Keywords
ascending transtentorial herniation - hydrocephalus - posterior fossa tumor - ventriculoperitoneal
shunt
Introduction
The phrase “cerebral herniation” refers to the movement of a section of the brain
from its usual anatomical location to a nearby area.[1] When the intracranial pressure (ICP) rises, pressure gradients form throughout the
craniospinal axis, causing brain tissue to herniate.[2] Ascending transtentorial herniation (ATH) is a well-known, although rare, consequence
of growing lesions in the posterior cerebral fossa.[3] The tentorium's inner margins separate, forming an oval orifice known as the tentorial
incisura. The midbrain is subject to displacement by forces from both below and above
the tentorium.[4] Pathogenesis is linked to the formation of a transtentorial pressure gradient ascending
from the posterior fossa to the supratentorial compartment.[5] Obstructive hydrocephalus is fairly prevalent, occurring in 71 to 90% of children
with posterior fossa tumors.[6] While cerebrospinal fluid (CSF) diversion is necessary for many patients during
surgery, preoperative placement of a ventriculoperitoneal (VP) shunt or endoscopic
third ventriculostomy (ETV) is generally discouraged, as only 25% of cases ultimately
require permanent CSF diversion.[7]
[8] CSF diversion may not only alleviate symptoms such as vomiting but also help stabilize
intracranial contents, resulting in a free operating field during definitive surgery.
However, postoperative deterioration in the patient's state following VP shunt should
alert the clinician to the risk of ATH of the brain.[9]
[10] ATH is the least understood of the brain herniation syndromes, occurring as a rare
consequence of VP shunt with a 3% incidence.[10] We report a case of left cerebellar solid cystic lesion with enhancing mural nodule
with obstructive hydrocephalus who developed ATH after VP shunt and recovered after
immediate and timely surgical decompression of the posterior cranial fossa.
Case Description
A 3-year-old boy presented with complaints of a 1-month history of headache without
associated fever or trauma. On examination, he was alert, playful, and obeying commands.
He displayed a tendency of swaying to the left and had dysmetria with cerebellar signs
on the left side. His pupils were 3 mm in diameter and reactive to light. Contrast-enhanced
magnetic resonance imaging (CEMRI) of the brain revealed a cystic lesion with an enhancing
mural nodule in the left cerebellar hemisphere likely with upstream obstructive hydrocephalus
([Fig. 1]). He was initially planned to undergo resection of the tumor without CSF diversion
to avoid shunt dependency. However, on day 2 of admission he had a hydrocephalic attack
and became unresponsive to pain stimulus with a Glasgow coma scale (GCS) score of
E1V1M1; both pupils were 5 mm in diameter and sluggishly reactive to light with bradycardia,
hypertension, and irregular breathing (Cushing's triad). He was immediately intubated
and put on ventilator. An immediate noncontrast computed tomography (NCCT) of the
brain revealed obstructive hydrocephalus with periventricular ooze ([Fig. 2]). He was immediately taken up for right medium-pressure VP shunt (Chhabra SH-202,
G. Surgiwear Ltd., India). Clear CSF under high pressure was tapped. Postoperative
brain NCCT showed optimal positioning of the shunt ([Fig. 3]). On postoperative day 0, he became alert, oriented, and started following commands.
The features of Cushing's triad had resolved. On postoperative day 4, he became drowsy,
but was still arousable and obeying commands. He still did not have features of Cushing's
triad. Repeat NCCT scan of the brain ([Fig. 4]) showed ATH with rostral displacement of the superior vermis through the tentorial
notch, complete obliteration of the quadrigeminal plate cistern, and flattening of
the posterior third ventricle along with anterior and superior displacement of the
third ventricle. He was immediately taken up for left paramedian suboccipital craniotomy
and gross total resection of the lesion via a transcortical approach to decompress
the posterior cranial fossa.
Fig. 1 Contrast-enhanced magnetic resonance imaging (CEMRI) of the brain: T2 and T1 contrast
sequences of sagittal, coronal, axial sections showing a cystic lesion with enhancing
mural nodule in the left cerebellar hemisphere; patent quadrigeminal (A,C;
solid arrows) superior cerebellar cisterns (A, B;
dashed arrows); and tonsillar herniation (D–F).
Fig. 2 Noncontrast computed tomography (NCCT) of the brain showing obstructive hydrocephalus
with periventricular ooze with patent superior cerebellar cistern (dashed arrow) and quadrigeminal cistern (solid arrow).
Fig. 3 Noncontrast computed tomography (NCCT) of the brain on postoperative day 0 of the
right ventriculoperitoneal (VP) shunt showing optimal position of the shunt with patent
superior cerebellar cistern (dashed arrow) and quadrigeminal cistern (solid arrow) with compression of brainstem against the clivus; the posterior border of the third
ventricle is highlighted with a dashed line.
