Kernohan notch confusion

[virajpatel]

A bit confused why compression of contralater superior peduncle causes ipsilateral paresis on the side of the lesion. Superior peduncle --> contralateral cortex (in this case side on the lesion) --> contralteral side of the body. So wouldn't the compression on contralateral penducle via herniation cause a contralateral paresis and ipsilateral paresis. I'm mainly confused by the reasoning in FA which states that the kernohan notch results in ipsilateral paresis but I'm wondering why it wouldn't be contralateral.

Apologize the wordiness, I did my best to explain my thought process.

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[NecrotizingFasciitis]

I was kind of confused on this too. Mainly commenting so someone can either confirm/deny my reasoning.

The way I rationalized it in my head is that the corticospinal tract decussates in the medulla, which is inferior to the midbrain, where the cerebral crus is located. So then when the uncus herniates & "compresses the contralateral cerebral crus," there is an ipsilateral hemiparesis.

As to why it only compresses only the contralateral cerebral crus, I think it's an indirect mass effect type of thing. Originally I thought the uncus translocated to the contralateral cerebral crus & made direct contact with the cerebral crus (or something?), but I think what actually happens is the contralateral cerebral crus moves through the kernham notch on it's respective side & is compressed by the structures it herniates towards. The ipsilateral cerebral crus (w/ respect to the herniated uncus) is moving away from it's respective kerham notch & not towards any structure that would compress it.

Again, don't know if this is correct, but that's kind of how I tried to work through the concept...

Here's an image that I looked at to try to understand:

 

[virajpatel]

Appreciate the explanation, that definitely makes sense. Just want to clarify though, so you're saying that when the uncut herniates it compresses the corticospinal tract (let's saying herniation on left) so right tract compressed causing left sided motor symptoms. However, isn't the cerebral crus a part of cerebellum peduncle so as a result the contralateral cerebellar output to the motor cortex (on left side) would be impaired, as well?