What is intracranial pressure (ICP)?

• A. The pressure in the ventricles alone
• B. the total pressures in the brain: arterial pressures + venous pressures + CSF pressures, etc.
• C. The arterial pressure in the brain
• D. The CSF pressure alone

Answer: (B)

Intracranial pressure is the pressure inside the skull that reflects the balance among all intracranial components. The Monro-Kellie doctrine says the total intracranial volume is fixed, so changes in one component (like blood, CSF, or brain tissue) affect the overall pressure unless compensated by others. Because of this, ICP isn’t just arterial pressure, or CSF pressure alone, or arterial pressure in the brain alone—it’s the combined pressure from arterial blood, venous blood, CSF, and brain tissue within the rigid skull. Clinically, ICP is measured inside the skull (often with intraparenchymal or ventricular monitors) and normally sits around 5–15 mmHg in adults; elevations indicate pathology such as edema, hemorrhage, or impaired CSF outflow.