Arterial anastomosis with Schlemm's canal: a rare cause of secondary open-angle glaucoma.

In four patients with open-angle glaucoma, blood was present in the region of Schlemm's canal. The blood had four characteristics that distinguished it from other more common causes of blood in Schlemm's canal: it was observed with a Zeiss goniolens, a lens that has no flange, and thus was not an artifact of the examination technique; the blood could not be expelled from Schlemm's canal by raising IOP with para-limbal suction unless the pressure in the eye exceeded the diastolic ophthalmic artery pressure level; as IOP approached this high level, the amount of blood in Schlemm's canal progressively decreased, eventually diminishing to a single point of reflux that was always in the same location; and at this point the blood pulsed in and out of Schlemm's canal with each beat of the heart. These findings suggest a pathological anastomosis between a deep limbal artery and Schlemm's canal. The high arterial pressure transmitted into Schlemm's canal could, in turn, lead to an elevation of IOP.