Early brain injury after subarachnoid haemorrhage
In the first hours after subarachnoid haemorrhage (a bleed into the CSF-filled gap around the brain), pressure inside the skull rises and the pressure driving blood through the brain falls.
Remember
In the first hours, the bleed itself raises intracranial pressure (the pressure inside the skull), and that lowers the pressure pushing blood through the brain; vasospasm comes later.
Memory aid
a balloon inflating inside a closed box
Picture the blood as a balloon filling the closed box of the skull: as the pressure rises, blood can no longer be pushed through the brain.
Early brain injury begins the moment the artery gives way. A large volume of blood is released into the subarachnoid space (the CSF-filled gap around the brain). The skull is closed, so the high volume of blood compresses the brain tissue directly. The blood also mixes with the CSF (cerebrospinal fluid, the clear fluid that cushions the brain), and that interferes with its drainage.
Compression and impaired drainage both raise the intracranial pressure (the pressure inside the skull). Impaired drainage can also cause acute hydrocephalus (CSF building up and enlarging the ventricles, the fluid-filled chambers in the brain) in roughly 20 to 30 per cent of patients, which adds to the pressure.
As the pressure climbs, the cerebral perfusion pressure (the pressure that drives blood through the brain) falls. Blood flow drops across the whole brain, giving a brief period of global ischaemia (too little blood reaching the whole brain).
When flow returns, the brain meets reperfusion injury (damage caused by the returning blood). This brings oxidative damage (harm from reactive oxygen species, which are highly reactive oxygen-containing molecules), inflammation and swelling.
Vasospasm (narrowing of the cerebral arteries) belongs to the days after the bleed, so it is not the mechanism of the global fall in blood flow in these first hours.
Key facts
- Timing
- Early brain injury begins straight away, when the vessel gives way, and comes before delayed cerebral ischaemia (a fall in blood supply to part of the brain in the days after the bleed).
- Early deaths
- Early deaths come mainly from the bleed itself, which compresses the brain and raises the pressure inside the skull.
- Pressure chain
- Raised intracranial pressure lowers cerebral perfusion pressure, so blood flow falls across the whole brain.
- Compensation
- Small arteries dilate to try to protect flow, but a critically low perfusion pressure is not restored.
- Order by time
- Hours bring compression and raised pressure, days bring vasospasm, and weeks bring chronic hydrocephalus, which may appear.
Exam traps
- Vasospasm does not explain the early fall in blood flow, because it is a delayed problem that peaks days later.
- Low blood pressure throughout the body is not what lowers perfusion in the first hours: perfusion falls because the pressure inside the closed skull rises.
- Chemical meningitis (inflammation of the meninges caused by the blood) causes neck stiffness and vomiting rather than early death.
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