Blood clots that temporarily block blood flow to the brain are the most common cause of transient ischemic attacks (TIAs). Blood clots may develop for a variety of reasons.

• A blood clot can form in an artery that supplies blood to the brain.
  • Blood clots usually form in arteries damaged by plaque buildup, which is a process called atherosclerosis.
  • Long-standing high blood pressure or diabetes may damage smaller blood vessels in the brain, causing a clot to form within the blood vessels and block blood flow. See a picture of how high blood pressure damages arteries.
• A blood clot can form in another part of the body (often the heart) and travel through the bloodstream to an artery that supplies blood to the brain. For example, clots may form:
  • After a heart attack.
  • As a result of other conditions that alter how blood flows through the heart, such as abnormal heart rhythms (especially atrial fibrillation), heart valve problems, patent foramen ovale, atrial septal defects, or heart failure.

In addition, an artery that is partially blocked with plaque can reduce blood flow to the brain and cause symptoms.

Rare causes of blood clots that can cause a TIA include:

• Clumps of bacteria, tumor cells, or air bubbles that move through the bloodstream.
• Conditions that cause blood cells to stick together. For example, having too many red blood cells (polycythemia), abnormal clotting factors, or abnormally shaped red blood cells, such as those caused by sickle cell disease, may cause blood clots to form.
• Inflammation in the blood vessels, which may develop from conditions such as syphilis, tuberculosis, or other inflammatory diseases.
• A head or neck injury that results in damage to blood vessels in the head or neck.
• A tear in the wall of a blood vessel located in the neck.