Herniation occurs when pressure of a mass lesion forces brain tissue to shift from one intracerebral compartment to another, and it can cause unconsciousness when pressure on the brainstem disrupts the reticular activating system.
The total volume contained in the cranial vault is limited, and a mass lesion must cause some shift of the intracerebral contents. Initially a mass lesion displaces cerebrospinal fluid, but eventually a limit is reached and intracranial pressure increases. Brain tissue is highly inelastic and so this pressure causes herniation.
Central herniation is presumed to be due to a pressure cone forcing the brain out towards its only exit, the foramen magnum. Uncal herniation occurs when the medial temporal lobe is forced over the tentorial edge into the space beside the lateral midbrain. This compresses the third cranial nerve and particularly the parasympathetic fibers to the pupil traveling around the outside of the nerve, causing unilateral dilation of the pupil on the side of the lesion, the 'blown pupil'.
Ro.p.p.er...(1.9.§6) has ascribed many of the signs of herniation to horizontal displacement of the brainstem. In this model, third-nerve paresis results from stretching of the nerve because of an increase in the distance from its origin on the anterior surface of the midbrain to the point at which it is tethered to the cavernous sinus. Acute pupillary dilation often corrects within minutes of initiating therapy for increased intracranial pressure, which is more consistent with lateral displacement than with irreversible uncal herniation.
Downward herniation can distract the basilar artery, which is tethered to the skull base, away from the brainstem and cause hemorrhage or infarction. Eventually, the downward movement increases subtentorial pressure and forces the cerebellar tonsils out through the foramen magnum. Tonsillar herniation can also occur if a lumbar puncture is performed on a patient with elevated intracranial pressure, thus generating a large transforamenal pressure gradient. This is often catastrophic, as acute pressure on the medulla causes sudden respiratory arrest. Upward herniation of brainstem structures through the tentorium is possible in the setting of large posterior fossa masses which increase subtentorial pressure.
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