Brain herniation: a revision of classical concepts.

This paper is an update on evolving ideas about brain herniations. Following observations on cerebellar pressure coning that raised doubts about its reputed lethal connotations, herniation at the tentorium was re-examined for its role in critically damaging the brain stem. Combining clinical, pathologic, computed tomography and magnetic resonance imaging data, it is concluded that temporal lobe herniation is not the means by which the midbrain sustains irreversible damage in acute cases, but rather lateral displacement of the brain at the tentorium is the prime mover and herniation a harmless accompaniment. Transtentorial herniation has been investigated with computed tomography using the three calcification relationship and descent through the tentorial opening could not be documented. Bilateral brain stem compression in acute bilateral cases must be distinguished from herniation. Upward cerebellar herniation is only the sign of an overfull posterior fossa. Subfalcial herniation is tolerated unless lateral displacement is excessive.