Ocular and cerebral ischemic mechanisms in disease of the internal carotid artery.

Stenosis of the internal carotid artery reduces the flow velocity in the ophthalmic artery. Lowered velocity permits increased red cell aggregation and decreased red cell deformability which increases viscosity. Contrary to the theory of remotely originating emboli, this is an alternate hypothesis regarding transient attacks of ocular and cerebral ischemia. The ophthalmic artery circulation time was measured in two groups of patients. The circulation time was defined as the interval between the appearance of contrast media in the siphon of the internal carotid artery and in the ocular choroid. The measurement was made on 151 angiograms of 108 subjects. These vessels were normal. An additional 76 patients had 108 angiograms which showed various amounts of internal carotid artery stenosis. These 76 patients had transient ischemic attacks; retinal, cerebral, or both. There is a significant difference in the ophthalmic artery circulation time in the two groups. The slowing in the ophthalmic artery is related to the degree of internal carotid artery narrowing. The circulation time in a cerebral branch of the internal carotid was not measured. It is presumed that stenosis of the internal carotid artery would have the same effect on a cerebral artery as on the ophthalmic artery.