The blood-brain barrier (BBB) is a highly selective semipermeable border formed by brain microvascular endothelial cells connected by tight junctions (claudins, occludins, ZO-1), surrounded by pericytes, astrocyte foot processes, and basement membrane. This neurovascular unit restricts paracellular transport, limiting entry of pathogens, toxins, and most hydrophilic drugs while allowing passage of small lipophilic molecules, glucose (via GLUT-1 transporter), amino acids, and gases. BBB disruption occurs through multiple mechanisms: (1) Inflammatory mediators (TNF-alpha, IL-1β, matrix metalloproteinases) degrade tight junction proteins during sepsis, meningitis, and neuroinflammation; (2) Ischemia causes ATP depletion and cytotoxic edema followed by BBB breakdown and vasogenic edema; (3) Hypertensive crisis overwhelms cerebral autoregulation (MAP >150 mmHg), causing forced diapedesis through endothelial cells (PRES — posterior reversible encephalopathy syndrome); (4) Osmotic demyelination occurs when rapid sodium correction creates osmotic gradients that damage myelinated tracts (central pontine myelinolysis). BBB disruption allows extravasation of plasma proteins and fluid into brain parenchyma (vasogenic edema), entry of neurotoxic substances, and immune cell infiltration. Clinical consequences include cerebral edema with elevated ICP, seizures, encephalopathy, and white matter damage. The NP recognizes conditions causing BBB...