What is Kernohan's notch?

What is Kernohan’s notch?

This is our mini-series name mini bites of pathology today our question is What is Kernohan’s notch and Where does the brain go when the pressure’s too high?

The brain does not have a lot of wiggle room when something else shows up.

It’s sitting there in the cranium with just a tiny bit of space between the meninges and the skull. So what happens when something like a tumor or a hematoma appears in the brain and takes up space?

Well, the brain tissue squeezes around rigid structures (like the falx) and gets pushed down into the foramen magnum. This displacement of brain tissue is called herniation, and it tends to occur in three places.

Some patients have just one type of herniation – but if the pressure is high enough, all three types of herniation will occur.

Here’s a nice diagram from Robbins that illustrates these three types of herniation.

Kernohan's notch

Subfalcine (cingulate) herniation

  • Cingulate gyrus squeezes under the falx cerebri
  • Can compress the anterior cerebral artery, leading to focal symptoms

Transtentorial (uncinate) herniation

  • Medial part of temporal lobe compressed against free margin of the tentorium
  • Can compromise third cranial nerve (leading to pupillary dilation and impaired ocular movements)
  • Can compress contralateral peduncle (leading to hemiparesis on the same side as the herniation), causing a notch (Kernohan’s notch) in the peduncle
  • With progression, can cause brainstem hemorrhages (Duret hemorrhages)

Tonsillar herniation

  • Cerebellar tonsils displaced through the foramen magnum
  • Can compress brainstem (damaging respiratory and cardiac centers)

This is exactly the kind of stuff that I cover in my neuropath mini-course. If you need a little review in this area, you may want to check it out (below).

credit: Kristine Krafts by Pathology student

 

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