Brain swelling is deadly because the skull has no room to spare
Most swollen tissue can simply expand. The brain cannot, because the skull is a rigid box. When fluid builds up inside it, something else has to give, so blood vessels, spinal fluid and brain tissue get squeezed. That is why cerebral edema is a major cause of brain damage after strokes and head injuries.
Cerebral edema means excess fluid collecting in or between the brain's cells. It accompanies a long list of problems: ischemic and haemorrhagic strokes, traumatic brain injury, tumours, infections such as meningitis, low blood sodium, high altitude and acute liver failure. Because it rides along with so many conditions, its overall frequency is hard to pin down, but one estimate found malignant swelling in roughly 31% of people within 30 days of an ischemic stroke.
The physics is summed up by the Monro–Kellie doctrine: inside a fixed, inelastic skull, any added volume displaces something else. Rising pressure brings headache, nausea, vomiting, drowsiness and visual trouble. The body may fight back by driving blood pressure up to keep blood reaching the brain; combined with irregular breathing and a slowing heart, this is called the Cushing reflex, and it often signals dangerous compression.
Doctors traditionally sort the swelling into two main kinds. In cytotoxic, or cellular, edema the cells themselves balloon. During a stroke, for example, starved cells run out of ATP, their sodium-potassium pumps fail, sodium builds up inside, and water rushes in by osmosis until neurons die. Some researchers prefer the term cellular edema, since no consistent toxin is involved. In vasogenic edema the blood–brain barrier breaks down, letting fluid and proteins such as albumin leak into the space between cells; it is typical of tumours such as glioblastoma, infections and the later phase of stroke.
Other varieties exist. In ionic edema the brain's solute concentration exceeds that of the blood, so water is drawn in, as can happen with severe hyponatraemia or when blood sugar falls too fast during treatment of diabetic ketoacidosis. In liver failure, ammonia crosses into the brain and is turned into glutamine, which pulls water into cells. Severe high blood pressure can force fluid through vessel walls, since the brain's circulation copes only up to a systolic pressure of about 150 mm Hg. Management depends on the cause and ranges from monitoring and fluid control to decompressive surgery.
Source: Cerebral edema
