Subdural haematoma Separation of the arachnoid and dura mater requires little physical force, which means that damage to small bridging veins in the space can result in subdural haematoma after even relatively mild head trauma. In cases of chronic or subacute subdural haematomas, the accumulation is of relatively low pressure and seldom presents as a medical emergency; even sizeable accumulations may be tolerated on a chronic basis with mild or no symptoms, and can be surgically drained through small openings in the skull (burr-holes). In many cases, there is some predisposing factor, such as cerebral atrophy or increased size of the underlying subarachnoid space. In cases of acute subdural haematoma, there is rapid accumulation of blood in the subdural space, typically after severe head trauma, which requires emergency neurosurgical drainage through a large craniotomy or craniectomy. The distinction between subdural and extradural haematoma on a CT scan relies on the anatomical features of the clot. Extradural collections tend to be lentiform in shape, reflecting the pressure required to separate the dura and periosteum. They will not pass deep to any major dural sinus and cannot extend along the falx cerebri or tentorium cerebelli. In contrast, acute subdural haematomas tend to be biconcave in shape and often follow the line of the dura along the falx or tentorium. Gray's anatomy