Stroke

A stroke, also known as a cerebrovascular accident, occurs when there is an interruption of the blood supply to a part of the brain. This interruption results in deprivation of oxygen and nutrients, leading to damage and death of brain cells within minutes^[1].

There are two primary types of stroke - ischemic stroke and hemorrhagic stroke - with a third - temporary ischemic attack (TIA) - often being referred to as a mini stroke^[2].

Ischemic strokes, which account for about 87% of all strokes^[4], are caused by a blockage that interrupts or reduces the flow of blood to the brain.

This blockage is usually a blood clot, and there are two types of blockage that can lead to an ischemic stroke:

1. Thrombotic stroke: This happens when a blood clot (thrombus) forms in one of the arteries that supply blood to your brain^[5]. A clot may be caused by fatty deposits (plaque) that build up in arteries and cause reduced blood flow (atherosclerosis) or other artery conditions^[6].

2. Embolic stroke: This occurs when a blood clot or other debris forms away from your brain — commonly in your heart — and is swept through your bloodstream to lodge in narrower brain arteries^[7]. This type of blood clot is called an embolus.

Certain factors can contribute to the formation of these clots, including:

• High blood pressure: This is the biggest risk factor for stroke. It can damage and weaken arteries throughout the body, making them more likely to block or burst. The risk of stroke increases as blood pressure rises^[8].

• Heart disease: Many types of heart disease can raise your risk of ischemic stroke. Examples include coronary artery disease, valve defects, irregular heart rhythm, and enlarged heart chambers^[9].

• Atherosclerosis: This is a condition characterized by the buildup of fatty deposits on the inner lining of the arteries, which can cause clots^[10].

• Blood disorders: Conditions that increase blood clotting can raise the risk of ischemic stroke. Examples include an increased level of red blood cells (polycythemia), increased platelets (thrombocythemia) and increased clotting factors (hypercoagulability)^[11].

• Diabetes: This disease can cause damage to blood vessels, making them more likely to become blocked^[12].

• Cholesterol imbalance: High levels of low-density lipoprotein cholesterol (the "bad" cholesterol) can increase the likelihood of clots forming in arteries^[13].

Other risk factors for stroke include smoking, age, gender, ethnicity, personal or family history of stroke or transient ischemic attack (TIA), obesity, physical inactivity, heavy or binge drinking, and use of illicit drugs^[14].

This is less common, but often more severe. A hemorrhagic stroke occurs when a blood vessel in the brain ruptures or leaks, leading to bleeding in the surrounding brain tissue^[1]. This bleeding can cause brain cells to die and can lead to brain damage, disability, or death.

There are two main types of hemorrhagic stroke:

1. Intracerebral Hemorrhage: This is the most common type of hemorrhagic stroke. It occurs when an artery in the brain bursts, flooding the surrounding tissue with blood^[16]. Hypertension (high blood pressure) is the most common cause of this type of stroke.

2. Subarachnoid Hemorrhage: This type of stroke involves bleeding in the area between the brain and the tissue that covers it (the subarachnoid space). This is most often due to a ruptured aneurysm, which is a weak or thin spot on an artery that balloons out and fills with blood^[17].

Hemorrhagic strokes can also be caused by conditions that affect the blood vessels, including:

• Aneurysms: An aneurysm is a weakened spot in a blood vessel wall that bulges out and can burst, leading to a hemorrhagic stroke. Aneurysms can be caused by high blood pressure, aging, and genetic factors^[18].

• Arteriovenous malformations (AVMs): AVMs are tangles of defective blood vessels and capillaries within the brain that can rupture. They are usually congenital, meaning a person is born with them^[19].

• Cerebral Amyloid Angiopathy: This is a build-up of amyloid protein within the walls of the blood vessels in the brain, which can weaken them and lead to a hemorrhagic stroke^[20].

• High Blood Pressure: Chronic high blood pressure can weaken the walls of the blood vessels, making them more prone to rupture^[8].

• Overuse of anticoagulants (blood thinners): These medications can help prevent blood clots, but they also increase the risk of a bleeding stroke^[21].

