Time to brain fixation and ischemic damage

[o2680236]

I've been reading neuropathological papers and asked myself: since we're told that irreversible hypoxic-ischemic damage to the brain occurs about 20-40 minutes from deprivation of blood supply to the brain, and since a very good time interval to brain fixation takes at least few hours and typically more than 12, then why don't we see massive ischemic damage to all brain structures in every neuropathological examination of a brain?

for example in this research on PVS patients you can see that most of them had apperently no ischemic damage on the neuropathological examination although it surely took at least few hours until brain fixation and all this time without blood supply to the brain we would have expected that global ischemic damage would have occured:
http://brain.oxfordjournals.org/content/123/7/1327/T1.expansion.html

What am I missing here?

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[KCShaw]

There are differences between ischemia, localized infarct, and global death. With ischemia there may be reversible or irreversible damage, which tends to become more visible as those cells/tissues respond to the ischemic insult. This is in contrast to global death, as in a brain specimen, where there -are- still changes that occur with time (autolysis/decomposition), some of which can be difficult to separate from pre-/peri-mortem ischemia but some of which can be fairly reliably interpreted. Sometimes one has to compare a localized change to whatever global changes are present to try to separate the two issues. But when the brain is removed, the typical response to ischemia by damaged or intact tissues doesn't take place.

Incidentally, it can take days to weeks to fully fix a whole brain; I've gotten consistently good fixation after about a week to 10 days, but some people push under that. Can be affected by technique I suppose, and formalin concentration. Some people split the corpus callosum and open the ventricles, some pre-cut the brain entirely in half, some string them up, some float them, some surround them with wads of paper or gauze, etc., but I'm not sure any of those things actually makes a major difference in fixation time.