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Family Doctor Books
Preview of Understanding Stroke

What is a stroke?
The brain controls our body movements, processes information from the outside world and allows us to communicate with others. A stroke occurs when part of the brain stops working because of problems with its blood supply. This leads to the classic symptoms of a stroke, such as a sudden weakness affecting the arm and leg on the same side of the body.

The brain is one of the most delicate parts of the body and, tragically, even a short time without a good blood supply can be disastrous. For example, although a finger or even a leg can be successfully saved after many hours without a blood supply, the brain is damaged within minutes. The symptoms of a stroke usually come on quickly and can be very severe.

Brain function

It is useful to describe the structure of the brain to help understand why different sorts of strokes occur.

The brain controls body movement, processes information from the outside world and allows us to communicate with others through a network of nerves that range throughout the body.

THE STRUCTURE OF THE BRAIN
The brain has two hemispheres: the left and the right. Each hemisphere is composed of four lobes. Each of the four lobes of each cerebral hemisphere has its own particular physical and mental functions. These can be impaired by brain damage.

The brain is encased in the bony skull and communicates with the rest of the body through the cranial nerves (which pass through openings in the skull) and the spinal nerves (which pass from the spinal cord through small gaps between the bones of the spine and control the arms, trunk and legs). The brain is made up of three main regions:
  • the brain stem
  • the cerebellum
  • the cerebral hemispheres.
The brain stem controls the breathing, heart rate and important reflexes, for example, the cough reflex to clear the breathing tubes. The ‘cerebellum’ is the centre for balance and coordination of movement. In evolutionary terms, these parts of the brain are quite old and provide the minimum brain power to survive. Connected to this more primitive part of the brain are the two cerebral hemispheres (left and right), which control speech, thinking, complex movements and vision.

The right and left hemispheres communicate through nerve fibre bundles that cross from one side of the body to the other. As a result, the left side of the brain controls the right side of the body and vice versa. A stroke affecting the left side of the brain therefore causes symptoms (for example, weakness) in the right side of the body. In most right-handed people, the left hemisphere is dominant and controls logic and speech, whereas the right hemisphere is involved with imagination and creative thought. This is called left-sided dominance.

The right and left hemispheres communicate with the muscles and sense organs through nerve bundles that cross from one side of the brain to the other. As a result, the left side of the brain controls the right side of the body and vice versa.

The blood supply to the brain is from four main blood vessels – two vertebral arteries and two carotid arteries. The vertebral arteries enter the skull from the backbone and mainly supply the brain stem and cerebellum, whereas the two carotid arteries enter the skull from the front of the neck and mainly supply the two cerebral hemispheres.

All four arteries join up in a rough circle, which helps to maintain an adequate supply of blood if one artery gets blocked. Water mains and electricity supplies operate on similar principles, to try to maintain adequate supplies even if part of the supply breaks down. Unfortunately, the effectiveness of the circle of arteries varies from person to person and often does not protect people from the symptoms of a stroke if one of the main arteries becomes blocked.

The blood is supplied to the brain from the front of the neck and the backbone. The arteries join in a rough circle which helps to maintain an adequate supply of blood if one artery is blocked.

Causes of a stroke

The brain uses large amounts of oxygen and nutrients (for example, glucose) which are supplied through the circulation. The most common cause of a stroke is when a blood vessel supplying these vital nutrients to the brain becomes blocked with a blood clot. The blood clot – known as a thrombosis – may form locally in a brain artery or form elsewhere (for example, in the heart) and travel in the bloodstream to lodge in the brain. This type of wandering clot is known as an embolus. When a blood vessel in the brain becomes blocked, the brain cells that it supplies quickly become starved of oxygen and glucose, and stop working properly. If the blood supply is not quickly resumed, these brain cells will die. This type of stroke is called an ischaemic stroke, or a ‘cerebral infarct’. The medical term ‘ischaemic ’ means a shortage of blood. ‘Cerebral’ is the medical term for the brain and ‘infarct’ is the medical term for death of a part of the body.

The second most common cause of a stroke is a brain haemorrhage, which occurs when a blood vessel bursts inside the head. As well as disrupting the supply of oxygen and glucose to some parts of the brain, the escaping blood can cause damage by clotting, swelling and triggering inflammation.

The most common cause of a stroke is a thrombosis – when a blood vessel supplying vital nutrients to the brain becomes blocked with a blood clot.

There are two types of brain haemorrhage: an intracerebral haemorrhage, when blood collects within the brain; and a subarachnoid haemorrhage, when blood collects between the skull and the brain. Doctors are now recognising some medical conditions that appear to weaken blood vessels and increase the chance of a rupture. High blood pressure certainly seems to be an important cause of an intracerebral haemorrhage. Subarachnoid haemorrhage is mainly caused by the rupture of small swellings – known as aneurysms – which can form in weakened blood vessels, and this problem can run in families.

The second most common cause of a stroke is a brain haemorrhage, which occurs when a blood vessel bursts inside the head.

Any bleed or thromboembolus within the head that causes a loss of function for more than 24 hours (assuming the patient survives) can be called a stroke. Symptoms that last less than 24 hours, and from which there is a complete recovery, are known as a transient ischaemic attack (TIA) or mini-stroke (see ‘Types of strokes’, page 11).

It is often difficult to tell an ischaemic cerebral stroke from a haemorrhagic stroke. Both can cause weakness, numbness or paralysis of part of the body, and may be associated with slurred speech and a loss of consciousness. A haemorrhagic stroke is often accompanied by a severe headache, however, and can be more severe, with widespread damage, so that a prolonged loss of consciousness (coma) is more likely.

KEY POINTS
  • Strokes result from problems affecting the blood supply to the brain
  • Some strokes are caused by the blockage of a blood vessel with a blood clot
  • Some strokes occur when a blood vessel ruptures