Increased Intracranial Pressure l: Introduction

Intracranial hypertension is a sustained elevation of intracranial pressure (ICP) above 22 mm Hg. In supine adults, normal ICP is ~7–15 mm Hg.

The rigid, nonexpandable cranium contains three components—brain tissue, blood, and cerebrospinal fluid (CSF)—that total ~1,700 mL in a typical adult: 1,400 mL brain (~80%), 150 mL blood (~10%), and 150 mL CSF (~10%). According to the Monro–Kellie doctrine, total intracranial volume is effectively fixed. When one component expands, CSF and venous blood are displaced to compensate; once that reserve is exhausted, ICP rises steeply.

Causes by compartment

Brain compartment — increased tissue volume (edema or mass effect)

• Diffuse cerebral edema from trauma, ischemia, hypoxia, hyperammonemia (liver failure), uremic encephalopathy, or acute hyponatremia.
• Space-occupying lesions with mass effect: hematoma, tumor, abscess, or large infarct with edema.

CSF compartment

• Overproduction (rare)—classically due to choroid plexus papilloma.
• Obstructed CSF flow—obstructive (noncommunicating) hydrocephalus from an intraventricular mass, aqueductal stenosis, or posterior fossa crowding.
• Impaired CSF absorption—communicating hydrocephalus after subarachnoid hemorrhage or meningitis due to dysfunction of the arachnoid granulations.

Vascular compartment — increased blood volume or pressure

• Arterial vasodilation with increased CBF: hypercapnia (elevated CO₂) or higher doses of some volatile anesthetics.
• High-flow shunts and venous hypertension: arteriovenous malformations or dural arteriovenous fistulas.
• Impaired venous outflow: cerebral venous sinus thrombosis or elevated central venous pressure (e.g., heart failure, prolonged Valsalva maneuver).
• Hemorrhagic processes: aneurysmal subarachnoid hemorrhage or intracerebral hematoma, raising ICP via acute mass effect and secondary hydrocephalus.