The brain is covered by the MENINGES of which there are nominally 3 – the PIA MATER, the ARACHNOID MATER and the DURA MATER (‘mater’ means ‘mother’). The pia is practically non existent and we may consider it no more. The brain sits within the skull (as if you didn’t know) but the significance of this is that there is no room for anything else; if ‘something else’ arises within the skull, then the brain has to make room for it. This is why doctors talk about ‘space occupying lesions’ and why they are so important; they squash the brain and that does the brain no good at all.
Bleeds in the skull are one such example of space occupying lesions. They may be associated with skull fractures, which may let in infection, but if there’s a bleed in the skull, it matters.
Such bleeds may be extra-dural (ie outside the dura and just under the skull), sub-dural (ie between the dura and the arachnoid) or subarachnoid. Extra-dural haemorrhages occur in babies when they are abused. Subdural haemorrhages may be ‘acute’ (that is, occurring quickly) or ‘chronic’ (that it occurring over a long time); they are always associated with a head injury but the head injury that leads to a chronic subdural may be so long ago and very slight, so that the afflicted swears blind that it never happened.
An acute subdural is caused by blood accumulating quickly and pressing on the brain. A chronic subdural starts out small and grows over weeks, months even years. It may grow so slowly that the brain is able to compensate, and may present after months or years with headaches. They occur in the elderly.
Subarachnoid haemorrhages are bad news. They may be associated with head trauma but they can occur spontaneously, when a tiny bubble on the wall of a basal artery (a ‘berry’ aneurysm) bursts. Because they are associated with arterial (ie high pressure) bleeds, they do a lot of damage.