Types of Cerebral Herniation and RX
Types of Cerebral Herniation and RX
Types of Cerebral Herniation and RX
org
1598
Brain Imaging
Anatomic Landmarks
is fixed and there is little room for expansion. As Anatomic landmarks are boundaries used as a
the Monro-Kellie hypothesis states, the sum of reference for the different brain compartments
volumes of the brain, CSF, and intracranial blood that help determine if a specific brain structure
is constant. An increase in the volume of one is displaced.
component will result in a decrease in the volume
of one or both of the other components (2). Direction of Mass Effect
When there is a change in the intracranial If there is any disease that causes mass effect, it is
volume that exceeds these compensation mecha- important to establish its location and determine
nisms, brain tissue will be displaced from one the direction of the vector force it creates. This will
compartment into another. It can be through point out the brain structures that may be dis-
anatomic or acquired spaces. Brain edema, placed or involved.
tumors, or hemorrhage are causes of cerebral
herniation secondary to an increase in volume Displaced Structure
and intracranial pressure (ICP). A decrease in Identifying the displaced structure is necessary to
ICP can also produce herniation, as in paradoxi- classify the type of hernia. Knowledge and ad-
cal herniation (1,3). equate evaluation of specific anatomic regions that
Brain herniation can be classified into two can herniate are fundamental.
broad categories: intracranial and extracra-
nial. Furthermore, intracranial hernias can be Indirect Signs
subdivided into three basic types: (a) subfalcine Sometimes the herniation can be subtle and dif-
hernia; (b) transtentorial hernia, which can be ficult to identify at first glance. Aside from look-
ascending or descending (lateral and central); ing at the specific brain structure that might be
and (c) tonsillar hernia (Table, Fig 1) (4,5). displaced, evaluating other potentially involved
Brain herniation may cause brain pressure structures can provide valuable information by
necrosis, compress cranial nerves and vessels showing indirect signs of the herniation.
causing hemorrhage or ischemia, and obstruct
the normal circulation of CSF, producing hydro- Herniation-related Complications
cephalus. Therefore, each type of hernia may be Brain herniation may cause different complica-
associated with a specific neurologic syndrome. tions secondary to compression of vessels, nerves,
Knowledge of the clinical manifestations ensures and the ventricular system. Stroke of the anterior
a focused imaging analysis. cerebral artery, posterior cerebral artery, or poste-
The most useful imaging modalities are CT rior inferior cerebellar artery occurs owing to vas-
and MRI. In the emergency setting, CT is regu- cular compression. Hydrocephalus manifests when
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Clinical Information
Type of and Neurologic Syn- Anatomic Direction of Displaced
Hernia dromes Landmarks Mass Effect Structure(s) Indirect Signs
Subfalcine Anterior cerebral Midline, falx Medial and Cingulate gyrus Dilatation of
artery syndrome cerebri, cingu- anterior, and CC contralateral
late gyrus, CC, beneath falx ventricle due to
and Monro compression of
foramen contralateral fo-
ramen of Monro
Transtentorial Paralysis of third Tentorium, Downward Anterior: uncus Displacement, rota-
descending nerve, compres- perimesence- from Posterior: parahip- tion, and elonga-
sion of PCA and phalic cisterns supratento- pocampal gyrus, tion of brainstem
choroidal arter- rial com- isthmus of for- Anterior or poste-
ies (occipital and partment, nical gyrus, and rior: widening of
medial temporal through anterior portion contralateral ven-
infarction) tentorial of lingual gyrus tricular atrium
incisura Central: dienceph- and temporal
alon, midbrain, horn
and pons Central: hydro-
cephalus
Transtentorial Manifestations of in- Tentorium, Upward from Superior cerebel- Obliteration of ip-
ascending creased ICP, brain- superior poste- lar hemispheres silateral perimes-
stem and cerebellar cerebellar and rior fossa and vermis, encephalic and
compression quadrigeminal through superior and contralateral
PCA and SCA cisterns tentorial inferior colliculi, crural cisterns
compression (oc- incisura midbrain Anterior displace-
cipital cerebral and ment of brain-
superior cerebellar stem, hydro-
infarction) cephalus
Tonsillar Manifestations of Foramen mag- Downward Cerebellar tonsils Effacement of peri-
brainstem and num (McRae through Pons medullary CSF
cerebellar com- line) foramen Medulla through foramen
pression Cerebellar tonsil magnum magnum
PICA compression Obliteration of cis-
(posterior infe- terna magna and
rior cerebellum, fourth ventricle
inferior cerebellar Vertical orientation
vermis, and lateral of folia of tonsil
medulla infarction)
Note.—CC = corpus callosum, PCA = posterior cerebral artery, PICA = posterior inferior cerebellar artery,
SCA = superior cerebellar artery.
