This is a guest article I wrote for Brainbook; check out the original on their site.

Why does an intracranial bleed matter?

The skull is an inflexible bony box. It has limited space inside, which is fully occupied by the brain, its covering layers, blood vessels, and a cushioning fluid known as CSF (cerebrospinal fluid). A bleed (or any extra mass) inside the skull quickly raises the intracranial (inside-skull) pressure, squishing these structures. This causes all sorts of neurological mayhem as the flow of blood and CSF is impaired.

After a head injury with bleeding inside the skull (intracranial haemorrhage), symptoms relate to the raised intracranial pressure (ICP). These include a progressive headache, nausea/vomiting, loss of consciousness, and seizures. When the bleed is large or ongoing, death will ultimately result from a stroke (reduced blood flow to an area of the brain), or herniation (bulging of brain outside its normal boundaries) – more on this later.

A subdural haemorrhage is one type of intracranial bleed. The name simply describes the location of the bleed under (sub) the dura (tough membrane on the inside of the skull).