the watershed zone and equatorial lattice degeneration in location and extent. Conclusion: A peripheral watershed zone is a regular feature in the far temporal sector of the choroid. The far peripheral choroid anterior to the watershed zone was perfused by peripheral retrograde choroidal arteries which originated from the long posterior ciliary arteries and/or the anterior ciliary arteries.