The tentorium cerebelli is a dural tent-like fold separating the supratentorial compartment (cerebrum) from the infratentorial compartment (cerebellum and brainstem). It attaches laterally to the petrous ridges and posterolaterally to the occipital bone, containing the transverse sinuses at its attached border. Its free anteromedial border creates the tentorial notch (incisura tentorii) through which the midbrain passes and which defines the boundary between the supra- and infratentorial spaces.
Provides structural support to the posterior cerebrum overlying the tentorium; creates the rigid division between supra- and infratentorial compartments that limits cross-compartmental brain displacement under normal conditions; houses the transverse sinuses.
Transtentorial (uncal) herniation occurs when a supratentorial mass pushes the medial temporal lobe (uncus) through the tentorial notch, compressing the ipsilateral CN III (producing pupil dilation — 'blown pupil') and then the contralateral cerebral peduncle (causing ipsilateral hemiplegia — Kernohan's notch). Posterior fossa surgery approaches to the cerebellum, brainstem, and posterior fossa tumours work in the infratentorial compartment defined by the tentorium.
Supratentorial mass effect herniates the medial temporal uncus through the tentorial notch, compressing the ipsilateral CN III against the free tentorial edge producing ipsilateral fixed dilated pupil, then compressing the midbrain causing progressive consciousness deterioration; emergency CT and neurosurgical intervention to decompress the mass are required.
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