Stroke

A stroke, also known as a cerebrovascular accident, occurs when blood flow to the brain is disrupted, resulting in tissue ischemia and death. A stroke can be ischemic or hemorrhagic. An ischemic stroke is when there’s an occlusion in an artery in the brain; while a hemorrhagic stroke is when an artery in the brain ruptures and causes blood to leak into the brain tissue.

Now, the brain is the communication and control center of the body, and to maintain neurologic function, all parts of the brain require a steady and consistent flow of oxygen- and nutrient-rich blood. The brain is supplied with blood through the internal carotid arteries and the vertebral arteries. The internal carotid arteries branch off into the middle cerebral arteries and the anterior cerebral arteries, while the vertebral arteries come together to form the basilar artery. The basilar artery then leads into the posterior cerebral arteries. The posterior cerebral arteries, along with the anterior cerebral arteries, internal carotids, and other smaller branched arteries form the circle of Willis. The circle of Willis is a group of arteries arranged in a circle that allows for collateral blood flow, meaning blood can circulate from one side of the brain to the other if a blockage occurs.

Okay, so with ischemic strokes, blood flow is disrupted by obstruction from a thrombus or embolus. A thrombus is a clot that forms within a vessel, whereas an embolus is when a clot breaks off from one area and travels downstream, lodging in a smaller vessel.

For example, a thrombus can occur from atherosclerotic plaque, where fats build up within a vessel supplying blood to the brain; and an embolus can occur from a blood clot in the atria of the heart due to atrial fibrillation that makes its way into cerebral circulation.

Risk factors for ischemic strokes include hypertension, hyperlipidemia, increased age, heart disease, and sleep apnea. There is also an increased risk with smoking, oral contraceptive use, and previous transient ischemic attacks, or TIAs, also called mini strokes, where blockage is temporary, usually lasting minutes to hours, and doesn’t cause long-term brain damage.

On the other hand, with a hemorrhagic stroke, blood flow is disrupted due to a rupture of a cerebral artery. This can occur due to conditions, such as hypertension, that cause weakening of the artery wall making it more prone to rupture.

Risk factors include advanced age, obesity, and the use of tobacco, alcohol, or other illicit drugs, like cocaine. Additionally, coagulation disorders such as disseminated intravascular coagulation; brain tumors; and other brain abnormalities like aneurysms can increase risk.

Alright, now when an ischemic stroke occurs, blood flow may be gradually or rapidly obstructed. For example, gradual obstruction of blood flow can occur as plaque slowly builds up in an artery; whereas, if an embolus lodges in a small artery, it may cause an abrupt obstruction. Now, the brain will usually be able to compensate for the reduced blood flow from one vessel, by increasing blood flow to other vessels or through angiogenesis, a process where collateral blood vessels are generated to go around the obstruction.

However, as blood flow decreases, but there’s an increased demand for oxygen- and nutrient-rich blood, like during exercise, TIAs may occur. This causes stroke-like symptoms that resolve once demands aren’t as high.