Fig. 4 Noncontrast computed tomography (NCCT) of the brain on postoperative day 6 of ventriculoperitoneal
(VP) shunt with ascending transtentorial herniation with complete obliteration of
quadrigeminal (solid arrow) and superior cerebellar cistern (dashed arrow) with flattening of the posterior third ventricle (dashed line)—appreciated better by comparing with [Fig. 3].
He was extubated after surgery (on postoperative day 0) and made an uneventful recovery.
NCCT of the brain ([Fig. 5]) following definitive surgery showed adequate decompression of the brainstem and
patent quadrigeminal and superior cerebellar cisterns. Definitive biopsy revealed
the lesion to be anaplastic ependymoma (CNS WHO grade 3). At discharge on day 5 after
definitive surgery, he was alert, playful, and moving all four limbs spontaneously.
At his last follow-up 3 months after surgery, he was undergoing radiotherapy with
the same functional status.
Fig. 5 Noncontrast computed tomography (NCCT) of the brain following left paraforaminal
paramedian suboccipital craniotomy and resection of tumor with resolution of ascending
transtentorial herniation (ATH) with patent quadrigeminal (solid arrow) and superior cerebellar cisterns (dashed arrow).
Discussion
Meyer described ATH for the first time in 1920, reporting an uncommon supratentorial
distention of the splenium.[11]
[12] In 1938, LeBeau described displacement of the cerebellar dome up through the tentorial
gap in a rare instance of cerebellar tumor.[13] The lateral ventriculogram revealed an apparent ablation of the posterior part of
the third ventricle. Vastine and Kinney discovered that the pineal body shifted upward
in 33% of the patients with subtentorial gliomas. Before the pineal gland is elevated,
there must be a significant upward shift in the midbrain.[4] Johnson and List mentioned that uncommon, large tumors of the pons may move the
brainstem anteriorly, causing a corresponding elevation of the third ventricle.[13] In 1979, Cuneo et al described this phenomenon as “the least understood of the brain
herniation syndromes.”[4]
The type of herniation that occurs is influenced by the width of the tentorial notch
and the direction of the mass effect (whether it is upward or downward). ATH is more
likely to happen in individuals with a broad tentorial notch and an upward mass effect.[1] A cerebellar lesion is the most common cause of ATH (65%), followed by cerebellopontine
angle, pons, and fourth ventricle lesions.[12] It is likely that posterior fossa lesions sufficiently large to cause considerable
upward displacement also block CSF pathways. By establishing an equilibrium of forces,
the ensuing hydrocephalus may prevent upward herniation.[14] Displacement of the cerebellum into the tentorial incisura is more likely to occur
when the mass originates near the incisura, for example, in the cerebellar vermis,
when drainage of the lateral ventricles relieves obstructive hydrocephalus and reduces
pressure above, and when the opening in the tentorium is wide.[4] The brainstem is initially squeezed against the clivus. As the process of enlargement
progresses, tonsillar herniation into the foramen magnum occurs, blocking the inferior
outflow of the posterior fossa and squeezing the medulla. When this is not lethal,
growing pressure in the posterior fossa will result in upward displacement through
the tentorial notch.[3] The protruding tissue endangers both the great vein of Galen and the superior cerebellar
arteries, pushing the midbrain forward and eventually compressing it from behind.[15] Cranial CT can precisely detect the rostral displacement of the superior vermis
through the tentorial notch. Early or impending upward herniation is indicated by
compression and slight posterior flattening of the quadrigeminal plate cistern. As
herniation worsens, it results in amputation of the peritectal CSF diamond, giving
the confluent quadrigeminal and superior cerebellar cisterns a triangular or “squared-off”
appearance. In severe cases, the herniated vermis fills the notch, completely eliminating
these cisterns and flattening the posterior third ventricle.[16]
The primary symptom of acute ATH herniation is a gradual decline in consciousness,[4]
[9]
[12]
[17]
[18] caused by impaired function of the reticular formation at the mesencephalopontine
junction.[17] It is well understood that midbrain ascending reticular activating circuits play
an important role in maintaining consciousness. In this regard, compression of the
midbrain with a related compromise of its blood supply may be a factor in inducing
and maintaining a state of unconsciousness that emerges and worsens as this complication
develops.[15]
[19] Components of Cushing's triad such as bradycardia and hypertension, and irregular
respiration, have been reported in other cases of ATH, particularly those involving
acute deterioration following sudden CSF drainage.[2]
[10]
[12]
[20]
[21]
[22] However, many authors perform direct tumor removal with an intraoperative external
ventricular drain (EVD) if needed for patients of posterior fossa tumor with hydrocephalus.
This avoids the need for a VP shunt and also avoids the dreaded ATH.
