Additional images courtesy of Dr Li Foong Foo
Last updated May 2001
CT is a vital tool in the assessment of patients with serious head injury, and revolutionised management when it was introduced. It remains the investigation of choice even following the advent of MRI, due both to the ease of monitoring of injured patients and the better demonstration of fresh bleeding and bony injury.
A blow to the skull results in compression injury to the adjacent brain (coup) and stretching on the opposite side (contrecoup). This may result in contusion, shearing injuries and rupture of intraaxial or extra-axial vessels, leading to haemorrhage.
CT in a comatose patient may reveal:
Skull X-rays may help management (there is much debate regarding their role), but if a CT is indicated, they are not necessary and may only cause delay.
These arise between the inner table of the skull and the dura. They usually develop from injury to the middle meningeal artery or one of its branches, and therefore are usually temporoparietal in location. A temporal bone fracture is often the cause, but is not essential. The expanding haematoma strips the dura from the skull; this attachment is quite strong such that the haematoma is confined, giving rise to its characteristic biconvex shape, with a well defined margin.
It may present as primary depressed consciousness or following a lucid interval. The bleeding is usually acute and so of high attenuation. There is often significant mass effect with compression of the ipslateral lateral ventricle and dilatation of the opposite lateral ventricle due to obstruction of the foramen of Munro. The basal cisterns may be effaced.
This is the typical appearance and location of an acute extradural haematoma. Note the high density of the haematoma. Slight midline shift is present.
This example shows a more unusual, lower location. Note also the gas within the haematoma - this indicates a basal skull fracture or, as in this case, it is post surgical. Note also the dilated lateral ventricle on the opposite side.
Right frontal acute extradural haematoma with an air bubble, and midline shift.
These arise between the dura and arachnoid, often from ruptured veins crossing this potential space. The space enlarges as the brain atrophies and so subdural haematomas are more common in the elderly.
This presents in a similar fashion to the extradural haematoma, and can have equally severe consequences due to mass effect, requiring urgent surgery. Differentiating the two is therefore not so important in the acute situation.
The blood is again of high attenuation, but may spread more widely in the subdural space, with a crescentic appearance and a more irregular inner margin.
These two illustrations show a falcine subdural haematoma, a more unusual distribution. Note the abnormally bright falx due to the adjacent fresh blood.
The aetiology is not always clear. It is probably due to trauma, often minor, in the preceding few weeks, but no such history is obtainable in 50% of cases. Symptoms are vague and often develop slowly with a gradual depression or fluctuation of conciousness. Subdural haematomas are bilateral in 10% of patients.
While acute subdural haematomas have increased attenuation, this decreases with time, becoming isodense after a week or so, and hypodense thereafter. Consequently chronic subdurals are often hypodense crescentic collections, often with mass effect. The collection may be more complex with layering of more dense material posteriorly and a gradual transition. Expansion due to osmosis may tear further veins leading to recurrent bleeds; hyperdense red blood cells from fresh bleeding may layer posteriorly, and complex septated collections may develop.
Isodense collections may be better demonstrated after intravenous contrast as the density will then be less than that of the brain. However this is rarely a problem with more modern scanners.
Note the crescentic low density collection typical of a chronic subdural haematoma, with associated midline shift.
This is the same case, higher up. Note the dilated opposite lateral ventricle. Midline shift often distorts the Foramen of Munro of the opposite side, causing obstruction.
This haematoma is not so old and is almost isodense. It is probably about one to two weeks old. This could be missed on older scanners with poorer quality images, but this is rarely a problem now.
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Dr A C Downie
Victoria Infirmary, Glasgow
andrew@radiology.co.uk