Brain Herniation into Arachnoid Granulations:
an Uncommon Entity with Variable Clinical and
Radiographic Manifestations
Greta B. Liebo, MD, Vance T. Lehman, MD, Laurence J. Eckel, MD, Kara M.
Schwartz, MD, John (Jack) I. Lane, MD.
Department of Neuroradiology, Mayo Clinic-Rochester.
ASNR, 2015 Annual Meeting, Chicago, IL, April 25-30.
Control #965
eEdE-141
Disclosures
No disclosures
Background
• Arachnoid granulations (AG) are normal structures,
created by invaginations of the arachnoid membrane
that extend through gaps in the dura, protruding into
the dural venous sinuses and occasionally the bone.1-13
• Rare cases of “encephaloceles,” or herniation/
invagination of brain parenchyma into these AGs have
been reported, but there are very few reports and little
analysis of this in the current literature.4-12
• Our series of 10 patients is the largest case series to
date.
Purpose
To characterize the variable radiographic
appearance and clinical significance of brain
herniation into AGs
Approach/Methods
• Retrospective imaging and clinical chart
review from our collective clinical experience
Approach/Methods
• Patient demographics, location of the AGs,
and the location of herniated parenchyma
were characterized
• The clinical presentations were reviewed
• The presence/absence of evidence of elevated
intracranial pressure at presentation was
recorded
• Associated MR signal change and adjacent
encephalomalacia were also documented
Approach/Methods
• Brain herniations into AG were categorized
into 3 broad clinical categories:
1) Incidental: No correlation of imaging findings
with clinical history
2) Indeterminate: Possible related clinical
symptoms
3) Likely Symptomatic : Clinical symptoms present
that seem to be related to AG with brain
herniation
CASE REPORTS:
Clinical Categories
• Incidental:
• Indeterminate:
• Likely symptomatic:
• Secondary to effects of the AG itself:
• Secondary to presumed incarceration:
# of Cases
6
1
3
(2)
(1)
INCIDENTAL CASES
CASE 1:
A 5 year old female with past medical history
significant for viral meningitis in infancy
presented for work up of newly diagnosed
absence epilepsy. Electroencephalogram
(EEG) demonstrated left frontal
predominant interictal epileptiform
discharges , raising additional concern for a
more focal process with complex partial or
secondary generalization of seizure activity.
A
B
MRI was performed at the time of initial
seizure workup. Coronal FIESTA (a), axial
FIESTA (b), coronal MPRAGE (c), and coronal
postcontrast T1 (d) demonstrate an AG in
the left transverse sinus containing a small
invagination of parenchyma from he
posteroinferior temporal lobe ().
The neurologists did not believe the
herniation of temporal lobe was
contributory to the patient’s seizures.
C
D
CASE 2:
A 70 year old female with
history of nephrectomy for
renal cell carcinoma
presented with multifocal
foot paresthesias involving
the right and left feet,
forearms, hands, and right
cheek.
MRI was ordered to
evaluate for a central
etiology. Axial T2 (a,b),
axial postcontrast T1 (c),
and coronal postcontrast
T1 (d) demonstrate
herniation of the left
cerebellar hemisphere into
the transverse sinus ().
No specific abnormality
was identified to account
for the patient’s
symptoms.
A
B
C
D
A
CASE 3:
A 60 year old male experienced an
indeterminate episode of
neurologic dysfunction
characterized by right upper
extremity monoparesis. He was in
the hospital at the time,
recovering from an orthopedic
knee procedure complicated by
lower extremity deep venous
thrombosis.
B
CT and follow up MRI were
ordered to evaluate for TIA or
stroke. Axial bone kernel CT (a),
axial T2 (b), coronal postcontrast
T1 (c), and sagittal postcontrast T1
(d) demonstrate cystic irregularity
with remodeling of the the left
occipital bone (), a classic
appearance for an AG. Herniation
of the left cerebellum into the AG
is seen, isointense to brain on all
sequences ().
C
D
A
B
C
CASE 4:
A 62 year old male presents for follow up status post
radiation and chemotherapy for a resected right
frontal anaplastic oligodendroglioma. His
postoperative course was complicated by
hemorrhage and intracranial hypertension.
