A large brain tumour in the right temporal and parietal lobe, with sudden weakness in the limbs on the right side.
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T1 post-gadolinium acquisition demonstrates a large enhancing mass with surrounding edema in the right hemisphere, causing compression of the lateral ventricle and herniation of brain tissue over the tentorium causing compression of the mesencephalon (arrows).
Due to the mass effect of the tumor, the medial part (uncus) of the right temporal lobe (arrow) is herniating over the tentorium (*) in the foramen magnum, thereby compressing the mesencephalon against the free edge of the contralateral tentorium and causing a notch on the left side.
Kernohan phenomenon is caused by an uncal transtentorial herniation causing compression of a cerebral peduncle (part of the mesencephalon). Consequently, the contralateral side of the brainstem is pressed against the free edge of the tentorium, resulting in an indentation called Kernohan notch.
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 notch in the left cerebral peduncle which results in right-sided motor impairment. This phenomenon can be clinically confusing since it is a secondary condition caused by a primary condition (tumor) on the opposite hemisphere of the brain.
With courtesy of Dr. R Noordveld.