Kernohan phenomenon

Last revised by Assoc Prof Frank Gaillard on 07 May 2022

Kernohan notch phenomenon is an imaging finding resulting from extensive midline shift due to mass effect, resulting in the indentation in the contralateral cerebral crus by the tentorium cerebelli. This has also been referred to as Kernohan-Woltman notch phenomenon and is a false localizing sign.

The integrity of the crus cerebri and its descending corticospinal tracts is disturbed, and a contralateral (to crus cerebri) motor deficit is produced with a deteriorating level of consciousness. A hemiparesis ipsilateral to the expanding mass is known as Kernohan phenomenon, which is a false localizing sign.

Transtentorial herniation is a well-described neurologic phenomenon caused by an expanding supratentorial mass lesion or hemorrhage. Uncal herniation is a common subtype of transtentorial herniation in which the innermost part of the temporal lobe, the uncus, is displaced beyond the free edge of the tentorium and applies pressure on the brainstem, most notably the midbrain.

Compressive forces cause the cerebral peduncle of 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 (to crus cerebri) motor deficit is produced with a deteriorating level of consciousness. 

Kernohan notch is demonstrated on cross-sectional imaging of the brain and is best seen in the coronal plane. 

On CT it is usually possible to show the mass effect of a tumor on the brain stem, but identification of an uncal herniation is much harder with CT.

MRI with its ability to image the brain in multiple planes and excellent resolution of the brainstem can reveal a deformity or injury of the cerebral peduncle resulting from a transtentorial herniation.  

The phenomenon was described by James Watson Kernohan (1896-1981), an Irish-born American pathologist in 1929 after an autopsy study revealed a notched cerebral peduncle from a contralateral herniation syndrome.

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