Fig. 23.4
Subfalcine (cingulate) herniation occurs when unilateral or asymmetric expansion of a cerebral hemisphere displaces the cingulate gyrus under the edge of the falx. This may compress the anterior cerebral artery.
Transtentorial (uncinate) herniation occurs when the medial aspect of the temporal lobe is compressed against the free margin of the tentorium. As the temporal lobe is displaced, the third cranial nerve is compromised, resulting in pupillary dilation and impaired ocular movements on the side of the lesion (“blown pupil”). The posterior cerebral artery may also be compressed, resulting in ischemic injury to tissue supplied by that vessel, including the primary visual cortex. With further displacement of the temporal lobe, pressure on the mid-brain may compress the contralateral cerebral peduncle against the tentorium, resulting in hemiparesis ipsilateral to the side of the herniation. The compression of the peduncle creates a deformation known as Kernohan’s notch. Compression of the midbrain and the ascending reticular activating system with transtentorial herniation leads to depressed consciousness. Progression of transtentorial herniation is often accompanied by linear or flame-shaped hemorrhages in the midbrain and pons, termed Duret hemorrhages (
Fig. 23.5 ). These lesions usually occur in the midline and paramedian regions and are believed to be the result of tearing of penetrating veins and arteries supplying the upper brain stem.
Tonsillar herniation refers to displacement of the cerebellar tonsils through the foramen magnum. This type of herniation causes brain stem compression and compromises vital respiratory and cardiac centers in the medulla, and is often fatal.
Fig. 23.3 Hydrocephalus.Dilatedlateralventriclesseeninacoronalsectionthroughthemid-thalamus.
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Fig. 23.4 Herniationsyndromes.Displacementofbrainparenchymaacrossfixedbarrierscanbesubfalcine,transtentorial,ortonsillar(intotheforamenmagnum).
EDEMA,HERNIATION,ANDHYDROCEPHALUS
Cerebraledemaistheaccumulationofexcessfluidwithinthebrainparenchyma.HydrocephalusisdefinedasanincreaseinCSFvolumewithinallorpartoftheventricularsystem.
Increasesinbrainvolume(asaresultofincreasedCSFvolume,edema,hemorrhage,ortumor)raisethepressureinsidethefixedcapacityoftheskull.
Increasesinpressurecandamagethebrainbydecreasingperfusionorbydisplacingtissueacrossduralpartitionsinsidetheskullorthroughopeningsintheskull(herniations).
Fig. 23.5 Durethemorrhage.Asmasseffectdisplacesthebraindownward,thereisdisruptionofthevesselsthatentertheponsalongthemidline,leadingtohemorrhage.