TIAs - Causes

What Causes a Transient Ischemic Attack?

A transient ischemic attack (also known as a TIA or "mini-stroke") is a sudden episode that is caused by a temporary lack of oxygen- or nutrient-rich blood to a part of the brain. This lack of blood supply occurs because a blood vessel becomes severely narrowed or blocked.

There are several reasons why a blood vessel in the brain may become severely narrowed or blocked. The most common causes of a transient ischemic attack involve either an embolism or thrombosis. An embolism develops when a blood clot or other tissue from another part of the body (such as the heart) moves through the blood into the neck or brain.

Thrombosis occurs when a blood clot (known as a thrombus) forms within a blood vessel of the brain or neck. Unlike an embolism, with thrombosis the blood clot does not break free.

A third way in which a blood vessel can become narrowed or blocked involves stenosis (a severe narrowing of an artery), which may occur in an artery in or leading to the brain. In most cases, stenosis is the result of a plaque buildup. Such narrowing may eventually completely block the artery.

These possible causes of a transient ischemic attack are the same as those for an ischemic stroke (see Stroke Causes). The difference between a TIA and a stroke is one of duration. By definition, a stroke produces symptoms that last for at least 24 hours. A TIA causes symptoms that improve after a shorter period of time (usually within 30 minutes).

Embolism

A blood clot or other tissue in the blood (such as fat) from a part of the body other than the brain can travel through blood vessels and become wedged in a smaller brain artery. This free-roaming clot or tissue is called an embolus (emboli is plural). Emboli often form in the heart. They also commonly form in the neck arteries or within the aorta.

There are several conditions that increase a person's chances for developing an embolus. One of the most common is in people with atrial fibrillation, sick sinus syndrome or other irregular heart rhythms. These conditions can cause poor blood flow which allows harmful clots to form. Emboli are also more likely to form in people with:

• Heart valve diseases (such as endocarditis, mitral valve prolapse, and rheumatic heart disease)
• A history of a recent heart attack
• Sickle cell disease
• Systemic lupus erythematosus (lupus or SLE for short)
• Other conditions that can cause inflammation of blood vessels, such as temporal arteritis, syphilis, or HIV/AIDS.

If the body is able to dissolve an embolus, allowing normal blood flow again, then the person will only suffer a TIA. If the body is not able to dissolve the embolus before brain tissue is damaged, then the person will suffer a stroke.

Thrombosis

A blood clot can also form in one of the brain arteries (called cerebral arteries) but instead of breaking free remains attached to the artery wall until it grows large enough to block blood flow. This type of clot is known as a thrombus.

Most of the time, a thrombus occurs within an area of the brain damaged by atherosclerosis. In atherosclerosis (also known as hardening of the arteries), deposits of plaque (a mixture of fatty substances, including cholesterol and other lipids) build up along the inner walls of large and medium-sized arteries, causing thickening, hardening, and loss of elasticity of artery walls and decreased blood flow. This increases the risk that a thrombus will form, causing severe narrowing or complete blockage of the affected blood vessel.

If the body is able to dissolve a thrombus that severely narrows or blocks an artery, allowing normal blood flow again, then the person will only suffer TIA symptoms. If the body is not able to dissolve the clot before brain tissue is damaged, then the person will have a stroke.

Stenosis

A third possible transient ischemic attack cause is a type of artery narrowing known as stenosis. This narrowing most often results from the buildup of plaque from atherosclerosis. Over time, this narrowing can completely block a brain artery.

Note:
embolism: 栓塞
thrombosis: 血栓形成
thrombus: 血栓
stenosis: 【醫學】(器官)狹窄
plaque: 【醫學】斑
atrial fibrillation: 心房纖顫 (Atrial fibrillation is a type of abnormal heart rhythm -- also known as an arrhythmia. )
endocarditis: 【醫學】心內膜炎
rheumatic: 風濕性
lupus: 紅斑狼瘡
Atherosclerosis: 【醫學】動脈粥樣硬化 (Atherosclerosis is a slow, progressive disease that causes hardening and narrowing of medium to large blood vessels, such as the aorta and the coronary arteries.)