Axial T2 CISS (a-c) and sagittal postcontrast T1 SPACE
(d) images demonstrate multiple tiny AGs eroding
into the occipital bones, containing small
invaginations of cerebellar parenchyma ().
D
July 2011, Ax T2
March 2013, Ax FLAIR
Nov 2013, Ax T2
CASE 5:
A 65 year old female presented with 3 year history of
progressive slurring and gait imbalance, now requiring the use
of a wheelchair. On exam, she was ataxic and dysarthric, with
left greater than right spastic hemiparesis and tremors.
Review of outside her MRIs dating back to 2011 demonstrated
progressive pontine and cerebellar atrophy with increased
signal in the pontine white matter tracts suggestive of multiple
system atrophy. Additionally, a prominent AG into the
transverse sinus was observed (), with focal fibrotic
appearing bands extending to the left cerebellar hemisphere,
and focal cerebellar encephalomalacia immediately subjacent
to the granulation ().
Oct 2014, Ax T2
CASE 6:
A 53 year old asymptomatic male
with history of metastatic
colorectal carcinoma presented
for restaging brain MRI.
Axial T2 (a), coronal CISS (b), axial
FLAIR (c), and sagittal postcontrast
T1 (d) demonstrate a cystic
abnormality with remodeling of
the right occipital bone, typical in
appearance for an AG ().
Herniation of the right cerebellar
hemisphere and multiple
septations are seen within the AG.
There is focal encephalomalacia in
the cerebellum, immediately
subjacent to the herniation ().
A
C
B
D
INDETERMINATE CASES
CASE 7:
A 20 year old female without significant past medical history presented with 8
months of incapacitating, medically refractory headaches. She described her
pain as typically right or left temporal, but infrequently posterior. She also
reported more recent “pressure-type sensation” that lasts only a few seconds
over her occiput.
A
B
Noncontrast head CT (a) demonstrates a complex predominantly CSF density
filling defect in the torcula. Axial T2 (b), sagittal pre-contrast T1 (c), and
sagittal post-contrast T1 (d) demonstrate herniation of the left cerebellar
hemisphere into a giant AG ().
C
D
LIKELY SYMPTOMATIC CASES
A
B
C
CASE 8:
A 70 year old woman presented history of recurrent fluid in the left ear, status post tympanostomy tube placement,
with persistent clear left ear drainage and positive beta-2 transferrin test.
Axial bone kernal CT (a) demonstrates erosion of the posterior petrous plate () and mastoid effusion. Axial T2 (b)
and postcontrast T1 (c) images show herniation of the left cerebellum through the bone defect ().
During left middle fossa craniotomy, the dura was described as very thin, but intact, without obvious defect leading
into the mastoid or middle ear space. On mastoidectomy, a fairly large meningoencephalocele was identified as the
obvious source of CSF leak.
Note: While no discrete cystic mass is identified, AGs have long been suspected to be responsible for erosions in this
location.6,11-13 This theory is also supported by the focal erosive appearance of the abnormality on CT.13
A
B
C
D
Figure: Preoperative Coronal CT of the right (a) and left (b) temporal bones, axial
CT of the right temporal bone (C), and Axial T2 SPACE of the right IAC (d).
E
Figure: Postoperative axial post-contrast T1 (e) and T2 SPACE (f).
CASE 9:
A 56 year old female without significant past medical
history presented with a 3 year history of progressive
right-sided hearing loss with tinnitus and recurrent
vertigo. Repeat brain MRIs over a 10 month period
showed a stable, multiloculated extra-axial cystic nonenhancing mass within the lateral aspect of the right
cerebellopontine cistern (). It was associated with
extension through the posterior cortex of the right
temporal bone. The lesion was isointense to CSF
without restricted diffusion and CT scan confirmed
bony erosion ().
A right suboccipital craniotomy was performed and the
radiographic bony defect was immediately appreciated
within the cerebellopontine angle. The defect was
superficial and inferior to the internal auditory canal
with a small vascular bundle extending from the
cerebellum into the bony defect, tethered by the dura.
