The basic reason a brain stroke attack, whether ischemic or hemorrhagic, occurs is due to loss of blood supply to a part of the brain. There are various causes for this interruption of blood supply and they relate to the type of stroke.
Due to the interruption of blood supply, the neurons (brain cells) are deprived of fresh blood and the oxygen and the nutrients it carries to them.
If blood supply is not restored within a few minutes, the brain cells begin to die and that part of the brain loses its function. This process is irreversible. The brain cells can survive only for 3 to 4 minutes without oxygen. After that, they begin to die.
Stroke, today, is the leading cause of disability worldwide. And, the disability could be of any type from physical to cognitive.
According to The National Institute of Neurological Disorders and Stroke, controlling the risk factors for stroke during the middle age, particularly those relating to high blood pressure may reduce the decline of cognitive function later in life.
What is an ischemic stroke?
A stroke (interruption of blood supply) may be caused by the blockage of an artery or its branches. This type of stroke is called an ischemic stroke. It is most common and accounts for 87 percent of all cases.
What is a hemorrhagic stroke?
Stroke may also be caused by the bursting open of an artery or its branch that causes the blood to spill out, thereby depriving the concerned brain cells of blood. This type of stroke is called a hemorrhagic stroke. It accounts for 13 percent of stroke cases.
Both the above types of stroke cause permanent damage to the part of the brain involved.
Transient ischemic attack (TIA)
There is a third type of stroke, which is transient or temporary in nature. It is called a transient ischemic attack (TIA) or a mini-stroke and is caused by the blockage of the artery by a clot.
Here, the disruption of blood supply takes place only for a brief period, about a minute or so, and is resumed almost immediately, not allowing the brain cells to die. The symptoms of TIA, however, can persist for hours. In this type of stroke, there is no permanent damage to the brain.
Strokes, therefore, are primarily of two types
1. Embolic strokes, which are again of two kinds:
- Thrombotic strokes
- Embolic strokes
2. Hemorrhagic strokes, which are again classified into two types:
- Cerebral hemorrhagic strokes
- Subarachnoid hemorrhagic strokes
Each of these strokes has its own causes and we shall discuss the causes of each of these strokes. However, before venturing further, you should know the risk factors that make you more prone to developing a stroke.
Causes of mild and massive strokes
The general causes described below can give you a mild stroke or a massive stroke. The causes are common for both types, mild and severe strokes. The difference lies in the severity of the cause.
In a mild stroke, the clot blocking the artery is small and does not lead to a complete occlusion of the arterial lumen. Or, it could affect a small arteriole where the area of the brain affected is very small.
Similarly, the hemorrhage could be from a small arteriole causing very little damage to the brain tissue. Usually, in a mild or a mini-stroke the damage is reversible because the cause of the stroke corrects itself before causing any permanent liability.
In a massive stroke, the clot fully occludes a big artery or the hemorrhage is massive where the amount of brain tissue damaged is over a larger area and for a longer period leading to permanent damage to the brain tissue and cells.
Causes of ischemic stroke
Ischemia means an inadequate blood supply and as the name suggests, this type of stroke is caused by an inadequate supply of blood to a part of the brain.
This is the most common type of stroke acco0unting in 87% of all the stroke cases.
Ischemia occurs in the brain due to the blockage of an artery, which results in severe reduction of the blood supply.
The most common causes of ischemic stroke are heart conditions such as atrial fibrillation and atherosclerosis, where clots form in the heart travel to the arteries blocking blood flow to the brain.
According to the type of blockage, ischemic stroke is classified into two types: thrombotic and embolic stroke.
Thrombotic stroke causes
Thrombotic stroke occurs when a thrombus (blood clot) forms in an artery of the brain and occludes it. This clot is usually formed by fat deposits (cholesterol plaques) on the inner wall of the artery. This is called atherosclerosis.
Atherosclerosis narrows the lumen of the artery reducing blood flow. Over time, as the plaque grows, it can completely occlude the artery.
Due to the narrowing of the artery, blood flow slows down and can cause collection of the blood cells, which then form a thrombus. This thrombus blocks the artery lumen disrupting blood supply and can cause a stroke.
