Mechanism
Kernohan's notch phenomenon is a result of the compression of the cerebral peduncle, which is part of the mesencephalon, against the tentorium cerebelli due to transtentorial herniation. This produces ipsilateral hemiparesis or hemiplegia
The skull is an incompressible closed space with a limited volume (Monro-Kellie Doctrine). When there is increased cranial pressure in the brain, a shift in the brain forms towards the only opening of the skull, the foramen magnum. Thus, when an increase of pressure in a hemisphere of the brain exists, the cerebral peduncle on the opposite hemisphere is pushed up against the tentorium, which separates the posterior fossa from the middle fossa. This produces a visible "notch" in the cerebral peduncle. Because of the fact that a Kernohan's notch is caused by an injury creating pressure on the opposite hemisphere of the brain, it is characterized as a false localizing sign.
The Kernohan's notch phenomenon is unique in that it is not only a false localizing sign, but is also ipsilateral or same-sided. The left cerebral peduncle contains motor fibers that cross over to the right side of the body. Thus, if you have a right hemisphere trans-tentorial herniation, it causes a Kernohan's notch in the left cerebral peduncle which results in right-sided motor impairment. Therefore you get, paradoxically, impairment of motor function on the same side of the body as the herniation which caused the Kernohan's notch on the controlateral side.
Read more about this topic: Kernohan's Notch
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