Brain, blood and cerebrospinal fluid occupy a fixed volume within a rigid skull
Expansion
The adult skull is rigid, containing:
- Brain: about 80 per cent
- Blood: about 10 per cent
- Cerebrospinal fluid: about 10 per cent
Any increase in one must be offset by a decrease in another, or pressure rises.
Compensation, in order:
- Displacement of CSF into the spinal subarachnoid space and increased absorption
- Displacement of venous blood
- Then compensation is exhausted
The intracranial pressure-volume curve is therefore flat then exponential: a small further increase in volume causes a large rise in pressure, which is why patients deteriorate suddenly after appearing stable.
Cerebral perfusion pressure = mean arterial pressure - intracranial pressure, and should be kept above about 60 mmHg.
Management targets each compartment: mannitol or hypertonic saline for brain water, CSF drainage, head elevation and avoiding venous obstruction for blood, controlled hyperventilation to reduce cerebral blood volume acutely, and decompressive craniectomy to remove the rigid constraint altogether.