Lifestyle factors such as smoking and excessive alcohol consumption can also increase the risk of hemorrhagic stroke.

A Transient Ischemic Attack (TIA), often called a "mini-stroke", is caused by a temporary disruption in the blood supply to part of the brain. The cause of this disruption is typically a small blood clot.

The mechanism of a TIA is very similar to an ischemic stroke, but in the case of a TIA, the blockage is temporary and the symptoms resolve within an hour, though they can last up to 24 hours. Despite the temporary nature of a TIA, it should be taken very seriously as it can be a warning sign of a future stroke. Up to one-third of individuals who experience a TIA will eventually have a stroke, with about half occurring within a year of the TIA^[22].

The risk factors for a TIA are the same as for an ischemic stroke.

The effects of a stroke can vary depending on the part of the brain affected and the extent of the damage. Some potential effects include^[23]:

1. Physical Effects: These may include paralysis or muscle weakness, usually on one side of the body. This can lead to difficulty in walking, swallowing (dysphagia), or performing movements (apraxia).

2. Speech and Language Issues: A person may experience problems with speaking (dysarthria), understanding speech (receptive aphasia), or expressing themselves (expressive aphasia).

3. Cognitive and Emotional Effects: Strokes can lead to cognitive problems including memory loss, attention deficits, and difficulties with reasoning or judgment. Emotional disturbances such as depression, anxiety, and mood swings can also occur.

Wernicke's area and Broca's area are two key brain regions involved in language processing, located in the dominant cerebral hemisphere, which is the left hemisphere for most people.

1. Wernicke's Area: Located in the superior temporal gyrus in the dominant cerebral hemisphere, Wernicke's area is responsible for the comprehension of speech. This area is typically supplied by the posterior division of the left middle cerebral artery (MCA). If a stroke occurs that blocks the MCA, or more specifically its posterior division, this can disrupt blood flow to Wernicke's area. Without adequate blood flow, the neurons in Wernicke's area do not receive the oxygen and nutrients they need to function and may die off, leading to Wernicke's aphasia. This condition is characterised by fluent, but nonsensical speech and difficulty understanding spoken and written language.

2. Broca's Area: Located in the posterior part of the frontal lobe, Broca's area is crucial for speech production. This region is usually supplied by the superior division of the left MCA. A stroke that disrupts blood flow in the superior division of the left MCA could therefore affect Broca's area, leading to Broca's aphasia. Individuals with Broca's aphasia typically understand speech well but struggle with speech production, resulting in non-fluent, effortful speech.

Both ischemic strokes, which occur when a clot obstructs blood flow to the brain, and hemorrhagic strokes, which occur when a blood vessel ruptures, can potentially cause these types of aphasia^[24]. The specific symptoms and their severity depend on the extent and exact location of the damage in the brain.

The situation can be more complex, as the areas of the brain responsible for language processing aren't confined to just Wernicke's and Broca's areas, but instead involve a broader network of regions across the brain. Nonetheless, damage to these areas is strongly associated with specific forms of aphasia.

Treatment for a stroke depends on the type, severity, and the time elapsed since symptom onset. The primary goals of treatment are to restore blood flow for ischemic stroke and control bleeding for hemorrhagic stroke^[25].

For ischemic strokes, immediate treatment includes:

1. Thrombolytics: Clot-busting drugs, like tissue plasminogen activator (tPA), can be used if the person reaches the hospital within 4.5 hours from symptom onset.

2. Endovascular procedures: Doctors may perform a procedure to deliver medication directly to the area of the blockage, or use a device to remove the clot.

For hemorrhagic strokes, treatment options include:

1. Medications: Drugs to lower blood pressure and prevent seizure and secondary damage caused by the bleeding.

2. Surgery: In some cases, surgery may be needed to repair blood vessel abnormalities and reduce the risk of further bleeding.

Following immediate treatment, rehabilitation is crucial to help the individual regain lost abilities and skills. This could involve physiotherapy, occupational therapy, and speech and language therapy, with the aim of restoring as much normal function as possible. Recovery varies from person to person, with some able to recover completely while others might have significant long-term disabilities.