Introduction

• The arterial walls slowly thicken, harden, and narrow until blood flow is reduced, a condition known as stenosis.
• These now abnormal arteries become vulnerable to injury. Such injuries signal the immune system to release white blood cells (particularly those called neutrophils and macrophages) at the site. This process is the first step in the inflammatory response, which may play a significant role in the stroke.
• Macrophages literally "eat" foreign debris and become foamy cells that attach to smooth muscle cells of blood vessels causing them to build up.
• The immune system, sensing further harm, releases other factors called cytokines, which attract more white blood cells and perpetuate the whole cycle.

As these processes continue, blood flow slows. In addition, other events contribute to the coming stroke:

• The injured inner walls fail to produce enough nitric oxide, a substance critical for maintaining blood vessel elasticity. The arteries become calcified and lose elasticity.
• The arteries, now hardened and rigid, become susceptible to tearing. In this event, the thrombus (blood clot) forms.
• The blood clot then blocks the already narrowed artery and shuts off oxygen to part of the brain. A stroke occurs.

Embolic Strokes and Atrial Fibrillation. An embolic stroke is usually caused by a dislodged blood clot that has traveled through the blood vessels (an embolus) until it becomes wedged in an artery. Embolic strokes account for about 25% of all strokes and may be due to various conditions:

• In about 15% of embolic strokes, the blood clots originally form as a result of a rhythm disorder known as atrial fibrillation. This abnormal rhythm is a rapid quivering beat in the upper chambers of the heart (the atria). Because of the irregular pumping, some blood may remain in the heart chamber where it forms clots, which can then break off and travel to the brain as emboli.
• Emboli can originate from blood clots that form at the site of artificial heart valves or as a result of heart valve disorders.
• Emboli can also occur after a heart attack or in association with heart failure.
• Rarely, emboli are formed from fat particles, tumor cells, or air bubbles that travel through the blood stream.