Uncal (transtentorial) herniation

What it is

Uncal, or transtentorial, herniation involves displacement of the temporal lobe uncus under the tentorium cerebelli. The uncus then presses on the third cranial nerve, which is why the syndrome is read from the eye. It is the herniation seen with an epidural hematoma, where rapid intracranial expansion under systemic arterial pressure leads to transtentorial herniation and CN III palsy.

Causes and risk

The named cause is the epidural hematoma: rupture of the middle meningeal artery, a branch of the maxillary artery, often after a skull fracture involving the pterion, the thinnest area of the lateral skull. On CT the epidural hematoma is a biconvex, or lentiform, hyperdense blood collection that does not cross suture lines, and a lucid interval belongs to the same story. More generally, uncal herniation through the tentorium is listed among the causes of a third nerve palsy if ICP is raised, where it is a false localizing sign.