herniations are rare in neonates because the sutures and fontanels are
Two types of transtentorial herniations occur in the neonatal period:
uncal and central. Uncal herniation acts as an extraaxial posterior fossa
mass lesion. In uncal herniation, the uncus of the temporal lobe goes
through the tentorial incisure and pushes the brainstem, displacing it
to the side and compressing it. The initial clinical sign of uncal herniation
is a dilated pupil followed by outward and downward deviation of the eye.
These findings occur because of common oculomotor nerve compression. Signs
of common oculomotor nerve compression are followed by bilateral brainstem
signs because the uncus compresses the brainstem on the side of the herniation
and the edge of the tentorium cerebelli compresses the brainstem on the
opposite side. Evidence of progressive rostrocaudal brainstem damage appears
as uncal herniation progresses. Uncal herniation occurs with supratentorial
lesions, particularly with those that produce hemorrhage and swelling
of the temporal lobe. Uncal herniation has been reported to occur with
meningitis in the neonatal period.
Central herniation behaves as
a midline posterior fossa lesion. In central herniation, the diencephalon
is displaced across the tentorial incisure.