Primary surgical excision of the tumor is strongly considered the preferred initial
strategy to directly address the mass effect and avoid ATH.[20] Patients with midline posterior fossa tumors may be considered for corrective surgery
of the lesion first, rather than palliative procedures like shunt surgery or ETV,
as these procedures have the potential to precipitate reverse herniation of the brain.[20] Since only 25% of patients need permanent CSF diversion, therapy with a ventriculoperitoneal
shunt or ETV before tumor excision is not advised, even if many patients need it at
the time of surgery.[7]
[8] If immediate excision is not feasible, controlled CSF drainage may be considered.[23] However, this strategy carries a significant risk of ATH, also known as reverse
brain herniation (RBH).[1]
[6]
[24] RBH/ATH is rare,[6]
[10]
[23] with incidence reported around 3%,[6]
[10]
[23] but it is associated with a significant high mortality.[6]
[10]
[23]
[24] ATH is caused by a sudden decrease in supratentorial pressure,[5]
[6]
[9]
[10]
[12]
[20]
[22] reversing the pressure gradient across the tentorium.[5]
[12]
[20]
[22] This sudden decompression, often following a shunt surgery or ETV, in the presence
of a space-occupying lesion in the posterior fossa, may cause upward herniation.[20] Allowing a liberal amount of CSF to leak during surgery or using low-pressure VP
shunt tubes may lead to reverse coning.[24] In such cases, it is crucial to slowly and carefully reduce the ventricular pressure.
Programmable shunt or even an EVD may be better alternatives in such cases as the
CSF output can be carefully monitored and controlled.[23] Throughout this process, one should closely monitor cardiovascular function and
heart rhythm for any changes. If arrhythmias are detected, it is advisable to immediately
halt the drainage of CSF. In case there is significant hemodynamic disturbances, one
should reinject the drained CSF or normal saline into the ventricular catheter.[2] Vigilance for ATH and prompt intervention are paramount regardless of the initial
approach.[10]
[23]
[24] Early detection and immediate intervention are crucial for reversing ATH and preserving
brain function. Surgical decompression should be undertaken as soon as possible, even
in cases of severe RBH.[6]
[10]
In the series by Cuneo et al, only 7 cases out of a total of 52 reviewed were diagnosed
antemortem and the mortality was 100%.[4]
[6] To our knowledge, four such cases have been reported in the literature recently
([Table 1]). More recent cases documented in the literature showed improved outcomes.[2]
[6]
[10] We believe that this condition is highly underdiagnosed because a delay in diagnosis
can prove to be fatal and not give time for imaging studies and further action. Despite
the high mortality associated with ATH, it is essential to perform surgical decompression
as soon as possible, even in severe cases. Ventricular drainage directly causes herniation
in approximately 25% of patients. Therefore, patients who undergo CSF diversion must
be closely monitored for ATH after the procedure.[6] In our case, the patient made an uneventful recovery following urgent and timely
decompression of the posterior cranial fossa. In hindsight, upfront tumor excision
should have been performed early. Additionally, when the patient deteriorated on the
second day following admission, inserting an EVD or placing a programmable VP shunt
could have been better as these options would have allowed for controlled CSF drainage.
Table 1
Case reports of ATH following CSF diversion in patients with posterior cranial fossa
tumors[6]
|
Sl. no.
|
Study
|
Age/gender
|
Diagnosis
|
Treatment
|
Outcome
|
|
1
|
Singha et al[20]
|
57 y/M
|
Midline posterior fossa (involving vermis and both cerebellums) hemangioblastoma with
hydrocephalus
|
ETV + suboccipital craniectomy and tumor decompression
|
Uneventful
|
|
2
|
Gurajala et al[10]
|
45 y/M
|
Right cerebellopontine (CP) angle tumors with hydrocephalus
|
VP shunt + tumor decompression
|
Discharged with nasogastric tube
|
|
3
|
Marappan et al[24]
|
3.5 y/F
|
Fourth intraventricular tumor
|
VP shunt
|
Not available
|
|
4
|
Tyngkan et al[6]
|
12 y/F
|
Left cerebellar lesion
|
VP shunt
|
Deceased
|
|
5
|
Present case
|
3 y/M
|
Left cerebellar anaplastic ependymoma
|
VP shunt followed by tumor excision
|
Discharged in good condition
|
Abbreviations: ATH, ascending transtentorial herniation; CSF, cerebrospinal fluid;
ETV, endoscopic third ventriculostomy; VP, ventriculoperitoneal.
Conclusion
Primary surgical excision of posterior fossa tumors is strongly considered the preferred
initial management strategy in patients with obstructive hydrocephalus. This directly
addresses the mass effect and aims to mitigate the significant risk of ATH, a serious
complication associated with CSF diversion procedures (VP shunt, EVD, ETV) due to
sudden pressure changes. Controlled CSF drainage might be considered if primary excision
is not feasible or hydrocephalus persists. Vigilance for ATH and prompt intervention
are paramount. Early signs of this condition must be promptly identified, and swift
action should be taken to reverse the process and preserve brain function.
