The displacement of the brain tissue between the intracranial compartments is called herniation. The intracranial cavity is divided by the tentorium cerebelli into two main compartments, supratentorial and infratentorial (posterior fossa).
Subfalcine Herniation (Cingulate Herniation)
• The cingulate gyrus medial to the frontal lobe crosses.
• The anterior cerebral artery remains under pressure.
• It has no specific clinical findings.
• It is the most common herniation type detected radiologically.
Transtentorial Herniation
• A lateral supratentorial mass may push the uncus and hippocampus through the tentorium opening.
• It is divided into two as axial (central) and uncal herniation.
1. Axial (central) herniation
It is seen in lesions located in the upper part of the cerebral hemispheres.
It may occur in lesions such as chronic subdural hematoma located bilaterally.
Bilateral myotic pupil, limitation of upward gaze, and Cheyne-Stokes respiration are clinical features.
2. Uncal herniation (lateral transtentorial hernia)
It is the most common symptom-producing herniation type.
It causes supratentorial masses.
Posterior cerebral artery, 3rd cranial nerve is under pressure.
Ipsilateral mydriasis develops.
Pyramidal irritation findings and hemiparesis develop on the opposite side of the lesion.
Later, the RAS is affected and unconsciousness occurs.
In the following period, the brain stem becomes compressed at the opposite tentorium edge and makes hemiparesis on the same side as the lesion (Kernohan's sign).
Transtentorial herniations may be accompanied by hemorrhages in the midbrain and upper pons that appear as linear or fountains; these are called "secondary brainstem hemorrhages" or "Duret hemorrhages".
Tonsillar Herniation
• It is caused by herniation of the cerebellar tonsils from the foramen magnum.
• They make brain stem compression.
• Sudden loss of consciousness, sudden respiratory loss, sudden intermittent opistotonus.
• Neck stiffness, loss of gag and cough reflexes are seen.
• Occurs most frequently in cerebellar tumors.