A brief introduction to Acute Vs Chronic Subdural haematoma in CT

Subdural haematomas arise between the dura and arachnoid, usually from ruptured veins crossing this potential space. The space enlarges as the brain atrophies and so subdural haematomas are more common in the elderly.
There are 2 types of subdural haematomas :

Acute subdural - Chronic subdural

Acute subdural

It is presents in a similar fashion to the extradural haematoma, and can have equally severe consequences due to mass effect, requiring urgent surgery. So Differentiating the two is 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.

Chronic subdural

It`s 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. In 10% of cases subdural haematomas are bilateral .

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.