How is the cranial cavity divided?
The cranial cavity is separated into compartments by infoldings of the dura. The right and left cerebral hemispheres are separated by the falx and the anterior and posterior fossa are separated by the tentorium.
What is a herniation?
When a part of the brain tissue is displaced into a compartment that it does not belong to, it is called as herniation.
Which is the most common type of herniation?
It is transtentorial herniation. Why is it called transtentorial? It is because the herniation happens from the supratentorial to the infratentorial compartments through the tentorial opening,
When the uncus, that is the anterior medial temporal gyrus herniates into the tentorial opening just anterior to and adjacent to the midbrain it is called as uncal herniation. Now the displaced brain tissue compresses the third nerve. What happens as a result of this? There is mydriasis and opthlamoplegia of the ipsilateral pupil
The other type of transtentorial herniation is the herniation of the thalamic medial structures through the tentorial opening with compression of the upper midbrain. Miosis and drowsiness are the heralding signs of this herniation
These are caused by herniation of the medial aspect of the cerebral hemisphere (cingulate gyrus) under the falx, which may compress the anterior cerebral artery.
Masses in the cerebellum may cause tonsillar herniation, in which the cerebellar tonsils are herniated into the foramen magnum. This may compress the medulla and respiratory centers, causing death.
Tonsillar herniation may also occur if a lumbar puncture is performed in a patient with increased intracranial pressure. Therefore, before performing a lumbar puncture, the patient should be checked for the presence of papilledema.