Intracranial pressure (ICP) is the pressure exerted by the tissues and fluids—such as brain tissue, blood, and cerebrospinal fluid (CSF)—within the rigid cranial vault. Under normal conditions, ICP ranges between 7 and 15 mmHg in a supine adult. ICP is a dynamic parameter that can fluctuate due to changes in body position, respiratory cycle, and systemic hemodynamics, but any sustained elevation above 20 mmHg is considered pathological and requires immediate intervention.
The Monro-Kellie doctrine is fundamental to understanding ICP regulation. This principle posits that the cranial cavity contains three main components—brain tissue (approximately 80%), CSF (10%), and blood (10%)—which exist in a state of equilibrium. If the volume of one component increases, the volume of one or both other components must decrease to maintain a constant ICP, given the fixed volume of the cranial vault. The brain compensates for minor volume changes through the displacement of CSF into the spinal canal and the compression of venous blood. However, when compensatory mechanisms are overwhelmed, ICP rises, compromising cerebral perfusion and leading to potential brain herniation.
In the pathological setting of increased ICP, several mechanisms may contribute to its elevation, including cerebral edema (vasogenic, cytotoxic, or interstitial), space-occupying lesions (e.g., tumors, hematomas), and obstructed CSF flow (e.g., hydrocephalus). As ICP rises, brain herniation can occur, wherein brain tissue is forced across structures like the falx cerebri or tentorium cerebelli, potentially leading to fatal outcomes. The most dangerous of these is uncal herniation, which can compress the brainstem and compromise vital autonomic functions.