Cerebral edema?

" Cerebral edema is the result of pressures over 160 mmHg, which dramatically increase brain capillary permeability" what does this mean? How come pressures over 160 mmHg will increase brain capillary permeability? won't the capillaries in the brain close to protect the brain tissue? thanks.
Answer:
When blood pressure shoots up to a danger level, many capillary beds are protected because the arterioles that supply the capillary bed contract and reduce the flow of blood to that capillary bed thus protecting it from dangerously high pressures, this, unfortunately is not the case in the CNS. The high pressure is transmitted to the cerebrovascular system and not only is the hydrostatic pressure increased but also the junction between the 2 ends of the endothelial cell (remember: a given section of capillary is only one cell that is joined at the end in a "tube-like" fashion by various intercellular junction) loosen up. This results in the movement of excess fluid from the intravascular spaces into the interstitium. Because the intersitium between the neurons is very small, even a small amount of fuid will make a big difference and increase the volume of the brain tissue, this is called cerebral edema. Note that the skull does not expand, since it is solid bone. As the brain is compressed a number of things happen
1. Blood flow to the brain becomes severely compromised
the neurons do not get the O2 and nutrients that they need
2. The increased pressure and compression starts to kill the
neurons
The brain will start to expand into any cavity pressent in the skull including the formane magnum. This causes herniation of the brain stem which controls the vital functions of the body
signs include
1. Pupil dilation
2. Increasing heart rate and BP to force blood into brain
3. Extensor posturing
4. Flaccid motor responses
5. Coma
6. Death
Causes of cerebral edema can be
1. Sever Hypertension (remeber NO autoregulation of arterioles in the brain)
2. Head Trauma
3. Poisons (dinitrophenol, isoniasid)
4. Osmotic edema (ie. Hyponatremia - decreased plasma osmotic pressure and loss of water to the interstitium)
5. Hydrostatic edema (your example - high BP)
6. Interstital Edema (rupture of CSF-brain barrier: permits CSF to penetrate brain and spread in the extracellular space of white matter)
7. Sever altitude sickness
The signs must be rapidly recognized and treated quickly, or the patient dies
1. Treat underlying cause (ie. rapidly decrease blood pressure)
2. Give osmotic diuretics (ie. Mannitol by IV Infusion)
3. Cool the brain and or body (induce mild hypothermia)
4. Craniotomy

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160 mm Hg is just a biological standard. Of course, it won't necessarily be *exactly* 160 mm Hg, though it will be close... there's almost always some variation.
Permeability relates to diffusion of fluids through the blood vessel walls, which creates the edema. The capillaries can't close to protect tissue, because the brain needs the blood supply. Without the blood supply (oxygen) a person or animal will die.
The edema can actually be a physiological means of protecting the brain. If the edema didn't occur, the blood vessels could rupture due to the increased pressue and cause hemorrhaging.

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