What is the meaning of Kernohan's notch?

Question

I was going through the CNS notes about transtentorial herniation and there was a term called Kernohan's notch which I couldn't understand. Please shed some light.

Answer 1

In Transtentorial or Uncinate herniation, **contralateral cerebral peduncle** is compressed resulting in **hemiparesis ipsilateral** to the side of the herniation. Because hemispheric lesions typically cause contralateral weakness, this ipsilateral hemiparesis can be a false localizing sign that would suggest to the examiner that the patient has a lesion in the opposite, unaffected hemisphere.

The changes in the peduncle in this setting are known as Kernohan's notch.

**Example:**
Herniation of right hemisphere → Compression of left hemisphere → Symptoms on right side of the body

Answer 2

Uncal herniation is a common sub-type of transtentorial herniation in which the innermost part of the temporal lobe (the uncus) can be squeezed so much that it goes by the tentorium and puts pressure on the brainstem, most notably the midbrain.

Compressive forces cause the midbrain to impinge on the contralateral edge of the tentorium cerebelli, forming an indentation in the crus known as Kernohan's notch. With time, the integrity of the crus cerebri and its descending corticospinal tracts is disturbed, and a contralateral motor deficit is produced with a deteriorating level of consciousness. A hemiparesis ipsilateral to the expanding mass is known as Kernohan's phenomenon, which is a false localizing sign.

Answer 3

**Kernohan's notch** is a cerebral peduncle indentation associated with some forms of transtentorial herniation . It is a secondary condition caused by a primary injury on the opposite hemisphere of the brain.[3] Kernohan's notch is an ipsilateral condition, in that a left-sided primary lesion evokes motor impairment in the left side of the body and a right-sided primary injury evokes motor impairment in the right side of the body. The seriousness of Kernohan's notch varies depending on the primary problem causing it, which may range from benign brain tumors to advanced subdural hematoma.