Main posterior watershed zone of the choroid. Variations of its position in normal subjects.

The main posterior watershed zone of the choroid is located between the nasal edge of the optic disc and the fovea and represents the area situated between the territories supplied by the temporal and nasal posterior ciliary arteries. In the fluorescein angiographies of 800 normal subjects a watershed zone was not observed in 33.1% due to technical reasons and in 22.3% due to the simultaneous filling of the peripapillar and macular choriocapillaris. In the remaining 44.6% the watershed zone was well outlined: it was straddling the optic disc in about half of these cases and involved the temporal half of the optic disc and the close choroid in the other half. Very rarely the watershed zone involved the nasal half of the optic disc or the papillo-macular area. The position of the watershed zone could be important to explain the visual field defect in anterior ischemic optic neuropathy and glaucoma.