A stroke occurs when the blood flow to an area of the brain is interrupted, resulting in some degree of permanent neurological damage. The two major categories of stroke are ischaemic (lack of blood and hence oxygen to an area of the brain) and haemorrhagic (bleeding from a burst or leaking blood vessel in the brain) stroke.
Pathophysiology of ischaemic stroke
The common pathway of ischaemic stroke is lack of sufficient blood flow to perfuse cerebral tissue, due to narrowed or blocked arteries leading to or within the brain.
Ischaemic strokes can be broadly subdivided into thrombotic and embolic strokes.
Narrowing is commonly the result of atherosclerosis – the occurrence of fatty plaques lining the blood vessels. As the plaques grow in size, the blood vessel becomes narrowed and the blood flow to the area beyond is reduced.
Damaged areas of an atherosclerotic plaque can cause a blood clot to form, which blocks the blood vessel – a thrombotic stroke.
In an embolic stroke, blood clots or debris from elsewhere in the body, typically the heart valves, travel through the circulatory system and block narrower blood vessels.
Based on the aetiology of ischaemic stroke, a more accurate sub-classification is generally used:
- Large artery disease – atherosclerosis of large vessels, including the internal carotid artery, vertebral artery, basilar artery, and other major branches of the Circle of Willis.
- Small vessel disease – changes due to chronic disease, such as diabetes, hypertension, hyperlipidaemia, and smoking, that lead decreased compliance of the arterial walls and/or narrowing and occlusion of the lumen of smaller vessels.
- Embolic stroke – the most common cause of an embolic stroke is atrial fibrillation.
- Stroke of determined aetiology – such as inherited diseases, metabolic disorders, and coagulopathies.
- Stroke of undetermined aetiology – after exclusion of all of the above.
In the core area of a stroke, blood flow is so drastically reduced that cells usually cannot recover and subsequently undergo cellular death.
The tissue in the region bordering the infarct core, known as the ischaemic penumbra, is less severely affected. This region is rendered functionally silent by reduced blood flow but remains metabolically active. Cells in this area are endangered but not yet irreversibly damaged. They may undergo apoptosis after several hours or days but if blood flow and oxygen delivery is restored shortly after the onset of stroke, they are potentially recoverable (figure 1).
Figure 1: Ischaemic penumbra – Potential to reverse neurologic impairment with post-stroke therapy
The ischaemic cascade
After seconds to minutes of cerebral ischaemia, the ischaemic cascade is initiated. This is a series of biochemical reactions in the brain and other aerobic tissues, which usually goes on for two to three hours, but can last for days, even after normal blood flow returns.
The goal of acute stroke therapy is to normalise perfusion and intervene in the cascade of biochemical dysfunction to salvage the penumbra as much and as early as possible.
Although it is called a cascade, events are not always linear (figure 2).
Figure 2: The ischaemic cascade
Important steps of the ischaemic cascade
- Without adequate blood supply and thus lack of oxygen, brain cells lose their ability to produce energy - particularly adenosine triphosphate (ATP).
- Cells in the affected area switch to anaerobic metabolism, which leads to a lesser production of ATP but releases a by-product called lactic acid.
- Lactic acid is an irritant, which has the potential to destroy cells by disruption of the normal acid-base balance in the brain.
- ATP-reliant ion transport pumps fail, causing the cell membrane to become depolarized; leading to a large influx of ions, including calcium (Ca++), and an efflux of potassium.
- Intracellular calcium levels become too high and trigger the release of the excitatory amino acid neurotransmitter glutamate.
- Glutamate stimulates AMPA receptors and Ca++-permeable NMDA receptors, which leads to even more calcium influx into cells.
- Excess calcium entry overexcites cells and activates proteases (enzymes which digest cell proteins), lipases (enzymes which digest cell membranes) and free radicals formed as a result of the ischaemic cascade in a process called excitotoxicity.
- As the cell's membrane is broken down by phospholipases, it becomes more permeable, and more ions and harmful chemicals enter the cell.
- Mitochondria break down, releasing toxins and apoptotic factors into the cell.
- Cells experience apoptosis.
- If the cell dies through necrosis, it releases glutamate and toxic chemicals into the environment around it. Toxins poison nearby neurons, and glutamate can overexcite them.
- The loss of vascular structural integrity results in a breakdown of the protective blood brain barrier and contributes to cerebral oedema, which can cause secondary progression of the brain injury.
Pathophysiology of haemorrhagic stroke
Haemorrhagic strokes are due to the rupture of a blood vessels leading to compression of brain tissue from an expanding haematoma. This can distort and injure tissue. In addition, the pressure may lead to a loss of blood supply to affected tissue with resulting infarction, and the blood released by brain haemorrhage appears to have direct toxic effects on brain tissue and vasculature.
- Intracerebral haemorrhage – caused by rupture of a blood vessel and accumulation of blood within the brain. This is commonly the result of blood vessel damage from chronic hypertension, vascular malformations, or the use medications associated with increased bleeding rates, such as anticoagulants, thrombolytics, and antiplatelet agents.
- Subarachnoid haemorrhage is the gradual collection of blood in the subarachnoid space of the brain dura, typically caused by trauma to the head or rupture of a cerebral aneurysm.
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