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. In some cases, surgery may be necessary to treat symptoms caused by internal carotid agenesis.

Some people with internal carotid agenesis may have other malformations involving the blood vessels, face, or ears. This typically occurs on the same side of the body as the internal carotid agenesis. People who have internal carotid agenesis are at an increased risk to develop an enlargement of other blood vessels in the brain (aneurysm). The risk for people with internal carotid agenesis to develop a brain aneurysm is estimated at 24-34%.

The cause of internal carotid agenesis is currently unknown. The malformation is present from birth and is believed to be caused when something happens early during the development of the baby that stops the carotid artery from forming correctly. Most of the time, the malformation is thought to occur by chance. However, less commonly, a person with internal carotid agenesis may also have other diseases or syndromes. People with internal carotid agenesis caused by other syndromes typically have other symptoms or medical problems.

Internal carotid agenesis is known to be associated with a few other diseases or syndromes. If an individual has internal carotid agenesis as a sign of another disease or syndrome, then it is possible that the malformation was inherited and that it could be passed on to future generations.

Internal carotid agenesis is diagnosed when one or both of the internal carotid arteries are found to be missing by specific imaging tests. These imaging tests may include angiography, computed tomography scanning (CT scan), and magnetic resonance imaging (MRI). These tests are used to visualize the blood vessels and the surrounding bones. The diagnosis may be confirmed with another imaging test called magnetic resonance angiogram (MRA) that looks specifically at the blood vessels. If people who have internal carotid agenesis do not have symptoms of the malformation, it is typically diagnosed by accident (or incidentally) when evaluating for other health concerns.

Due to the increased risk of aneurysms, many doctors recommend that a person with internal carotid agenesis be screened for the development of aneurysms. However, the condition is rare, it is not clear how often such screening should take place and if screening is actually helpful. If a brain aneurysm is observed, further monitoring or surgery may be necessary.

The long-term outlook for people affected by internal carotid agenesis is typically good. Although the malformation may be associated with complications such as brain aneurysm, there are imaging techniques and surgical procedures that can be used to identify and treat people with aneurysms. People with this malformation may be more likely to experience complications during certain types of surgery, so it is important for a person with internal carotid agenesis to tell all of their doctors, especially surgeons, about the diagnosis. In some cases, internal carotid agenesis may be associated with other diseases or syndromes. In these cases, the long-term outlook may depend on the associated syndrome or disease.