Important topic for neurosurgery residents

1. Operative approaches for 4th
ventricular tumours
Dr. Paresh Kumar Gouda
FINAL YEAR RESIDENT
NEUROSURGERY

2. Fourth ventricle viewed from the side (A) and from
behind (B). Landmarks such as the vertical median sulcus and oblique
calamus scriptorius give the caudal floor the appearance of a fountain
pen. (Modified from Cohen AR. Surgical Disorders of the Fourth Ventricle.
Cambridge, MA: Blackwell Science; 1996.)

4. Operative approach

5. • The safest and most direct approach to the fourth
ventricle is the midline suboccipital approach.
• Preoperative:
• All imaging and labs should be reviewed carefully.
• Antibiotics should be given with incision.
• Preoperative steroids can decrease vasogenic edema,
alleviate headache and neck pain, decrease the
incidence and severity of aseptic meningitis and the
posterior fossa syndrome, and decrease nausea and
vomiting allowing for better hydration and nutrition
prior to surgery.
• Ensure availability of automatic retractor system.

6. Intraoperative monitoring
• Intraoperative monitoring may be helpful if there is
danger of violating the brain stem or cranial nerves.
• The most sensitive measure of alteration of brain stem
function is the pulse and blood pressure, since
cardiovascular reflexes are mediated by structures
near the fourth ventricle such as the nucleus tractus
solitarius and dorsal motor nucleus of the vagus.
• Any alterations in vital signs while working near the
floor of the fourth ventricle should be considered a
serious warning sign to stop manipulation.

7. • The best option for direct monitoring of brain-stem function
is brain-stem auditory- evoked potentials (BAEP), in which
an auditory click is measured at earlobe and vertex
electrodes.
• This produces five waves that correspond, respectively, to
the proximal cochlear nerve, distal cochlear nerve, cochlear
nucleus, superior olive, and lateral lemniscus/inferior
colliculus.
• Evidence of pontomesencephalic transmission of the
impulse implies that the brain stem has not been
compromised.
• However, this pathway is fairly lateral and may be preserved
in spite of serious damage to the central core of the brain
stem.

8. • Another monitoring technique, somatosensory
evoked potentials (SSEP), follows sensory signals
through the medial lemniscus, but this is also
some distance from the floor of the fourth
ventricle, and SSEP is even less sensitive than
BAEP.
• Finally, EMG with direct stimulation of the facial
nerve or lateral rectus can be used to verify
integrity of the cranial motor nerves if tumor
abuts or envelops them.

9. Positioning
• There are three possibilities for positioning: prone, lateral oblique, or
sitting.
• Each of the positions requires the head to be pinned using a Mayfield or
Sugita head holder as long as the patient is more than 2 years old.
• The pins are coated with an antibiotic ointment and placed two
centimeters above the ear in the unshaven scalp.
• It is important to avoid the squamous temporal bone and shunt tubing if
present.
• Use of pins in infants can lead to skull penetration producing depressed
fracture, dural laceration, hematoma, or postoperative abscess.
• Therefore, rather than using pins, very young children should be placed
face down with the head on a padded horseshoe, ensuring there is no
pressure on the eyes.
• All three positions require a certain amount of neck flexion, so caution
should be used if there is known preexisting neck pathology, especially a
craniocervical anomaly, spinal instability, significant cervical spondylosis,
or herniation of the cerebellar tonsils on preoperative imaging.

11. Prone position
• The most commonly used position for the midline suboccipital approach
(especially in very young patients) is the prone position, in which the
patient is rolled after induction of anesthesia so that the face is toward
the floor.
• There are many advantages to this position: the anatomy is clearly
visualized, it is easy for two to work together since one operator can stand
on either side, and the multiple complications of the sitting position do
not occur.
• The most significant disadvantage of the prone position is venous
congestion that can lead to more significant blood loss, pooling of blood in
the operative field, and soft tissue swelling of the face.
• This congestion is much worse if the head is rotated and flexed, and is
improved somewhat by elevating the head above the level of the heart.
• Also, nasotracheal rather than orotracheal intubation can minimize
compression of the base of tongue and impairment of venous drainage of
the tongue and pharynx.

