A 68 year-old woman with a known glioblastoma multiforme presented with approximately one week of confusion and drowsiness. Examination showed word-finding difficulty, a right homonymous hemianopsia, and right arm apraxia. The left pupil was larger than the right and poorly reactive.

The tentorium is a dural structure that separates the cerebrum from the brainstem and cerebellum that lie in the posterior cranial fossa below. The opening in the tentorium through which the brainstem, specifically the midbrain, is connected to the cerebrum is called the tentorial incisura. The presence of a large supratentorial mass in one hemisphere often increases pressure from above, resulting in herniation, whereby part of the cerebrum herniates, or is pushed through the tentorial incisura. The structure that herniates first is usually the uncus of the medial temporal lobe - thus the term "uncal herniation", a subset of descending transtentorial hernations. As the uncus herniates, it first presses against the midbrain, resulting in an ipsilateral third nerve palsy. Because the parasympathetic fibers lie on the outside of the third nerve, the first sign of uncal herniation is usually pupillary dilatation. Further compression results in paralysis of extraocular muscles.

Because transtentorial herniation occurs most commonly from a supratentorial mass, the patient usually already has a contralateral hemiplegia. As the brainstem becomes torqued, the contralateral cerebral peduncle may become compressed against the tentorial notch (i.e., Kernohan’s notch), resulting in quadriplegia (contralateral hemiplegia from the initial lesion, ipsilateral hemiplegia from Kernohan’s notch phenomenon). As herniation proceeds, dysfunction of both cerebral hemispheres occurs, followed by dysfunction of the brainstem. As this occurs, abnormal posturing is seen.

The displacement of brain structures described above illustrates the Monro-Kellie doctrine, which states that in an adult the cranial volume is a constant. The cranial contents consist primarily of brain, cerebrospinal fluid (CSF) and blood vessels. If a mass such as a hematoma, tumor or edema develops, these elements must shift to accommodate the mass. Since the cranial volume is a constant, part of the cranial contents will herniate through the tentorial incisura to make room for the mass. The opposite occurs in a patient with loss of brain mass, such as occurs after a stroke, wherein the CSF spaces often enlarge to fill the void.