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Subdural haematomas are usually caused by a head injury.
Head injuries that cause subdural haematomas are often severe, such as from a car crash, fall or violent assault.
Minor bumps to the head can also lead to a subdural haematoma in a few cases.
A subdural haematoma develops if there's bleeding into the space between the skull and the brain (the subdural space) caused by damage to the blood vessels of the brain or the brain itself.
As this blood starts to build up in the subdural space, it can place pressure on the brain (intracranial hypertension) and can cause brain damage.
Anyone can develop a subdural haematoma after a severe head injury.
Chronic subdural haematomas form gradually a few weeks after a minor head injury.
These are more commonly seen in older people and those who take anticoagulant ("blood-thinning") medicine, drink excessively, or have another medical condition.
Most chronic subdural haematomas affect people over 60, and the chances of developing one increase with age.
This is thought to be because most people's brains shrink to some degree as they get older.
This places the brain's blood vessels under increased tension, like a stretched rubber band, and means they're more vulnerable to damage from minor injuries.
Drinking too much alcohol over a long period of time can also gradually cause the brain to shrink and make the brain's blood vessels more vulnerable to damage.
Read more about alcohol misuse.
Taking medicine to reduce your risk of blood clots can increase your risk of developing a chronic subdural haematoma.
This is because it means your blood clots less easily and any bleeding caused by a head injury is likely to be more severe.
Both anticoagulant medicines and antiplatelet medicines, such as aspirin and clopidogrel, may increase your risk.
An increased risk of chronic subdural haematoma has also been linked with:
