How does an LP cause a tonsillar herniation?

[bassfishindoc]

I understand that performing a LP on a pt with elevated ICP can cause a tonsillar herniation, however, I only understand this because that is what my notes tell me. Can anybody explain the pathophysiology behind this? Thanks.

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[FutureInternist]

I understand that performing a LP on a pt with elevated ICP can cause a tonsillar herniation, however, I only understand this because that is what my notes tell me. Can anybody explain the pathophysiology behind this? Thanks.

Closed system with high pressure inside....you release the pressure from one end only....the other end pushes down due to pressure difference.

 

[putyou2sleep]

What he said.

The assumption in saying that it can cause a herniation is that there is increased pressure. This is not always the case, but in instances of a cerebral mass, bleed, swelling, or hydrocephalus you may have increased pressure at the top - leading to downward push as you open the bottom.

Pretty rare occurrence as I understand it, but is a lot of the justification that people use for ordering a CT before an LP if they are concerned about a mass-occupying lesion.