Snapshot A 70-year-old man presents to the emergency department with confusion after having a convulsive episode. Prior to having a seizure, the patient reported to having a progressively worsening headache that awoke him from sleep, and right-sided weakness over the course of 7 months. On physical exam, there is weakness 2/5 strength throughout the right-side, and a left pupil that is unresponsive to light. A computerized tomography (CT) scan of the head shows a ring-enhancing lesion with surrounding cerebral edema. (Uncal herniation) Introduction Clinical definition brain tissue herniation that can result in compression of brain tissue vascular supply Pathogenesis space occupying masses can result in mass effect (intracranial structure displacement) and they include tumor edema hemorrhage Herniation Syndromes Three Clinically Important Brain Herniation Syndromes Herniation Syndrome Mechanism Clinical Findings Transtentorial herniation The medial temporal lobe (especially, the uncus) herniates through the tentorial notch Uncal herniation triad ipsilateral unresponsive ("blown") pupil hemiplegia typically contralateral; however, if midbrain is compressed on the opposite side it can result in ipsilateral hemiplegia(Kernohan's phenomenon) decreased level of consciousness secondary to compression of the midbrain reticular formation and can progress to coma Central herniation The brainstem becomes downwardly and centrally displaced Unilateral or bilateral lacteral rectus palsy in cases of mild central herniation that compresses the abducens nerve Bilateral uncal herniation in cases of significant central herniation Tonsillar herniation cerebellar tonsils herniates through the foramen magnum that can result in compression of the midbrain that leads to respiratory arrest cardiovascular instability death Subfalcine herniation The cingulate gyrus (as well as other structures) herniates under the falx cerebri At times this can lead to anterior cerebral artery compression under the falx cerebri resulting in infarction