Transtentorial Herniations
Transtentorial herniations are rare in neonates because the sutures and fontanels are open. 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.