Strokes can have a variety of causes, depending on the type of stroke involved.

Ischemic strokes result from a restriction of blood flow to the brain. Blood clots are the most common cause of arterial blockage, but blood vessel blockage can also result from a narrowing of the arteries (stenosis), which often results from a buildup of material in the lining of an artery (atherosclerosis).

Hemorrhagic strokes result from bleeding within or around the brain. This may be caused by the bursting of an artery within the brain. This can occur when a weak spot on an artery wall expands (cerebral aneurysm) or when artery walls begin to lose elasticity, leaving them brittle, thin and prone to cracks.

Various risk factors are associated with strokes. They include:

• History of transient ischemic attacks (TIA). These “mini-strokes” are caused by brief, temporary interruptions in blood flow to the brain. People who experience a TIA are 10 times more likely to have a stroke than their counterparts who do not experience a TIA. A stroke may sometimes occur as soon as two days after a TIA.
  
  
• Advanced age. While strokes can strike at any age, nearly three-quarters of strokes occur in people older than 65, according to the Centers for Disease Control and Prevention (CDC). The risk of a stroke doubles with each decade over the age of 55. Young people are most likely to have strokes resulting from cocaine use, high blood pressure, a traumatic head injury, carotid artery dissection or other causes.
  
  
• Gender. At younger ages, men are more likely to experience and die from a stroke, although overall more women are killed by stroke, perhaps because women tend to live longer. Among older people, the incidence of stroke is roughly equal.
  
  
• High blood pressure (hypertension). Studies have shown that people with hypertension have a risk of stroke that is 4 to 6 times higher than it is for people without hypertension. Hypertension is considered an important risk factor for stroke because it usually presents no symptoms or warning signs. Studies have shown that patients with a systolic/diastolic blood pressure of less than 120/80 mmHg have about half the lifetime risk of stroke compared to people with high blood pressure.
  
  
• Coronary artery disease (CAD) (narrowing of the arteries that supply blood to the heart). There is a strong correlation between CAD and ischemic stroke because both are caused by the same underlying process, atherosclerosis. Atherosclerosis occurs when arteries become obstructed by plaque deposits in the lining of an artery.
  
  
• Atrial fibrillation. This heart rhythm disorder can cause the blood to clot. If these blood clots travel from the heart up into the brain, a stroke may occur. The disorder is associated with 15 percent of all strokes, according to the CDC.
  
  
• Diabetes. People with diabetes have two to four times the risk of stroke compared to people who do not have the illness, according to the CDC. Having diabetes also appears to worsen the outcome of a stroke in many cases.
  
  
• Smoking. Smoking doubles a person’s risk for ischemic stroke, according to the CDC. Smoking promotes atherosclerosis, in which plaque builds up on the walls of arteries. In addition, smoking increases the levels of certain blood clotting factors, such as fibrinogen. Nicotine also raises blood pressure, and the carbon monoxide in cigarette smoke (if inhaled) reduces the amount of oxygen that blood can carry to the brain.
  
  
• Family history of stroke. The risk of stroke increases if a person's parents, grandparents, sisters or brothers have had a stroke.
  
  
• Low levels of HDL (“good”) cholesterol and high levels of LDL (“bad”) cholesterol. Poor levels of good and bad cholesterol can lead to atherosclerosis, which significantly raises the risk of stroke.
  
  
• Obesity. A body mass index (BMI) of 30 or greater (indicating a person is obese) increases the risk of stroke. BMI is usually related to the amount of body fat.
  
  
• Lack of exercise. Not maintaining a moderate level of physical activity can lead to obesity, which increases the risk of stroke.
  
  
• carotid artery dissection. A tear in the inner lining of the carotid artery, creating a space through which blood could leak, causing a stroke.
  
  
• Excessive use of alcohol. This can lead to an increase in blood pressure, which increases the risk of stroke.
  
  
• Illegal drug use. Drugs associated with an increased risk of stroke include cocaine, heroin and amphetamines.
  
  
• Head injury, neck injury. Physical trauma to these areas can damage blood vessels and increase the risk of stroke.
  
  
• Birth control pills and hormone therapy have been linked to an increased risk of stroke. Taking birth control pills increases the risk of stroke, especially for women who smoke and are over the age of 35. Low-dose birth control pills appear to carry a lower risk for stroke. Data from large studies have also demonstrated that hormone replacement therapy, using either estrogen alone or estrogen plus synthetic progesterone, raises the risk of stroke among healthy postmenopausal women.
  
  
• Prior stroke or heart attack.
  
  
• Living in the southeastern United States. For several decades, the southeastern United States has had the highest stroke mortality rate in the nation, according to the CDC. The reason for this increased risk factor remains unknown.
  
  
• Undergoing catheter-based procedures. People who have surgeries that involve placing certain types of catheters (small, flexible tubes) into the heart have a higher risk of blood clot formation.

Other possible contributors to the risk of stroke include:

• Sleep apnea. A condition in which breathing involuntarily stops and starts during sleep. Interruptions in oxygen to the heart and brain can significantly increase a person’s risk of heart disease, high blood pressure and stroke.
  
  
• Atrial flutter. A type of atrial tachycardia, which is an unusually fast heart rhythm that originates in the upper chambers of the heart (atria). Atrial flutter is characterized by a very rapid but regular electrical signal in the atria, which causes a very rapid heartbeat.
  
  
• Depression. Although the exact mechanism is not clearly understood, studies continue to show an association between chronic depressive symptoms and increased risk of stroke.
  
  
• Acute inflammation in the body. Studies have shown that people that test positive on a blood test for certain proteins that indicate acute infection have an increased risk of stroke. The test used is called a C-reactive protein test. A positive result may indicate the presence of certain inflammatory conditions or diseases (e.g., rheumatoid arthritis, lupus).

Although stroke affects all races and ethnicities, African Americans have the greatest risk of stroke. This includes a higher likelihood of stroke at an earlier age and greater risk of overall mortality.

Medical researchers do not completely understand why African Americans have an increased risk of stroke. Some studies have suggested that racism and poverty may play a role. In addition, African Americans are more likely to have medical risk factors (e.g., diabetes, high blood pressure, sickle cell anemia, smoking, obesity) known to be closely associated with stroke.