Intracranial Pressure Physiology
Overview
Intracranial pressure (ICP) normally ranges from 5-15 mmHg in adults. The skull is a rigid, closed box, meaning any increase in the volume of its contents must be compensated by an equal decrease in another, as described by the Monro-Kellie Doctrine.
The Monro-Kellie Doctrine
Intracranial volume is constant: Brain tissue (80%) + Blood (10%) + CSF (10%). An increase in one compartment is compensated in two phases:
- Phase 1: CSF displaced into the spinal subarachnoid space.
- Phase 2: Cerebral venous blood displaced via jugular veins.
- Phase 3 - Decompensation: Compliance exhausted. Any further volume increase causes an exponential, uncontrolled rise in ICP.
Cerebral Perfusion Pressure
CPP = MAP - ICP. Normal CPP is 70-80 mmHg. Cerebral autoregulation maintains constant CBF over a MAP range of 50-150 mmHg. This protective autoregulation is lost after severe TBI.
WarningThe Cushing Reflex: A Late and Dire Sign
As ICP approaches MAP, CPP falls to near zero. The brainstem responds with the Cushing Triad: Hypertension (to maintain CPP), Bradycardia (baroreceptor response), and Irregular respirations (brainstem compression). This is a pre-terminal sign indicating imminent catastrophic herniation.
Imaging Signs of Raised ICP
- CT: Effacement of sulci and basal cisterns, loss of grey-white differentiation, midline shift, uncal herniation.
- MRI: Downward displacement of the diencephalon, Duret haemorrhages in the pons (secondary to central herniation).
- Ultrasound/MRI: Optic nerve sheath dilation >5 mm reflects raised ICP.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
Hyperventilation (reducing pCO2) causes cerebral vasoconstriction and rapidly lowers ICP, but this effect is exhausted within 30 minutes. Mannitol lowers ICP via osmotic extraction. Head-of-bed elevation to 30 degrees promotes jugular venous drainage and reduces ICP.