This document discusses different types of brain herniation seen on imaging. The most common types are subfalcine herniation and descending transtentorial herniation. Subfalcine herniation occurs when one hemisphere pushes across the midline under the falx cerebri. Descending transtentorial herniation occurs when the temporal lobe and hippocampus herniate through the tentorial incisura. Other types discussed include tonsillar herniation, ascending transtentorial herniation, and rare transdural herniations. Complications of herniations include hydrocephalus, nerve compression, and infarcts.
6. Subfalcine herniation
•most common
•supratentorial mass in one hemicranium
•affected hemisphere pushes across the midline under the inferior "free" margin of the falx, extending into the contralateral hemicranium
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9. Subfalcine herniation: imaging
Axial and coronal images show that
•cingulate gyrus
•anterior cerebral artery (ACA)
•internal cerebral vein (ICV)
are pushed from one side to the other under the falx cerebri.
The ipsilateral ventricle appears compressed and displaced across the midline
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11. Complications
•unilateral obstructive hydrocephalus
–foramen of Monro occlusion
•Periventricular hypodensity with "blurred" margins of the lateral ventricle
–Fluid accumulates in the periventricular white matter
12. Complications
•When severe, the herniating ACA can be pinned against the inferior "free" margin of the falx cerebri
secondary infarction of the cingulate gyrus
16. Descending transtentorial herniations
•the second most common
•a hemispheric mass
•initially produces subfalcine herniation
•As the mass effect increases, the uncus of the temporal lobe is pushed medially
begins to encroach on the suprasellar cistern
hippocampus follows
hippocampus effaces the ipsilateral quadrigeminal cistern
both the uncus and hippocampus herniate inferiorly through the tentorial incisura
20. unilateral DTH: imaging
early
uncus is displaced medially
Ipsilateral aspect of the suprasellar cistern effaced
Ipsilateral prepontine + cerebellopontine angle cistern enlarged
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22. Descending transtentorial herniation
As DTH increases
hippocampus also herniates medially
quadrigeminal cistern compression
midbrain pushed toward the opposite side of the incisura
23. Descending transtentorial herniation
severe cases
entire suprasellar and quadrigeminal cisterns are effaced.
The temporal horn can even be displaced almost into the midline
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25. bilateral DTH
both hemispheres become swollen
the whole central brain is flattened against the skull base
All the basal cisterns are obliterated
hypothalamus and optic chiasm are crushed against the sella turcica
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28. Complete bilateral DTH
both temporal lobes herniate medially into the tentorial hiatus
midbrain and pons displaced inferiorly through the tentorial incisura
The angle between the midbrain and pons
is progressively reduced from 90° to almost 0°
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30. Complications
•CN III (oculomotor) nerve compression
–CN III palsy
•PCA occlusion as it passes back up over the medial edge of the tentorium
–secondary PCA (occipital) infarct
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33. Kernohan notch
•As the herniating temporal lobe pushes the midbrain toward the opposite side of the incisura
–contralateral cerebral peduncle is forced against the hard edge of the tentorium
•Pressure ischemia ipsilateral hemiplegia
–the "false localizing" sign
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37. Duret hemorrhage
"Top-down" mass effect displaces the midbrain inferiorly
closes the midbrain-pontine angle
Perforating arteries from basilar artery
are compressed and buckled
secondary hemorrhagic midbrain infarct
40. hypothalamic and basal ganglia infarcts
complete bilateral DTH
perforating arteries from the circle of Willis compression against the central skull base
hypothalamic and basal ganglia infarcts
46. Tonsillar herniation
•The cerebellar tonsils are displaced inferiorly and become impacted into the foramen magnum.
•congenital (e.g., Chiari 1 malformation)
– mismatch between size and content of the posterior fossa
•Acquired
–an expanding posterior fossa mass pushing the tonsils downward—more common
–intracranial hypotension: abnormally low intraspinal CSF pressure
•tonsils are pulled downward
47. Tonsillar herniation: imaging
•Diagnosing tonsillar herniation on NECT scans may be problematic.
Cisterna magna obliteration
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49. Tonsillar herniation: imaging
•MR: much more easily diagnosed
•In the sagittal plane
–the tonsillar folia become vertically oriented
–the inferior aspect of the tonsils becomes pointed
–Tonsils > 5 mm (or 7 mm in children) below the foramen magnum are generally abnormal
•especially if they are peg-like or pointed (rather than rounded)
50. Tonsillar herniation: imaging
•In the axial plane, T2 scans show that the tonsils are impacted into the foramen magnum
–obliterating CSF in the cisterna magna
–displacing the medulla anteriorly
61. OTHER LESS COMMON HERNIATION: TRANSALAR TRANSDURAL/TRANSCRANIAL HERNIATIONS
62. Transalar Herniation
•brain herniates across the greater sphenoid wing (GSW) or "ala"
•ascending > descending
63. Ascending transalar herniation
•caused by a large middle cranial fossa mass
•An intratemporal or large extraaxial mass
Temporal lobe + sylvian fissure + MCA
up and over the greater sphenoid wing
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67. Descending transalar herniation
•caused by a large anterior cranial fossa mass
Gyrus rectus is forced posteroinferiorly over the GSW
displacing the sylvian fissure and shifting the MCA backward
71. Transdural/Transcranial Herniation
•MR best depicts these unusual herniations.
•The disrupted dura
–discontinuous black line on T2WI
–Brain tissue, blood vessels, and CSF, are extruded through the defects into the subgaleal space
80. References
•Osborn, Anne G. "Secondary Effects and Sequellae of CNS Trauma."Osborn's Brain: Imaging, Pathology, and Anatomy. Salt Lake City, UT: Amirsys Pub., 2013. N. pag. Print.
•Osborn, Anne G. "Cerebral Vasculature: Normal Anatomy and Pathology."Diagnostic Neuroradiology. St. Louis: Mosby, 1994. N. pag. Print.
•Kaewlai, R. Imaging of Traumatic Brain Injury. 2013. Web.