Internal carotid agenesis occurs when one or both of the blood vessels that supply blood to the brain (internal carotid arteries) do not develop (agenesis). The missing carotid artery can be on either side of the body. People missing one or both of the internal carotid arteries may not have any symptoms, because the body develops other blood vessel pathways to carry blood to the head. Some people do have symptoms including headaches, blurred vision, paralysis of some of the nerves in the head (palsy), recurrent seizures (epilepsy), or muscle weakness on one side of the body (hemiparesis). People who have Internal carotid agenesis have an increased risk for enlargement of the other blood vessels (aneurysm) in the brain. The exact cause of Internal carotid agenesis is not known. Diagnosis of Internal carotid agenesis often occurs accidentally when a person is having a brain MRI or CT scan. The diagnosis can be confirmed with magnetic resonance angiography (MRA). Due to a higher risk of brain aneurysms seen in people who have Internal carotid agenesis, screening for aneurysms may be recommended.