The lesion had the gross appearance of thickened
arachnoid membrane resembling and arachnoid cyst.
Pathology confirmed a benign process comprised of
several components including arachnoid cells, fibrous
tissue, portions of bone, and neuroglial tissue. At 3month follow-up, MRI showed complete resection and
continued right sided hearing loss.
F
Focal encephalomalacia of the cerebellum was
observed, immediately adjacent to the abnormality,
both before and after resection, suggestive of prior
strangulation and infarction ().
A
C
CASE 10:
A 46 year old woman presented with 10 days of
headache, which rapidly progressed from mild to
“excruciating, stiff neck, and new onset diplopia. A
Head CT was ordered in the ED, but showed no
acute abnormality. She stayed in bed for 2 more
weeks, with persistent headache, diplopia, and
fatigue, before returning for follow up with a
neurologist. A brain MRI was ordered, but
symptoms resolved spontaneously prior to her MRI
appointment.
B
D
The initial screening head CT (a,b) demonstrates
large erosions of the occipital bone (), typical in
appearance for AGs. Ax FLAIR (c), Axial T2 (d),
sagittal FLAIR (e), and sagittal postcontrast T1 (f)
images demonstrate protrusion of the cerebellar
hemispheres into the AGs () with adjacent edema
() and enhancement () the medial right
cerebellar hemisphere.
Initial differential considerations included tumor,
infection, inflammation, or vascular etiology.
However, there was no leptomeningeal
enhancement or other abnormality to account for
the cerebellar findings. Over the course of the next
3 months, the cerebellar signal abnormality
resolved, suggestive of prior incarceration without
strangulation/infarction.
E
F
Discussion
• Background & Literature Review:
– One of the major difficulties in reviewing the literature
regarding this topic is the inconsistency of language and
description across different case reports.
• Some authors prefer the term invagination5 over brain
herniation7,10,11, perhaps trying to emphasize the assumed
benignity of this entity.
• Others designate the abnormality as an encephalocele8,9,12 or
meningoencephalocele6, particularly if it occurs in the temporal
bone, along the posterior petrous face.
– These inconsistencies highlight the variability in patient
presentation and outcome.
Discussion
• Proposed Pathophysiology:
– Most authors believe that brain herniation into AGs occurs
spontaneously 8-10
– Some authors have suggested that increased intracranial
pressure may be a predisposing factor. 7,8,10,12
– In our experience:
• None of the patients in our series had signs or symptoms of
increased intracranial hypertension at the time of exam.
– Only 1 patient (case 4) had history of prior intracranial hypertension, but
the imaging appearance of the AGs was unchanged from earlier
examinations suggesting that herniation preceded the period of
intracranial hypertension.
• However, we noticed a marked propensity of herniations into AGs
occur in the posterior fossa, involving the distal transverse sinuses,
posterior petrous ridges, or occipital bones.
Discussion
• Proposed Pathophysiology:
– Most authors believe that brain herniation into AGs occurs
spontaneously 8-10
– Some authors
Basedhave
onsuggested
our experience,
that increased intracranial
we
pressure may be a predisposing factor. 7,8,10,12
favor that herniation occurs
– In our experience:
• None of the patients in our series had signs or symptoms of
increased intracranial hypertension at the time of exam.
•
spontaneously, primarily
– Only 1 patient (case 4) had history of prior intracranial hypertension, but
dependent
onwas
the
location
the
the imaging appearance
unchanged
from earlierof
examinations
suggesting that herniation preceded the period of intracranial
hypertension.
arachnoid
granulation.
However, we noticed a marked propensity of herniations into AGs
occur in the posterior fossa, involving the distal transverse sinuses,
posterior petrous ridges, or occipital bones.
Discussion
• Epidemiology: Our overall experience
– Age: 5-70 years; mean: 50 year.
– Sex: 7 female, 3 male
Discussion
• Epidemiology: Our overall experience
– Age: 5-70 years; mean: 50 year.
appears to increase
– Sex:Prevalence
7 female, 3 male
with age, correlating with
concurrent increase in size and
number of AGs.
