Extradural haematoma forms a lens-shaped collection
Subdural haematoma forms a crescent-shaped collection
Blood due to subarachnoid haemorrhage occupies the CSF spaces - sulci, fissures, ventricles, basal cisterns
Intracranial bleeding is either intra-axial (in the brain) or extra-axial (outside the brain).
There are three types of extra-axial haemorrhage: extradural haematoma, subdural haematoma, and subarachnoid haemorrhage.
Extradural haematoma
Extradural (or epidural) haematoma is a post-traumatic event resulting from injury to an intracranial artery, most commonly the middle meningeal artery. Leakage from an injured artery results in collection of blood which strips the dura mater away from the inner table of the skull.
Extradural haematoma results in formation of a lens-shaped collection. This is because the dura mater is strongly adhered to the skull in the region of the sutures. Stripping of the dura from the skull is limited at these points resulting in limitation of the extent of a collection.
A lens-shaped collection has formed following trauma to the left side of the head
Note the collection of blood is limited at the points of the sutures
This is the typical appearance of an extradural haematoma
There was no accompanying fracture in this case (don't forget to check the bone windows)
Subdural haematoma
The cerebral veins are fragile. Risk of injury to these veins is increased in elderly and anti-coagulated patients. Subdural haematoma may result from minor trauma. There is often no clear history of trauma at all.
A subdural collection is not limited by the attachment points of the dura to bone, as is seen in an extradural haematoma. The result is the formation of a crescent-shaped collection. The arachnoid remains intact and so blood does not pass into the sulci.
A large crescent-shaped collection is seen over the left side of the brain
The collection is of high density material due to the presence of clotted blood
There is no blood extending into the sulci
The sulci on the side of the haematoma are partially effaced indicating 'mass effect'
Subarachnoid haemorrhage
Subarachnoid haemorrhage is commonly associated with trauma or spontaneous bleeding from an intracranial aneurysm. Aneurysms are only visible on conventional CT images if they are large.
Less frequently, subarachnoid haemorrhage is due to spontaneous bleeding from a congenital arteriovenous malformation or spontaneous bleeding from the veins around the brain stem (perimesencephalic subarachnoid haemorrhage).
Small foramina connect the subarachnoid space with the fourth ventricle. Blood due to a subarachnoid haemorrhage can, therefore, pass into any part of the CSF spaces: sulci, fissures, basal cisterns or ventricles.
