They are secondary to raised intracranial pressure with formation of a transtentorial pressure cone involving the cerebral peduncles (crus cerebri) and other midbrain structures caused by raised pressure above the tentorium.
Kernohan’s notch is a groove in the cerebral peduncle that may be caused by this displacement of the brainstem against the incisura of the tentorium. The resulting ipsilateral hemiparesis is a false localising sign, known as the Kernohan-Woltman syndrome. This may succeed or accompany temporal lobe (uncal) herniation and subfalcian herniation secondary to a supratentorial mass.
Imaging can be difficult.
The mechanism is uncertain but is probably caused by the displacement of the brainstem stretching and lacerating pontine perforating branches of the basilar artery; venous infarction may play a role.
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- Images of Duret Hemorrhage from MedPix