12. • The weight is distributed to minimize pressure points that can lead to skin
breakdown and neuropathy, especially at the ulnar nerve at the elbow, common
peroneal nerve across fibular head, and lateral femoral cutaneous nerve at the
iliac crest.
• Two longitudinal padded roles are placed under the patient, and the knees and
ankles are padded.
• The neck is placed in the “military tuck position” with moderate flexion of the
upper cervical spine (to open up the space between the foramen magnum and the
arch of C1) and less flexion of the lower cervical spine (to bring the occiput parallel
with the patient’s back).
• The chin and chest at least two fingers apart. Finally, the table is positioned so that
the neck is parallel to floor and the head is above the heart.
• The shoulders can be gently retracted toward the feet with some tape, and a strap
under the buttocks is helpful to prevent sliding.
• The surgeon and assistant then operate from either side using the microscope, and
the scrub nurse’s Mayfield table can be placed over the patient’s back.

13. Lateral position
• The lateral oblique or lateral decubitus position is similar to the prone
position, except that the patient is lying on his or her side.
• This allows superior visualization of pathology high in the fourth ventricle,
in the lateral recesses, and in the cerebellopontine angle.
• The posterior fossa contents do not sink inward as they do in the prone
position and the operative distance is more comfortable for the surgeon.
• The principle disadvantage of the lateral oblique position is that the
anatomy is not centered so the surgeon must visualize all structures
rotated.
• Also, it is constantly necessary to support the upper cerebellar
hemisphere to maintain exposure, although the lower hemisphere
naturally falls away.
• The patient is placed on the side with the dependent arm ventral on the
table. A soft roll or IV bag wrapped in foam is placed in the axilla of the
dependent arm to prevent brachial plexus injury or vascular compression,
and the dependent leg is padded with special attention paid to the fibular
head of the upper leg to avoid peroneal palsy.

14. Sitting position
• The third option for positioning is the sitting position, in which the patient is
positioned sitting upright so that the operative corridor is parallel to the floor.
• The sitting position offers a very clear operative field since blood and cerebrospinal
fluid drain out of the operative site.
• However, there are many risks to the sitting position. The most significant dangers
are cardiovascular instability and hypotension, air embolism, and subdural
hematoma.
• All patients should have an agitated saline echocardiogram to exclude right to left
shunt through a patent foramen ovale that could complicate air embolism and
presence of such a shunt is an absolute contraindication for the sitting position.
• Precordial Doppler ultrasonic flow and end-tidal CO2 should be monitored
throughout the case.
• The risk of subdural hematoma is greatly increased by presence of a shunt, and if
possible the shunt should be occluded prior to attempting an operation in the
sitting position.
• Other risks of the sitting position include tension pneumocephalus, cervical
myelopathy, thermal loss (especially in children), surgeon fatigue, and sudden loss
of CSF from enlarged lateral and third ventricles after removal of a fourth ventricle
mass lesion.

16. • When applying the head holder, the pin sites must be
covered with Vaseline gauze to minimize entry of air
and the head taped to the head-holder for extra
support in case the pins become dislodged.
• The patient is elevated slowly into the sitting position
so that the foramen magnum is at the surgeon’s eye
level with both of the patient’s legs flexed at the knees
to prevent postoperative sciatica.
• The instrument table is placed over the patient’s head.
• Infants too young for pins may be taped to a padded
headrest to support the forehead and chin, but it is
probably safer to use the prone position.

17. • Throughout the case the patient should be carefully
monitored for signs of hypotension or air embolism.
• If air embolism occurs, the wound should be packed with a
saline-soaked sponge, and anesthesia should aspirate the
atrial catheter to attempt to remove the embolus from the
left atrium.
• If the embolus is severe, the patient should be placed in left
decubitus position; otherwise, as soon as the patient is
stable, the wound may be slowly exposed while covering
the potential source of air with Gelfoam and Surgicel.
• If careful preparation is undertaken and complications
dealt with promptly, the sitting position can be relatively
safe.

18. Preparation
• After positioning, the back of the head is shaved to expose
the suboccipital region and the scalp degreased with
acetone and alcohol then cleansed with a povidine iodine
solution.
• A linear midline incision is outlined 1-2 cm above the
external occipital protuberance down to the level of C4.
• The operative field is walled off with towels, draped with
iodoform adhesive, and infiltrated with 0.25% lidocaine with
1/400,000 epinephrine (or 0.1% lidocaine with 1/1,000,000
epinephrine in infants less than 1 year old).
• If there is concern that it will be necessary to rapidly
decompress the lateral ventricles intraoperatively or
postoperatively, a burr hole may be drilled in the right
posterior parietal region.

