Internal carotid agenesis
occurs when one of the blood vessels which supply blood to the brain does not develop (agenesis). It is rare, occurring in less than 0.01% of people. Usually there is a pair of internal carotid arteries, one on the left side and one on the right side; agenesis occurs on the left side three times more frequently than on the right side. Individuals with this condition may not have any symptoms because there are several blood vessels transporting blood to the brain such that if one does not develop, the others can function in its place. However, if symptoms occur, they may include headache, blurred vision, paralysis of some of the nerves in the head (palsy), epilepsy, or muscle weakness on one side of the body (hemiparesis). The main concern with internal carotid agenesis is the increased risk for enlargement of the other blood vessels (aneurysm), which can occur in up to 67% of people with this condition (compared to 2-4% of individuals without this condition). Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.