|ACR Index: 1.3|
Brain edema means an increase in brain water. There are three types of cerebral edema:
â€¢ Cytotoxic (intracellular)
â€¢ Vasogenic (extracellular or interstitial)
â€¢ Hydrostatic/Hydrocephalic (extracellular)
The water may accumulate in between the cell - interstitial edema (vasogenic or hydrostatic); or, water may accumulate inside of the cells - intracellular (cytoxic) edema.
Cytotoxic edema occurs when the cell-membrane pump fails and there is a net movement of water from the interstitial space into the cell. This most commonly occurs after cerebral ischemia - causing "red, dead, swollen neurons". The cell membrane may also fail with hypoxia, metabolic poisons, and hypoglycemia.
Vasogenic edema is, by definition, edema created by a breakdown in the blood-brain-barrier with leakage of water and solutes into the interstitial spaces of the brain. Therefore, we should expect to see a lesion with contrast enhancement as the cause of the edema. The edema itself is not observed to enhance on routine imaging.
Vasogenic edema spreads most easily through the association fiber tracts and is virtually excluded from the tight compact commisures - such as the corpus callosum. The projection tracts (e.g. the corticospinal tracts, internal capsule, and corona radiata) have an intermediate ability to absorb and hold interstitial edema. Vasogenic edema is commonly described as "finger-like" or as forming "pseudopods" as it spreads from the centrum semiovale into the subcortical white-matter and extends into individual cerebral gyri.
The white matter of the cerebellum is less able to absorb and hold interstitial water, so that vasogenic edema is much less common as compared to the cerebral hemispheres. This can be confusing. For example, cerebral metastasis often have extensive surrounding vasogenic edema - much less so in the cerebellum. Pilocytic astrocytomas of the cerebellum are infrequently surrounded by edema; whereas the same tumor in the cerebral hemisphere may have an alarming amount of vasogenic edema.
"Ischemic edema" is actually a combination of both cytotoxic and vasogenic edema. Cytotoxic edema occurs rapidly after cerebral ischemia. With infarction, vasogenic edema can develop, and takes hours to days, typically reaching a peak at 3-7 days. This is also associated with the peak mass effect of the infarct.
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Hydrostatic edema results from increased intraventricular pressure, with rupture of the ependymal lining. This allows CSF to leak into the interstitium, and it then spreads through the periventricular white matter. This is usually most pronounced around the frontal horns and the trigone of the lateral ventricles.
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Brain Swelling, Herniation, and Edema:
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