Subdural haematoma - Causes of subdural haematoma

A subdural haematoma is most often caused by a head injury that tears a blood vessel in the space between the skull and the brain (the subdural space).

Blood escapes from the ruptured blood vessel, leading to the formation of a blood clot (haematoma), which places pressure on the brain and may cause brain damage.

Intracranial pressure

As the bleeding continues, the subdural haematoma will grow, taking up more room inside the skull. The haematoma may press on the brain, leading to a build-up of pressure inside the skull. This is referred to as intracranial hypertension.

As the pressure increases, it is thought to have a harmful effect on the cells of the brain. This causes the brain tissue to swell, leading to a further increase in pressure. The increased pressure squashes the brain against the wall of the skull, resulting in symptoms such as headaches, confusion and muddled speech.

Left untreated, a subdural haematoma may damage your brainstem. The brainstem is the part of the brain that helps to regulate many of the body’s vital functions, such as consciousness and breathing. Therefore, brainstem damage can sometimes result in coma and possibly death.

In most cases, subdural haematomas develop after a head injury causes a large amount of blood to escape from a torn blood vessel.

Acute subdural haematomas

The most common cause of acute subdural haematoma is probably the brain being subjected to an "acceleration-deceleration force". This is when the skull and the brain inside the skull are propelled in one direction with great force (acceleration) before coming to a sudden stop (deceleration).

The most common ways that the brain can be subjected to this type of injury are:

motor vehicle accidents
falls
violent assaults

These types of injury can damage the brain in three main ways:

they can cause tearing of the veins that carry blood from the brain to the heart and lungs
they can damage the arteries that provide the brain with oxygen-rich blood
they damage the tissue of the brain, resulting in bleeding

As well as the direct damage caused by the pressure of the haematoma on the brain, the damage to the blood vessels inside the brain can disrupt the blood flow to the brain. This can often result in secondary brain damage, which is more severe than the damage caused by the initial injury.

Chronic subdural haematomas

The fact that most people’s brains tend to shrink to a certain extent as they grow older is thought to play an important part in the development of a chronic subdural haematoma.

This shrinkage places the veins that carry blood out of the brain under increased tension, much like a rubber band that has stretched to its maximum extent. The increase in tension makes the veins much more vulnerable to damage, so that even a minor injury can result in the veins tearing.

Unlike acute haematomas, these tears are usually minor, causing low levels of bleeding. However, over the course of several weeks, the amount of blood gradually builds up and a haematoma develops.

A number of factors may increase your risk of developing a chronic subdural haematoma. These are discussed below.

Age

Over half of all cases of chronic subdural haematoma affect people aged over 60. The chance of developing one increases with age.

This is mainly because as a person gets older, their brain reduces in size, creating a larger subdural space (more space between the brain and the skull).

Alcohol misuse

Drinking too much alcohol can shrink the brain over a period of time, which can result in a larger subdural space being created.

Anticoagulant medicines

Taking anticoagulant medicines can increase the risk of developing a subdural haematoma. Anticoagulants are often used to treat or prevent conditions that are caused by the blood clotting too quickly, such as deep vein thrombosis (DVT) and heart attacks.

Anticoagulant medicines, such as warfarin and heparin, work by slowing down the body’s blood clotting process. This can prevent a bleed in the subdural space from clotting quickly enough, so you'll bleed for longer than usual into the subdural space.

Seizures

A seizure occurs when the normal electrical activity of the brain is disrupted, which can cause the brain and body to behave strangely, such as losing conciousness or shaking uncontrollably.

Exactly why seizures increase the risk of developing a chronic subdural haematoma is unclear. It may be that the abnormal electrical activity during a seizure makes the brain more vulnerable to damage.

Ventriculoperitoneal (VP) shunt

A ventriculoperitoneal (VP) shunt is a thin tube that is implanted in the brain to drain away any excess fluid to another part of the body. VP shunts are used to treat a condition called hydrocephalus.

Occasionally, a VP shunt can overdrain, which can cause a chronic subdural haematoma.

Certain health conditions

Some health conditions, such as haemophilia and thrombocytopenia prevent the blood from clotting properly, which may mean that you bleed more heavily and for longer than usual.