This diagram shows common forms of herniation due to raised intracranial
pressure:
1. Cingulate or subfalcine herniation. Unilateral expansion of a cerebral
hemisphere displaces the cingulate gyrus (part of the cerebral cortex) and
also may cause compression of the anterior cerebral artery.8
2. Central herniation. Injury to, or tumour growth within, the cerebral cortex
causes direct downward displacement of the thalamic structures onto the
brainstem, heralded by constricted pupils and decreased level of consciousness.
There follows rapid loss of consciousness, Cheynes-Stokes respirations
progressing to apnoea, and fi xed, dilated pupils.9
3. Uncal herniation. Displacement of the temporal lobe or hippocampus
causes compression of the third cranial nerve, causing sluggish and then
fi xed dilated pupils (initially on the same side as the herniation – ‘blown
pupil’). Compression of other structures may cause an ipsilateral hemiparesis,
and Cheynes-Stokes respiration, progressing to a neurogenic hyperventilation8,9.
4. Tonsillar herniation arises when the lower parts of the cerebellum (the
‘tonsils’) are compressed downward through the foramen magnum. Brainstem
respiratory and cardiac centres are affected, causing life-threatening disruption
of heart and lung function.