there is involvement of the foramen of Monro or the skull. Anteriorly, it is fixed to the crista galli;
aqueduct of Sylvius. Cranial nerves may be af- posteriorly, it widens and adheres to the tento-
fected when there is involvement of the brainstem rium. Immediately inferior to the free edge of the
and basal cisterns. falx is the corpus callosum and cingulate gyrus.
The pericallosal artery runs through the perical-
Relevant Anatomy losal sulcus (Fig 4a) (7,10).
The cranial cavity is divided by bony landmarks The tentorium cerebelli extends inferiorly and
and reflections of the dura mater. The main dural laterally from its confluence with the falx (10).
reflections are the falx cerebri and tentorium It has a U-shaped opening called the tentorial
cerebelli, which divide the cranial cavity into right incisura, which provides communication between
and left cerebral hemispheres and the posterior the supratentorial space and the posterior fossa,
fossa, thus defining the supra- and infratentorial a potential herniation site. The midbrain and
compartments (Fig 3) (7,9). cerebral peduncles pass through the incisura. The
The falx cerebri has an anteroposterior orien- uncus and hippocampus are located just superior
tation and is attached superiorly to the inside of to the medial edges of the incisura.
RG • Volume 39 Number 6 Riveros Gilardi et al 1601
Figure 1. Drawings depict different kinds of brain herniation. ATH = ascending transtentorial
hernia, DTH = descending transtentorial hernia.
Figure 2. Approach to diagnosing brain herniation syndromes. Diagram shows the
six-point guideline for analysis of cerebral herniation cases. ACA = anterior cerebral ar-
tery, PCA = posterior cerebral artery, PICA = posterior inferior cerebellar artery.
The basal cisterns are spaces filled with encephalic cistern, close to the free edge of
CSF and located in the subarachnoid space. the tentorium. The oculomotor nerve exits the
They contain the proximal portions of some midbrain anteriorly and courses medially to the
cranial nerves and basal cerebral arteries. They uncus on its way to the cavernous sinus. These
are in close contact with the main intracranial structures are at risk of compression by the
structures (Fig 4b, 4c) (11). Basal cisterns are herniated tissue.
involved in almost any hernia type, making them Finally, the ventricular system is a set of cavi-
a key anatomic landmark. ties that produce and circulate CSF through the
The posterior cerebral arteries, anterior central nervous system. It consists of two lateral
choroidal arteries, and basal veins of Rosenthal ventricles divided by the septum pellucidum.
pass around the midbrain through the perimes- They communicate with the third ventricle via
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Intracranial Hernias
Subfalcine Hernia
Subfalcine hernia, also known as midline shift
or cingulate hernia, is the most common type of
cerebral hernia. It is generally caused by unilat-
eral frontal, parietal, or temporal lobe disease
that creates a mass effect with medial direction,
pushing the ipsilateral cingulate gyrus down and
under the falx cerebri. Figure 3. Drawing shows the main dural reflections.
The anterior falx, although rigid, is displaced
secondary to the mass effect. On the other hand,
the posterior falx, wider and more rigid, will DTH may be divided into two types: lateral
resist the displacement. This explains why subfal- (anterior and posterior) and central hernias. Lateral
cine hernias occur anteriorly. hernias involve the medial temporal lobe. In the
The septum pellucidum deviates at the level of anterior subtype, the uncus is herniated downward
the foramen of Monro, which serves as a land- into the ipsilateral crural cistern. In the posterior
mark for quantification of the degree of midline subtype, the parahippocampal gyrus is displaced
shift (12). This shift can be measured on axial im- downward into the posterolateral part of the tento-
ages by drawing a central line at the level of the rial incisura (9). Finally, in central hernias, there
foramen of Monro and measuring the distance is descent of the diencephalon, midbrain, and pons
between this line and the displaced septum pel- (12). This classification can be understood as a con-
lucidum (Fig 5). In subfalcine hernias, the degree tinuum representing the progression of DTH.
