The brain is in constant need of oxygen and nutrients which are transported via the blood vessels, specifically the arteries. If the blood supply is interrupted for a few seconds, there may be some disruption of brain activity without any permanent damage to the brain tissue. This leads to temporary symptoms that resolves usually within minutes. Longer lasting impairment of blood supply will lead to severe tissue injury and localized areas of cell death that can permanently affect brain function.
A stroke occurs when the blood supply to the brain is affected, either from a blockage of an artery or bleeding in/around the brain, and leads to dysfunction of normal brain activity. This is further explained under lack of oxygen to the brain. The interruption of blood supply leads to death of brain cells in a specific part of the brain (infarct). There are two main types of strokes – an ischemic stroke and a hemorrhagic stroke. An ischemic stroke occurs when an artery is narrowed and eventually obstructed to a significant degree or even completely, usually by a blood clot. A hemorrhagic stroke occurs when a blood vessel in the cranial cavity (part of the skull where the brain is housed) is ruptured and blood leaks out, in and around the brain.
Many people experience a warning sign in the days, weeks or even months before an actual stroke. This occurs because the blood supply is gradually reduced and at times of increased demand, the brain is momentarily starved of oxygen. The warning sign of an impending ischemic stroke is known as a transient ischemic attack. However, it may not be apparent in some people and a high-risk person without the symptoms of a TIA should not be comforted by its absence.
What is a transient ischemic attack?
A transient ischemic attack or TIA is the short-lived disruption of brain function due to a temporary blockage of blood to the brain. The brain tissue is starved of oxygen and there normal functioning of brain cells is affected. There is usually no permanent damage to the brain tissue since there is no death of brain cells. However, it has been postulated that there could be death of some cells, too few to cause a permanent disruption in brain activity. Normal functioning returns once blood flow to the brain is restored. Although various other terms such as mini-stroke and warning stroke are used for this condition, these may be incorrect because there may be no actual brain tissue death as is characteristic of a stroke. Instead the common term pre-stroke, is a better option to describe a transient ischemic attack.
Brain tissue is highly sensitive to changes in blood supply. Although this organ is light in weight, accounting for only 1% to 2% of the body weight, its oxygen and energy demands are significant in comparison to the entire body. The brain requires about 20% of the available oxygen in the body. Oxygen deprivation for even a few seconds is sufficient to cause tissue injury (ischemia). Within minutes of oxygen deprivation, the brain cells of the compromised area begin to die (infarct). Whereas some level of tissue injury may be reversible, cell death is irreversible.
Meaning of a Pre-Stroke / TIA
A transient ischemic attack (TIA) or pre-stroke is a warning sign of an impending stroke. It indicates that blood supply to the brain has been reduced, usually from a narrowing of the artery. The blood supply may be sufficient to sustain normal brain functioning for now. However it can gradually or rapidly progress to the point where the blood supply is inadequate and the brain tissue may die (infarct ~ stroke), sometimes without any further warning.
A person experiencing these attacks needs to be aware that it is only a matter of time before a stroke occurs. There is no way of estimating with any degree of certainty whether this will occur within days, months or years. Seeking immediate medical attention can reduce the chances of a stroke occurring or at the very least, minimize its impact.
Causes of a TIA
The brain derives its blood supply from two main arteries – the carotid artery at the front of the neck and vertebral artery at the back of the neck. The branches of these arteries provide blood to the brain and any narrowing or blockage will affect the brain to varying degrees. The causes and risk factors of a transient ischemic attack are the same as those of a stroke.
- Atherosclerosis is the build up of fatty plaques in the wall of the arteries. The narrowed artery may eventually be blocked when the plaque bursts and a clot forms at the site or a clot from elsewhere gets lodged in the narrowed artery.
- Blood clot formation elsewhere in the body, most notably within the heart, may break away and lodge in the arteries to the brain . This runaway clot is known as an embolus and is more likely to be seen with heart valve disorders and arrhythmias. Sometimes the embolus is not a clot but some other semi-solid or solid material.
- Vasculitis is the inflammation of the wall of a blood vessel and when it specifically affects an artery then it is known as arteritis. Inflammation leads to swelling of the walls and therefore narrowing of the artery. Turbulent blood flow due to the narrowing and/or damage of the inner lining of the artery (endothelium) may encourage clot formation.
- Arterial dissection is a tear in the wall of the artery allows blood to accumulate within it. This can affect the blood flow through the artery and even lead to clot formation.
- Sympathomimetic drugs may cause vasoconstriction (narrowing of the blood vessels) or sudden elevation of the blood pressure (hypertension). It may be found in many prescription drugs, however, its ease of availability and unsupervised use in over-the-counter (OTC) cough and cold medication by high risk patients is of a greater concern. Other sympathomimetic drugs include narcotics like cocaine or ecstasy (MDMA).
- Hypotension (low blood pressure) which arises suddenly may limit the oxygen availability to the blood especially in conditions like anemia (low blood hemoglobin) or where the artery is already narrowed and there is minimal blood supply to sustain the brain.
- Masses may be solid tumors or collections of fluid like blood (hematoma) which are constantly enlarging may compress arteries or impact on the brain itself.
Signs and Symptoms
The clinical features of a transient ischemic attack (TIA) usually lasts for less than 5 minutes but can persist up to an hour. TIA’s may occur several times in a year or in more severe cases, several times in a day. Signs and symptoms include :
- Tingling, numbness or other abnormal sensations in the face, arms or legs, usually on one side.
- Muscle weakness or even paralysis usually on one side.
- Slurred speech
- Confusion – difficulty comprehending others.
- Visual disturbances – double vision, blurred vision or sudden loss of vision – may affect one or both eyes.
- Loss of balance and coordination
- Loss of consciousness (fainting/”blackouts”)
It is important to note that all these symptoms are temporary and will quickly resolves within minutes or at most, in less than an hour. Not all of these symptoms may be present in a TIA – some patients may only experience one or two symptoms at most.