The skull is a rigid, non-expandable container. The total volume within it — the intracranial volume — is therefore fixed. This has a fundamental consequence that is known as the Monro-Kellie doctrine: the total volume of the intracranial contents must remain constant, and any increase in the volume of one component must be accompanied by a corresponding decrease in another, or intracranial pressure (ICP) will rise.

The three components of intracranial volume are brain parenchyma (approximately 80% of total volume), blood — both arterial and venous — (approximately 10%), and cerebrospinal fluid (approximately 10%). In a healthy person, small increases in the volume of one component — for example, a small intracranial haematoma — can be accommodated by displacement of CSF into the spinal subarachnoid space and partial compression of venous sinuses. This compensatory capacity is finite, however, and once it is exhausted, even small additional increases in volume cause large rises in ICP. This non-linear relationship between volume and pressure is described by the intracranial compliance curve.

The clinical implication is that a patient’s ICP can remain normal or only mildly elevated while their compensatory mechanisms are functioning, and then rise precipitously once those mechanisms are exhausted. This is why patients with expanding intracranial haemorrhages or cerebral oedema can appear relatively stable and then deteriorate suddenly — they have crossed the inflection point of the compliance curve.