Thrombus or Embolus
Arteries can be plugged by thrombus or embolus in the lumen
A thrombus is a solid mass of platelets and/or fibrin (and other components of blood) that forms locally in a vessel. Thrombi form when the clotting mechanism is activated. This is supposed to happen when you are injured. However, it can also occur at the site of an ulcerated atherosclerotic plaque or wherever the endothelial cells lining the inner surface of an artery have been damaged.
These are cross-sections of the common carotid artery through its bifurcation into the internal and external carotids and of the smaller internal carotid. You have already identified plaque in the walls that narrows the vessels. Can you find a section in which stringy reddish-brown thrombus further occludes the lumen? Remember that thrombus tends to form on the surfaces of plaques where the lining of the vessel has been damaged.
This patient suffered sudden blockage of his right internal carotid artery and survived for several years although he was extremely disabled by paralysis of his left lower face, arm, and leg, loss of sensation on his left side, vision problems, and left-sided neglect. Can you find the infarcted brain tissue?
Thrombus formation also occurs in places where blood flow is sluggish, enabling clotting factors to accumulate and giving platelets more opportunity to stick together. Disorders of blood cells (for instance sickle cell disease) or blood proteins can increase the chance of thrombus formation and therefore contribute to the risk of ischemic stroke.
An embolus is most often a piece of a thrombus that has broken free and is carried toward the brain by the bloodstream. The term thromboembolus is used a lot because it turns out that most emboli arise from thrombi. However, bits of plaque, fat, air bubbles, and other material also qualify as emboli. Presumably an embolus floats along with the flowing blood until it encounters a narrowing in an artery through which it cannot pass. When the embolus gets stuck, it blocks the artery. This reduces blood flow to downstream tissues and causes them to become ischemic.
Three important sources of emboli to the brain
- Thrombus that initially formed within the diseased heart
For example, the irregular, ineffective contractions of the heart muscle seen in atrial fibrillation lead to blood pooling in the left atrium and increased thrombus formation. Bits of these thrombi enter the systemic circulation as emboli that can travel to any organ, including the brain. - Thrombus that forms on a heart valve
This occurs more frequently in the left heart, because the mitral and aortic valves are slammed shut by higher pressures. When the endothelium that usually covers a valve is damaged, it exposes the underlying tissue which is highly thrombogenic. A thrombus on a heart valve is sometimes called a vegetation. This is because it often looks like a branching bush that is attached to the valve by its main stem. - Thrombus associated with atherosclerotic plaque formed in extra- or intracranial parts of a carotid or vertebral artery
A bit of the thrombus breaks off and is carried more distally in the same vessel by the flowing blood (this is sometimes called artery-to-artery embolism).
Regardless of its source, an embolus does its damage by getting stuck in a large artery or branch and blocking blood flow beyond that point.
The diameters and branching patterns of the large arteries seem to have a lot to do with where embolic material tends to travel and where it tends to ultimately lodge. For instance, the large diameter, gently curving course, and rapid blood flow in the middle cerebral artery put it at particular risk for embolism--and therefore the regions of brain that it supplies at risk for embolic stroke. The smaller anterior cerebral artery, which originates from the internal carotid at a sharper angle, captures emboli less often--emboli apparently don't corner well!