HEADACHES: SUB-DURAL HAEMORRHAGE
After passing through the brain, used blood is collected in large veins lying directly on the inside of the skull. Sometimes, as a result of a minor head injury, these veins can snap or tear. When this happens, blood slowly leaks out between the two outermost coverings of the brain (the dura). This is called a sub-dural haemorrhage. Because veins work at a much lower pressure than arteries do, blood leaks out only slowly, so the effects of a sub-dural haemorrhage are much slower in onset than with bleeding from an artery. With a sub-arachnoid haemorrhage (which occurs in the next layer of the brain covering), the ruptured vessel is an artery under high pressure and blood leaks out very quickly indeed. With a sub-dural haemorrhage it is more of an ooze.
The large veins draining the brain can be stretched and broken, particularly by blows to either the front or the back of the head. It doesn’t need a particularly violent bang to do it, either – even a blow which isn’t enough to cause concussion or loss of consciousness is still capable of causing a sub-dural haemorrhage. In older age the brain sometimes shrinks, and a sub-dural haemorrhage is more likely to happen simply because the brain can move around more inside the skull: hence sub-dural haemorrhages are more common in older people.
Bleeding inside the skull compresses the brain, causing it to function less well; the higher the pressure inside the brain, the more it cuts off the normal flow of blood through brain arteries, and so the less oxygen can pass into the nerve cells -which therefore don’t function as well as they used to. Patients with a sub-dural haemorrhage gradually think more slowly, become less alert, and eventually become drowsy: they may go into coma. Typically, the degree of drowsiness comes and goes. In the worst cases the patient can develop a stroke-like condition. If untreated, a sub-dural haemorrhage may end in death.
Pressure like this is likely to cause a severe, continuous headache. A slow bleed into the inside of the skull is ultimately just as devastating as a fast one but is far less dramatic, and the symptoms come on more slowly. In the older person, where the brain has already shrunk slightly, the extra blood may create very little extra pressure at first, because there’s more space available within the skull. However, in younger people, excess pressure can develop to a much greater degree.
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