A SUBDURAL haematoma, which carries a high risk of death, occurs when blood collects between the skull and the surface of the brain.
It is usually caused by a head injury and symptoms include a worsening headache, feeling sick or vomiting, confusion and a loss of consciousness.
According to the NHS website, the symptoms can develop soon after a severe head injury or sometimes a few days or weeks after a more minor head injury.
Actress Natasha Richardson died from a very similar type of bleed following a ski accident in 2009.
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According to the NHS website, a subdural haematoma occurs when blood escapes from a blood vessel in the space between the skull and the brain.
This forms a blood clot (haematoma) which places pressure on the brain.
A severe head injury - such as from a car crash or fall - is usually the cause, although more minor head injuries can cause a subdural haematoma.
In these latter cases, the people tend to be over the age of 60, are taking blood-thinning drugs or have experienced problems with alcohol.
Immediate medical attention is needed for a subdural haematoma.
If the patient reaches a medic in time, then surgery can be performed to remove part of the skull and remove the haematoma.
Alternatively, a small hole can be drilled into the skull and a tube inserted to drain the clot.
Acute subdural haematomas are the most serious type because they are often associated with significant damage to the brain.