of midline shift correlates with the prognosis; In this type of hernia, the pressure caused by
less than 5-mm deviation has a good prognosis, the crowding of tissue within the incisura compro-
whereas a shift of more than 15 mm is related to mises the third cranial nerve, posterior cerebral
a poor outcome (13). artery, and midbrain. Hydrocephalus develops
In more severe hernias, the displaced tissue because of the compression of the cerebral aque-
may compress the corpus callosum and contra- duct. In cases with severe and abrupt downward
lateral cingulate gyrus, as well as the ipsilateral displacement of the brainstem, stretching and
ventricle and both foramina of Monro, causing shearing of perforating branches of the basilar
dilatation of the contralateral ventricle (Fig 6). artery occur, resulting in ischemia and hemor-
There may also be focal necrosis of the cingulate rhage in the brainstem. Usually, these findings are
gyrus due to direct compression against the falx located near the pontomesencephalic junction.
cerebri (7,12). Compromise of these structures However, the effect can be multiple or even extend
manifests clinically as hypobulia, apathy, and into the cerebellar peduncles. This is called Duret
indifference (14). Subfalcine hernias are best hemorrhage; it is a late finding and portends a
demonstrated at coronal MRI (Fig 7). Another poor prognosis, usually death (12).
potential complication is compression of the an- It is important to note that different types
terior cerebral artery, specifically the pericallosal of cerebral hernias can be present at the same
artery, with consequent infarction of the cor- time. In DTH, if there is further descent of brain
responding vascular territory (4,7) (Fig 8). The tissue, a tonsillar hernia might occur. Also, a sub-
most common clinical manifestation of anterior falcine hernia may be present, depending on the
cerebral artery–territory infarction is contralateral location of the disease.
leg weakness (14).
Lateral Hernia
Descending Transtentorial Hernia Lateral hernias occur when the medial temporal
Descending transtentorial hernia (DTH) is the lobe is displaced downward through the tento-
second most common type of cerebral hernia. It rium incisura. They can be divided into anterior
occurs when brain tissue is displaced downward and posterior hernias, depending on the portion
through the tentorial notch (9). that is displaced.
RG • Volume 39 Number 6 Riveros Gilardi et al 1603
Figure 4. Relevant radiologic anatomy in cerebral hernias. (a) Coronal T2-weighted MR image shows the cere-
bral falx in the interhemispheric fissure (arrow), tentorium (white arrowheads), tentorial incisura (dashed oval),
corpus callosum (CC), cingulate gyrus (CG), hippocampus (H), and pericallosal sulcus with the pericallosal artery
(black arrowhead). (b) Sagittal T1-weighted MR image at the midline of the cranial cavity depicts the cisterna
magna (CM), interpeduncular cistern (IPC), medullary cistern (MC), pontine cistern (PC), quadrigeminal cistern
(QC), and supracerebellar cistern (SCC). The corpus callosum (CC), cingulate gyrus, and clivus (*) are also noted.
The brainstem divisions are as follows: medulla (M), midbrain (Mb), and pons (P). (c) Axial T2-weighted MR im-
age shows the aqueduct (arrowhead), posterior cerebral artery (arrow), crural cistern (CrC), hippocampal gyrus
(HG), interpeduncular cistern (IPC), perimesencephalic cistern (PMC), quadrigeminal cistern (QC), and uncus (U).
Anterior Hernia.—Anterior (or uncal) hernia is the is widening of the ipsilateral perimesencephalic
better understood subtype of DTH (12). Usually, cistern, with displacement and rotation of the
a unilateral supratentorial lesion (particularly in brainstem (Figs 9, 10). With more advanced
the middle cranial fossa) causes an inferior and herniation, the midbrain and opposite cerebral
medial mass effect that pushes the uncus over the peduncle are compressed against the tentorial
free edge of the tentorium (7). It is the first event edge (Fig 9b) (7,12). Descending corticospinal
in most cases of DTH, usually followed by her- and corticobulbar tracts may be affected above
niation of more posteriorly located brain tissue. the medullary decussation, resulting in motor
However, the distribution and sequence of the weakness on the same side as the lesion, known
DTH will also depend on certain factors, such as as the Kernohan notch phenomenon (false lo-
the location of the disease and the size and con- calizing sign) (15).
figuration of the incisura (9). Compression of the posterior cerebral artery,
The initial displacement of the uncus results third cranial nerve, and aqueduct of Sylvius may
in effacement of the suprasellar cistern, the ear- result in medial temporal and occipital lobe in-
liest finding in this type of hernia. Often, that is farcts, blown pupil, hemiparesis, and hydrocepha-
all it effaces. As the herniation progresses, there lus (Fig 11) (1).
