The carotid arteries provide blood supply to the head. There are two common carotid arteries, located on each side of the neck, that divide into the internal and external carotid arteries. The external carotid artery provides blood supply to the scalp, face, and neck while the internal carotid artery supplies blood to the brain.

Narrowing of the internal carotid artery may decrease blood supply to half of the brain that it supplies. This narrowing called atherosclerosis (atheroma = lump of plaque + sclerosis = hardening) occurs because of the accumulation of plaque on the inside of the artery wall. Plaque begins as a soft, waxy collection of cholesterol and triglycerides but over time begins to harden and calcify. Often it occurs where increased blood pressure and blood turbulence irritates the inner lining of the artery and this often occurs where the internal and external carotid arteries split (bifurcation).

As the carotid artery begins to further narrow, the pressure within the artery continues to increase. This may cause the plaque to rupture, causing blood clots to form. A large clot may completely block (occlude) the artery or smaller clots and bits of plaque may travel into smaller arteries within the brain and cause disruption of blood supply to parts of brain tissue.

It is only when a stroke or transient ischemic attack occurs, or a bruit (blowing or whistling sound) is found on physical examination by your doctor, that the diagnosis of carotid artery disease is usually made. Otherwise, the narrowing carotid artery does not cause symptoms.