One of the most serious complications of abnormal cerebral fluid dynamics is the development of brainedema. Because the brain is encased in a solid cranialvault, accumulation of extra edema fluid compresses the blood vessels, often causing seriously decreased blood flow and destruction of brain tissue.
The usual cause of brain edema is either greatly increased capillary pressure or damage to the capillary wall that makes the wall leaky to fluid. A very common cause is a serious blow to the head, leading to brain con-cussion, in which the brain tissues and capillaries aretraumatized so that capillary fluid leaks into the trau-matized tissues.
Once brain edema begins, it often initiates two vicious circles because of the following positive feedbacks:
(1) Edema compresses the vasculature. This in turn decreases blood flow and causes brain ischemia. The ischemia in turn causes arteriolar dilation with still further increase in capillary pressure.The increased cap-illary pressure then causes more edema fluid, so that the edema becomes progressively worse. (2) The decreased cerebral blood flow also decreases oxygen delivery. This increases the permeability of the capillaries, allowing still more fluid leakage. It also turns off the sodium pumps of the neuronal tissue cells, thus allowing these cells to swell in addition.
Once these two vicious circles have begun, heroic measures must be used to prevent total destruction of the brain. One such measure is to infuse intravenously a concentrated osmotic substance, such as a very con-centrated mannitol solution. This pulls fluid by osmosis from the brain tissue and breaks up the vicious circles. Another procedure is to remove fluid quickly from the lateral ventricles of the brain by means of ventricular needle puncture, thereby relieving the intracerebral pressure.