19. Incision
• The incision is made with a number 10 blade applying firm digital
compression, and bleeding points are coagulated.
• The incision should be midline, but if the tumor is lateral, a hockey-stick
incision can be used to allow for a wider craniectomy.
• The skin is undermined superficial to the fascia on both sides of the
superior half of incision in preparation to create a fascial flap for closure.
• The skin is then elevated with toothed forceps or a skin hook and a plane
of dissection developed with knife or monopolar coagulation, sparing the
occipital artery and nerve whenever possible.
• Even a slight deviation off midline will produce brisk bleeding from the
muscles once deeper tissues are exposed.
• When anatomical landmarks are identified to confirm that the operative
course is truly midline, cerebellar or Weitlaner retractors are placed to
maintain exposure.
• As deeper layers are exposed, curved retractors may be used.

22. • Next, the fascia is incised using a Y-shaped incision,
keeping the lateral ends of the Y below the
ligamentous insertion (nuchal ligament).
• While a linear midline fascial incision without the
upper limbs of the Y allows use of the avascular plane
between the splenius capitus and semispinalis capitis
muscles, it is often difficult to reapproximate such an
incision tightly at the superior nuchal line.
• Muscle flaps are then developed with monopolar
cautery and periosteal elevators, stripping the muscle
from the bone as far as the mastoid emissary vein.

24. • This exposure is maintained with two curved
cerebellar retractors and the rostral flap is placed
under tension using a 3-0 silk suture to reflect it
rostrally.
• The muscle insertions are stripped off the
spinous process and laminae of C2.
• Finally, the junction between the pericranium and
dura at the foramen magnum is sharply
dissected, and then the posterior fossa dura
separated from the inner table of the occipital
bone using a curette.

26. Craniotomy
• The suboccipital craniotomy is begun with burr holes on
either side of midline just below the transverse sinuses,
about three centimeters from midline.
• A third burr hole can be placed below the torcular in
older patients.
• In children, the dura is not firmly adherent to the skull
so it is safe to drill close to or even on top of the sinuses,
but more caution must be used with adults.
• The dura near the burr hole is then stripped using a
Penfield and the bone removed using a high speed drill.
• The superior and lateral limits of the craniotomy are the
transverse and sigmoid sinuses.

29. • Inferiorly, the craniotomy should always include the posterior edge
of the foramen magnum to prevent laceration of the brain against
the closed bony rim when cerebellar elements are retracted
downward and minimize damage from herniation if hematoma or
swelling should occur postoperatively.
• The midline bone is removed last since it is often very vascular and
contains a keel that can be quite deep.
• This keel must be stripped of dura with a Penfield, using extreme
caution near the occipital sinus in the midline and the annular sinus
near the foramen magnum.
• All exposed bone edges should be waxed, especially in the sitting
position.
• Because of the irregular contour of the inner bone surface in adult
patients, it is sometimes necessary perform a craniectomy rather
than a craniotomy, removing the bone in a piecemeal fashion.

30. • To expose the posterior arch of C1, the soft tissues
overlying it are reflected laterally using a small
periosteal elevator, stripping the inferior arch first since
the vertebral artery is on its superior aspect.
• It is sometimes easier to do this after C2 has been
exposed.
• The periosteum can sometimes be swept off the arch
of C1 using an index finger covered with gauze.
• Monopolar cautery should be used with caution when
dissecting the soft tissue over C1 (especially at the
superolateral surface) to prevent injury to vertebral
artery.

31. • It is important to remember that C1 can be bifid and is
often cartilaginous in infants and young children.
• C1 laminectomy is helpful for lesions that herniate beneath
the foramen magnum.
• To remove the lamina, small angled curettes can be used to
strip the deep surface of the bone, and then the bone itself
removed with an angled Kerrison punch or Leksell rongeur.
• Because extending a laminectomy below C2 in young
children increases the risk of swan neck deformity, it is
prudent to remove the smallest amount of bone possible.
• For most tumors, it is usually only necessary to remove as
far as one level above the most caudal aspect of the tumor.

33. • Prior to the dural incision, the wound should be irrigated
and retractor systems and microscope prepared.
• If the dura is tense, the intracranial pressure can be
reduced with external ventricular drainage (if available),
hyperventilation, or mannitol, although* mannitol should
be used with caution in the sitting position as it has been
implicated in the development of subdural hematomas.
• All techniques for dural incision require crossing the
occipital and annular sinuses, which may be very large in
infants under age 2 years and can persist until 25 years of
age.