Discussion
• Clinical Associations:
– Headaches, tinnitus, and rarely intracranial venous
hypertension have all been reported with giant
AGs WITHOUT BRAIN HERNIATION, though no
direct correlation has been established. 1,6,7,10
Discussion
• Clinical Associations: Our overall experience
– 60% Incidental (unrelated to clinical symptoms)
– 10% Indeterminate
– 30% Associated directly with clinical symptoms
and/or evidence of incarceration/strangulation
Discussion
• Clinical Associations: By Location
Location
Cases
Incidental
Distal
Transverse
Sinus
Case 1
x
Case 2
x
Case 5
x
Torcula
Case 7
Posterior
Petrous Face
Case 8
x
Case 9
x
Occipital
Bones
Symptomatic
x
Case 3
x
Case 4
x
Case 6
x
Case 10
Indeterminate
x
Discussion
• Clinical Associations: By Location
Brain herniation into AGs along the posterior
petrous plate seem to have the highest rate of
symptomatic presentation.
• Symptoms are likely related to associated bony remodeling,
dehiscence, and subsequent mastoid/middle ear effusion or
CSF leak rather than the brain herniation itself.
• The presence of brain herniation in this location may be
symptomatically inconsequential, though important to
document, as it may aid in presurgical planning, altering the
surgical approach.
Discussion
• Clinical Associations: Strangulation and
Infarction
• Our experience:
– 30% of cases (3/10) were discovered incidentally, with subjacent
encephalomalacia suggestive of old strangulation and infarction.
– 10% of cases (1/10) had radiographic signs and symptoms
suggestive of acute ischemia from incarceration without complete
infarction.
• This association has not been previously reported to our
knowledge.
Discussion
• Clinical Associations: Strangulation and
Infarction
• Our experience:
– 30% of cases (3/10) were discovered incidentally, with subjacent
This suggests
there
is
a
propensity
for
the
herniated
encephalomalacia suggestive of old strangulation and infarction.
brain–tissue
strangulate
and infarct.
10% ofto
cases
(1/10) had radiographic
signs and symptoms
suggestive of acute ischemia from incarceration without complete
infarction.
• This association has not been previously reported to our
knowledge.
Discussion
• Clinical Associations: Strangulation and
Infarction
• Our experience:
– 30% of cases (3/10) were discovered incidentally, with subjacent
This suggests
there
is
a
propensity
for
the
herniated
encephalomalacia suggestive of old strangulation and infarction.
brain–tissue
strangulate
and infarct.
10% ofto
cases
(1/10) had radiographic
signs and symptoms
suggestive of acute ischemia from incarceration without complete
lackinfarction.
of relevant patient history in these cases
The
suggests
that manyhasofnot
these
were “silent”
• This association
beeninfarctions
previously reported
to our
or subclinical,
knowledge. perhaps presenting only with
headache or no symptoms at all.
Discussion
• Clinical Implications:
– We expect detection of brain herniation into AGs to increase over
time, owing to improved MR imaging techniques and spatial
resolution.
– The propensity for incarceration or strangulation raises the question of
necessary follow up or management.
• If discovered incidentally, no further follow up may be indicated, particularly
because the majority of strangulation/infarction events are subclinical.
• If symptomatic at presentation, there may be a role for follow up imaging or
elective surgical decompression.
– Recommended imaging sequences: FIESTA or CISS with multiplanar reconstructions.
– However, larger population-based and prospective case studies are
necessary to fully understand the prevalence, natural history, and
clinical implications of this entity.
– In the meantime, the focus should be on recognition and education,
particularly in cases undergoing pre-operative evaluation for related or
unrelated surgical planning.
Conclusion
• Brain herniation into AGs is a rare finding with
variable clinical and radiographic manifestations.
• While the pathophysiology is not well
understood, we favor that herniation occurs
spontaneously, with incidence related primarily
to the location of the AG.
• While most cases are thought to be
asymptomatic, patients may present with a
variety of symptoms related to bony remodeling
of the temporal bone or parenchymal
incarceration/strangulation.
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