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Figure 9. DTH in a 38-year-old man with an acute head injury and left subdural hematoma. There is
right shift and mild rotation of the brainstem. (a) Axial nonenhanced CT image shows widening of the
left basal cistern (arrow) and effacement of the right basal cistern (dashed line), as well as dilatation of the
temporal horn of the right lateral ventricle (*). (b) CT image shows compression and rotation of the mid-
brain, which appears elongated (arrowheads). There is complete obliteration of the perimesencephalic
cisterns. Note the widening of the opposite ventricular atrium and temporal horn (*).
fossa with an upward direction, displaces the tissue will slide through the foramen magnum,
cerebellar vermis and hemispheres superiorly causing tonsillar herniation. On the other hand,
through the tentorial incisura. It is more likely to when the tentorial opening is large, upward
occur when the mass originates near the inci- herniation of the superior cerebellar vermis will
sura, like in the cerebellar vermis (10). Another occur before tonsillar herniation (10).
possible cause is sudden relief of supratentorial As there is upward herniation of the cerebel-
intracranial hypertension (7). lar vermis, anterior displacement of the midbrain
The tentorial incisura size is variable and and cerebral aqueduct takes place. The normal
influences whether ascending transtentorial her- concave configuration of the quadrigeminal plate
nia or tonsillar hernia occurs. In the context of cistern is distorted, taking on a flat or convex
increased intracranial pressure, brain tissue is dis- morphology. If the posterolateral aspect of the
placed toward the site that offers less resistance. midbrain is compressed bilaterally, the classic
When the tentorial incisura is small, cerebellar “spinning top” configuration will appear (16).
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Tonsillar Hernia
Tonsillar hernia is inferior displacement of the
cerebellar tonsils through the foramen magnum
into the cervical spinal canal. It may be congeni-
tal (Chiari spectrum) or acquired.
Normal tonsillar position relative to the fora-
men magnum varies with age. Mikulis et al (18)
described the normal position of tonsils below
the foramen magnum for different age groups. Figure 12. DTH in a 64-year-old man with a history of
ischemic heart disease who presented with symptoms
In the 1st decade of life, the presence of cerebel- of sudden neurologic deterioration due to a stroke. DTH
lar tonsils more than 6 mm below the foramen occurred secondary to brain edema. Axial nonenhanced
magnum is considered abnormal. In the next 2 CT image at the level of the midbrain shows left dis-
decades, the reference value is 5 mm; for the 4th placement and rotation of the mesencephalon (curved
arrow), left displacement of the quadrigeminal plate
to 8th decades, the threshold is greater than 4 (arrowhead), and obliteration of the perimesencephalic
mm; and at age 80 years or older, 3 mm is the cisterns. In addition, dilatation of the atrium and tempo-
limit (18,19). ral horn of the left lateral ventricle is present (*).
The McRae line is used as a reference for this
measurement. It is obtained by drawing a line
from the basion to the opisthion. The degree of
RG • Volume 39 Number 6 Riveros Gilardi et al 1607
Figure 13. (a) Ascending transtentorial hernia in a 26-year-old man after resection of a medulloblastoma. Sagittal
gadolinium-enhanced T1-weighted MR image shows obliteration of the quadrigeminal, superior cerebellar, and inter-
peduncular cisterns. There is associated folding of the inferior colliculi (arrowhead) under the superior colliculi, and both
structures have shifted upward. Anterior displacement of the brainstem reduces the space of the pontine and medul-
lary cisterns (curved arrow). Note the superior displacement of the third ventricle roof (straight arrow) and the ante-
rior displacement of the mamillary bodies and tuber cinereum (circle), which are in close contact with the midbrain.
(b) Ascending transtentorial hernia in a 33-year-old man suspected of having intracranial lesions. Axial T2-weighted MR
image shows the cerebellum (white *) ascending through the right side of the incisura (arrowheads), causing oblitera-
tion of the right perimesencephalic and left crural cisterns, diminished space of the quadrigeminal cistern, enlargement
of the temporal horns of the lateral ventricles (black *), and edema in the tectum and right cerebral peduncle (arrow).
