What causes stroke?
Brain cells are incredibly delicate and require a steady supply of oxygen. Even brief interruptions in the delivery of oxygen can cause injury.
Strokes are caused by three main mechanisms:
- The blood supply to the brain may be blocked. This may be caused by tiny clots (emboli or thrombi) plugging the blood vessels of the brain. Or the vessels themselves may be narrowed, reducing the amount of blood delivered to the brain (as is seen in Moyamoya Syndrome, for example).
- The blood may not have enough oxygen in it to begin with. For example, cases when someone can't breathe for long periods of time or in the setting of carbon monoxide poisoning. Or the blood may not be circulating fast enough to bring fresh oxygen to the brain. For example as might occur if the heart is not beating appropriately.
- The brain itself might be under pressure. For example as occurs in brain swelling after trauma or when there is bleeding around the brain. Or there may be bleeding within the substance of the brain itself, which directly injures the brain tissue and also makes it harder for blood with oxygen to be delivered to it. This increased pressure often prevents oxygenated blood from the heart from entering the confines of the skull.
In all of these cases, the injury to the brain tissue and the lack of oxygen cause some of the brain cells to die, resulting in a stroke.
What are some conditions that are associated with strokes?
The following genetic conditions may lead to stroke:
- Deficiencies of the anticoagulant (blood clot preventing) proteins C and S
- Deficiency of antithrombin-III, another anticoagulant
- A mutation in the blood clotting factor prothrombin G20210A
- Sickle cell disease, in which red blood cells are abnormally shaped
- Activated protein C resistance, where the body resists the anticoagulant protein C
- Dyslipoproteinemias, abnormal concentrations or abnormal lipoproteins (cholesterol molecules) in the blood
- Homocystinuria, where the body is unable to process amino acids properly
- Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), a condition that affects the brain and nervous system and is a result of defect in the mitochondrial genome (inherited exclusively from the mother's side).
- Fabry disease, which leads to an inability to metabolize fat
- Menkes disease, which interferes with copper metabolism
- Tangier disease, a severe reduction in the body's good cholesterol
Embolic strokes are usually caused by an embolus - a blood clot that forms elsewhere in the body and travels to the brain. Embolic strokes may be associated with the following conditions:
- Congenital heart disease - refers to heart defects acquired at birth
- Bacterial endocarditis - an infection of the heart's inner lining or valves
Thromboticstroke are caused by a thrombus, or blood clot, that develops in the arteries that supply blood to the brain. Thrombotic strokes may be associated with the following conditions:
- Factor V Leiden mutation, which leads to abnormal blood clotting
- Antiphospholipid antibodies, which leads to blood clots in both arteries and veins
- Hyperhomocysteinemia, which refers to an abnormally high level of the amino acid homocysteine in the blood
- Elevated lipoprotein (a), a type of cholesterol
Or acquired, such as deficiencies in clotting pathways resulting from:
- Hepatic (liver) or renal (kidney) disease.
Vasculitis, which refers to blood vessel inflammation, may be caused by infectious or noninfectious conditions.
Infections causes include:
- Sepsis, which refers to when the bloodstream becomes overwhelmed with bacteria, of any type (particularly meningitis)
- Varicella (chickenpox)
- Human immunodeficiency virus (HIV)
- Mycoplasma, a type of pneumonia
Noninfectious causes encompass a wide variety of autoimmune disorders, including:
- Behcet disease, which causes chronic inflammation of blood vessels throughout the body
- Sarcoidosis, which causes inflammation I the lymph nodes, lungs, liver, eyes, skin or other tissues
- Sjögren syndrome, an immune system disorder
- Ulcerative colitis, a form of inflammatory bowel disease
- Kawasaki disease, caused by inflammation in the walls of small and medium sized arteries throughout the body
- Henoch-Schonlein Purpura (HSP), a group of disorders that cause blood vessel inflammation
Stokes may also be caused by
- Illicit drug use
- Fibromuscular dysplasia, an abnormal cluster of cells growing in the artery wall
- Arteriovenous Malformations (AVMs), an abnormal cluster of blood vessels
- Arteriovenous Fistulas (AVFs), an abnormal connection between an artery and vein
- Vein of Galen Malformations (VOGMs), a blood vessel malformation in the brain
- Aneurysms, a bulge or ballooning in an artery wall
- Moyamoya Syndrome, a disease in which the brain arteries are constricted
- Venous thrombosis, a blood clot that forms within a vein
- Arterial ischemic stroke, blockage of blood flow in an artery
- Arterial dissection (vertebral dissection, carotid dissection), a tear in the artery lining
What are the symptoms of a stroke?
Stroke symptoms vary widely depending on which part of the brain has been injured. Some parts of the brain can suffer strokes with little or no recognizable symptoms, referred to as "silent" strokes. Other strokes, even very small ones, can cause devastating symptoms, such as paralysis, blindness or even death, if they occur in sensitive areas of the brain.