This type of stroke is generally seen in older persons, particularly those who have developed high cholesterol or have diabetes.
Other causes that can form a thrombotic stroke are:
- Traumatic injuries to the head that can result in clot formation
- Bleeding or coagulation disorders related to blood clot formation
- Use of certain drugs and substance abuse. This is the cause most common among the young adults. Such drugs include opiates, cocaine, amphetamine, hallucinogens like LSD and phencyclidine, marijuana, barbiturates and such other sedatives, alcohol, and tobacco.
- Certain types of headaches like migraines can cause spasm of an artery, slowing down the blood flow that can result in the formation of a clot and subsequently a stroke.
Cardiac causes of embolic stroke
An embolic stroke occurs when a blood clot formed elsewhere in the body breaks free and travels through the bloodstream and gets lodged in an artery or its smaller branches in the brain, disrupting blood supply and causing a stroke. This type of clot that has traveled from elsewhere is called an embolus.
Embolic strokes usually occur from emboli that result from heart disease or after a heart surgery. Another cause seen in about 15 percent of people with embolic strokes is atrial fibrillation.
Atrial fibrillation is an abnormal, rapid and irregular beating rhythm of the upper chambers of the heart. Due to this type of beating of the heart, some blood pools in the heart, which increases the risk of clot formation in the chambers of the heart. These clots can travel through the arterial network to the brain.
Causes of hemorrhagic or bleeding stroke
Hemorrhagic or bleeding strokes are less common than ischemic strokes. They account for 13 percent of all strokes, but for 40 percent of all deaths due to stroke.
The main cause of a hemorrhagic stroke is the rupture of a weakened artery causing blood to spill into the neighboring brain tissue. This deprives the distal part of the brain of blood supply, oxygen, and nutrition.
Hypertension is the most common cause of a hemorrhagic stroke. This occurs due to the chronic damage to the wall of the blood vessels due to high blood pressure. Seventy-seven percent of Americans treated for a hemorrhagic stroke have high blood pressure.
The artery can also weaken and rupture due to two other reasons: aneurysms and arteriovenous malformations.
1. An aneurysm of an artery, which is the ballooning of the weakened section of the arterial wall, which over time, if left untreated, ruptures and bleeds.
2. Arteriovenous malformation refers to a cluster of abnormally formed blood vessels. Any of the blood vessels can rupture and bleed.
The blood that spills accumulates and increases the intracranial pressure. This presses on the adjoining brain tissue damaging the brain cells of the compressed area of the brain.
Hemorrhagic stroke is of two types: intracerebral (inside the brain) hemorrhage and subarachnoid hemorrhage. Both are critical emergencies and can cause coma and death.
Why you get an intracerebral hemorrhagic stroke?
If the amount of leaked out blood increases rapidly, the sudden increase in pressure can lead to unconsciousness or death. Intracerebral hemorrhage usually occurs in certain selected parts of the brain, which include the basal ganglia, cerebellum, brain stem, or cortex.
Intracerebral hemorrhagic stroke is the most common type of hemorrhagic stroke in which the bleeding occurs inside or within the brain. It can occur in persons of all age groups.
Risk factors for intracerebral stroke include:
- Hypertension (most common cause)
- Cerebral cavernous malfunctions – when blood vessels do not develop correctly in the brain
- Arteriovenous malformations (AVMs) – a genetic condition where blood vessels develop malformation leading to an abnormally tangled web.
Causes of subarachnoid hemorrhagic stroke
In this type of stroke, the bleeding occurs outside the brain in the subarachnoid space – the area between the brain and the membranes that cover it. This is the area where the cerebrospinal fluid circulates.
It is known to cause a headache of intense severity, probably the worst headache you can ever get.
Sudden buildup of intracranial pressure can cause loss of consciousness and even death.
Causes of subarachnoid hemorrhage stroke include:
- Arteriovenous malformations
- Bleeding disorders
- Head injury
- Medication such as blood thinners
- Cerebral aneurysms are often seen in the arteries at the base of the brain. An aneurysm is a rounded or irregular swelling in the arteries, which causes the arterial wall to become weak and more prone to rupture.
Subarachnoid hemorrhage is seen in all age groups including teenagers and young adults. However, it is more common in women than in men.