A temporal bone fracture is often the cause, but is not essential. The expanding haematoma separates 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 and slight midline shift .