Transalar Hernia
Transalar hernia is an uncommon and less
described type of hernia. It is usually associated
with subfalcine and transtentorial hernias (21).
It can be divided into descending and ascend-
tonsillar herniation is the perpendicular length ing transalar hernias. In the descending type, the
from the McRae line to the tip of the displaced frontal lobe is displaced posteriorly and inferiorly
tonsil (Fig 14) (20). over the sphenoid wing. It manifests secondary to
The most common cause is an infratentorial frontal lobe disease. It can cause compression of
mass creating a downward mass effect. It may the middle cerebral artery against the sphenoid
also be secondary to a supratentorial mass, in ridge with a middle cerebral artery infarction.
which case it is usually associated with a DTH. It With ascending transalar hernia, the temporal
can cause severe neurologic damage followed by lobe is displaced superiorly and anteriorly across
sudden respiratory arrest (12,19). the sphenoid ridge owing to a middle cranial fossa
Visualization of tonsils extending below the mass effect. This displacement can compress the
foramen magnum, anterior brainstem displace- supraclinoid internal carotid artery against the
ment, and loss of CSF surrounding it are com- anterior clinoid process with infarction of the
mon features (Figs 15, 16). The fourth ventricle anterior cerebral artery and middle cerebral artery
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Figure 15. Chiari I malformation. Axial (a) and coronal (b) T2-weighted MR images show tonsillar descent (arrow in b)
with anterior brainstem displacement (arrow in a) and effacement of the CSF in the foramen magnum (arrowheads in a).
Extracranial Hernia
External hernias are less common than other
types of hernias. They are most frequently caused
by postsurgical and posttraumatic cranial defects
that allow brain tissue to pass through.
Craniectomy may be performed to decompress
intracranial contents in patients with intracranial
hypertension after medical management fails (22).
Brain edema is common in the 1st week after de-
compressive craniectomy. It may correspond to hy-
perperfusion and loss of resistance in brain tissue,
causing a higher hydrostatic pressure gradient that
favors transcapillary leakage of fluid (23).
A large craniectomy defect allows the brain
to expand without constriction. If the defect
is too small, swollen brain may herniate with a
“mushroom cap” appearance. This can result in
compression of cortical veins and lead to venous
infarction and contusion of the brain at the crani-
ectomy margins. Both CT and MRI are effective
in depicting this hernia (3,12) (Fig 17).
Paradoxical hernia is a rare and potentially
fatal complication of decompressive craniectomy.
It is a neurosurgical emergency. Atmospheric
Figure 16. Drawings show tonsillar hernia. (a) Sagittal view
demonstrates tonsils extending below the foramen magnum pressure exceeding ICP at the site of the crani-
(curved arrow), brainstem compression against the clivus ectomy causes a pressure imbalance and conse-
(straight white arrow), obliteration of the medullary cistern quent subfalcine and/or transtentorial hernia. The
(black arrow), and obstructive hydrocephalus (*). (b) Axial brain tissue is displaced from the craniectomy
view at the level of the foramen magnum. The displaced tonsils
(*) cause obliteration of the surrounding CSF, anterior displace- defect (Fig 18). It is often triggered by an acute
ment of the medulla (arrow), and compression of the spinal imbalance of ICP secondary to CSF drainage or
(black arrowhead) and vertebral (white arrowhead) arteries. lumbar puncture (3).
Symptoms include a depressed level of con-
sciousness, autonomic instability, signs of brain-
stem release, and focal neurologic deficits (3).
RG • Volume 39 Number 6 Riveros Gilardi et al 1609
Figure 17. Extracranial hernia. (a) Axial diffusion-weighted image shows an acute infarction in the left frontal
lobe. Also note the small focus on the right (arrowhead). A few days later, the patient developed vasogenic
brain edema, and decompressive craniectomy was performed. (b) Axial CT image after craniectomy shows
brain parenchyma herniating through the defect of the frontal and temporal lobes (arrows). The sylvian fissure
is enlarged, and the frontal and occipital horns of the left lateral ventricle are retracted.
Figure 18. Paradoxical hernia in a 32-year-old man who was shot in the head three times. He underwent
decompressive craniectomy and developed sinking flap syndrome. (a) Axial nonenhanced CT image shows
left midline displacement and right ventricle compression with associated edema. There is metal artifact in the
left frontal lobe from remnants of the bullet (arrowheads). (b) Sagittal CT image demonstrates the frontal and
temporal lobe displacement.
TM
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