Increased Intracranial Pressure

The skull is a rigid container holding brain tissue, cerebrospinal fluid (CSF), and blood. The Monro–Kellie principle describes what happens when one of these volumes increases: the others can compensate only briefly. Once compensatory capacity is exhausted, a small additional volume can produce a steep rise in intracranial pressure (ICP). A haematoma, tumour, abscess, cerebral oedema, excess CSF, or impaired venous drainage can all raise pressure. The result is not simply “a high number.” Increased ICP compresses cerebral vessels, reducing blood flow to brain tissue. Cerebral perfusion pressure (CPP) is calculated as: CPP = MAP − ICP Therefore, either a falling mean arterial pressure (MAP) or a rising ICP can deprive the brain of oxygenated blood. Continued pressure may displace brain tissue through rigid openings in the skull, producing herniation and brainstem compression. This physiology explains why hypotension, hypoxaemia, fever, agitation, coughing, straining, and neck compression are dangerous in a patient with suspected ICP elevation: each can worsen the mismatch between the brain’s metabolic needs and its available perfusion.