tentorium cerebelli

**Anatomy** - A rigid, tent-shaped fold of meningeal dura mater. It lies between the occipital lobes above and the cerebellum below. - It attaches posteriorly and laterally to the occipital bone along the transverse sinus grooves. It attaches laterally to the petrous ridge (superior border of the petrous temporal bone). Anteriorly it attaches to the anterior and posterior clinoid processes. - The free (anterior-medial) edge forms the **tentorial notch (incisura)**. The midbrain passes through the notch. - It is continuous with the falx cerebri at the midline. The **straight sinus** runs along this junction. The falx cerebelli attaches inferiorly. - It contains or borders several dural venous sinuses: the transverse sinus (posterolateral attachment), the superior petrosal sinus (petrous attachment), and the straight sinus (midline junction with the falx cerebri). - **Innervation:** Mainly the tentorial branch (recurrent meningeal branch) of the ophthalmic division of the trigeminal nerve (CN V1). This is why supratentorial headaches are referred to the frontal/orbital region. Infratentorial pain refers to the occiput/neck via upper cervical nerves (C1-C3) and the vagus (CN X).

**Divisions** - Supratentorial: cerebral hemispheres, diencephalon. Innervation is via V1. - Infratentorial: cerebellum, pons, medulla. Innervation is via C1-C3 and CN X.

**Clinical significance: herniation syndromes** - **Uncal (transtentorial) herniation:** The medial temporal lobe (uncus) herniates over the free edge of the tentorium. Causes include a supratentorial mass, epidural or subdural hematoma, tumor, or edema. - **CN III compression** gives an ipsilateral "blown" pupil (fixed and dilated, "down and out"). Parasympathetic fibers are superficial and are affected first. - **Posterior cerebral artery compression** causes contralateral homonymous hemianopia with macular sparing (occipital infarct). - **Compression of the contralateral cerebral peduncle** against the tentorial edge produces ipsilateral hemiparesis. This is **Kernohan's notch phenomenon**, a false-localizing sign. Compression of the ipsilateral cerebral peduncle gives contralateral hemiparesis. - **Duret hemorrhages:** Midline and paramedian pontine/midbrain hemorrhages from stretching of the basilar artery perforators. They signal brainstem compression and a poor prognosis. - Late: coma, decorticate then decerebrate posturing, Cushing triad (hypertension, bradycardia, irregular respiration). - **Upward (reverse transtentorial) herniation:** Posterior fossa mass effect pushes the cerebellum upward through the notch. It can compress the midbrain and cause obstructive hydrocephalus (aqueductal compression). - **Tentorial (subdural) hemorrhage:** Neonatal tentorial tears from excessive molding or forceps delivery can cause subdural hemorrhage. Tears of the tentorium or falx lead to rupture of bridging veins or sinuses. - **Obstructive hydrocephalus:** Posterior fossa lesions (for example medulloblastoma in children, cerebellar hemangioblastoma) can obstruct the 4th ventricle. - **Other:** Meningiomas can arise from the tentorium. Cavernous sinus and petrosal sinus thrombosis relate to nearby dural venous sinuses.

**High-yield pearls** - The tentorial notch contains the midbrain. CN III exits the interpeduncular fossa, travels near the free edge of the tentorium, and runs lateral to the posterior communicating artery. This is why it is compressed so easily. - CN IV is the only cranial nerve that exits the brainstem dorsally. It travels along the tentorial edge. - Ipsilateral pupil dilation precedes motor findings in uncal herniation. - Subfalcine herniation (cingulate gyrus under the falx) compresses the ACA. Tonsillar herniation (through the foramen magnum) compresses the medulla and causes respiratory arrest.