Clinical meaning
Traumatic brain injury (TBI) involves primary injury (mechanical disruption at the moment of impact: contusion, laceration, diffuse axonal injury/DAI, hemorrhage) and secondary injury (evolving cascade over hours to days: cerebral edema, ischemia, excitotoxicity, neuroinflammation, oxidative stress, mitochondrial dysfunction). The Monro-Kellie doctrine states that the cranium is a fixed volume containing brain parenchyma (~80%), CSF (~10%), and blood (~10%). An increase in any component (edema, hemorrhage, mass lesion) must be compensated by a decrease in another — when compensation is exhausted, intracranial pressure (ICP) rises exponentially. Normal ICP is 5-15 mmHg. Cerebral perfusion pressure (CPP) = MAP - ICP; CPP <60 mmHg is associated with ischemia, and CPP >70 mmHg may increase risk of ARDS from excessive vasopressor use. Target CPP 60-70 mmHg. The Brain Trauma Foundation guidelines stratify management by GCS: mild TBI (GCS 13-15) — CT head if risk factors present (LOC >5 min, amnesia, vomiting, age >60, coagulopathy, dangerous mechanism); moderate TBI (GCS 9-12) — CT head mandatory, ICU admission, serial neurological exams, neurosurgical consultation; severe TBI (GCS ≤8) — intubation for airway protection, ICP monitoring, tiered ICP...