35. • A Y-shaped incision allows wide visualization and can
be extended if necessary.
• One superior limb should be incised first with a
number-15 blade.
• The incision should start just inferior to the transverse
sinus and travel obliquely to the midline, stopping
short of the occipital sinus.
• The other superior limb is incised next, and then they
are connected over the midline.
• If there is significant bleeding from the midline
occipital sinus, it should be controlled with obliquely
placed hemostatic clips or suture ligatures.

38. • Either way, both the superficial and deep layer of the
dura must be incised or the sinus will be tented open.
• The vertical limb of the Y is opened last using scissors
so that the dura can be tented if bleeding is seen.
• The vertical incision extends to the foramen magnum
so that it will extend below the falx cerebelli, which is
occasionally present in childhood.
• If bleeding is very troublesome, the dura can be
opened paramidline.

39. • The dura is then covered with a moist collagen
sponge or wet Gelfoam sandwich to prevent
desiccation and anchored to the fascia with 4-0
neurolon suture.
• This allows wide exposure of the cerebellar
vermis and hemispheres.
• The arachnoid is opened next over the cisterna
magna to allow drainage of CSF.
• If the tumor is in the cerebellar hemisphere,
another dural incision can be extended laterally
to more fully expose the involved cerebellum.

43. • Techniques for intradural exposure and resection of the
tumor will vary depending upon the location and size
of the tumor.
• Gentle separation of the cerebellar tonsils will expose
the cerebellomedullary fissure through the opened
vallecula giving an unimpeded view of the inferior roof
of the fourth ventricle.
• Narrow malleable automatic retractors can be used to
maintain separation of the tonsils; the retractor system
should be kept close to the patient so as not to
interfere with the subsequent operation.

45. • The operating microscope is brought into the
field and the anatomy is identified.
• In particular, the location of the caudal loops of
PICA should be carefully noted since they are
often tethered to the tonsils and the walls of the
cerebellomedullary fissure by small perforating
branches.
• The foramen of Magendie and the small tuft of
choroid plexus protruding from it will be clearly
seen, as well as any tumor that protrudes from
the foramen.

46. • The thin layers forming the lower part of the roof can be
opened to expose sufficient exposure, but if not, it is
sometimes helpful to retract the inferior vermis rostrally or
incise the caudal vermis, avoiding the gutter between the
vermis and the hemisphere to prevent injury to the inferior
vermian veins there.
• Lateral lesions may require removal of one tonsil by
dividing the pedicle attaching the superolateral margin of
the tonsil to the biventral lobule.
• To reach the lateral roof or lateral recess, part of the
cerebellar hemisphere can be resected without significant
morbidity as long as the dentate nuclei are not violated.

48. • If the tumor is not adherent to the floor of the
fourth ventricle, cottonoid patties should be
placed beneath the tumor to protect the delicate
brain-stem structures just beneath the floor.
• These cottonoids should be placed under direct
vision and never used as a tool to dissect the
tumor from the floor of the fourth ventricle.
• After the tumor has been removed, the glistening
white floor of the fourth ventricle should be
clearly visible.

49. • The retractors are then removed and the
cerebellar hemispheres allowed to fall back into
place.
• If there is extension of the tumor through one of
the foramina of Luschka into the cerebello-
pontine cistern, the ipsilateral tonsil and
cerebellar hemisphere can be retracted medially
to expose it.
• Sometimes it is necessary to do a secondary
retromastoid approach to completely resect the
tumor.

50. • The dura is closed using a running 4-0 neurolon
or polypropylene after approximating the dural
edges with interrupted sutures.
• A Valsalva maneuver will identify potentially
dangerous venous bleeding.
• The dural closure should be watertight if
possible, starting peripherally then working
centrally to gradually overcome the tension.

51. • If the dura is not watertight, there is increased
risk of pseudomeningocele due to a ball-valve
effect or hydrocephalus from arachnoid
adhesions produced by blood from the muscles.
• Sometimes the dura will be dried and shrunken
by the end of the case, especially if measures
have been taken to obliterate the occipital sinus.
In this case, the remaining defect can be covered
with a pericranial or fascial graft.

54. • Freeze-dried bovine pericardium or human
allograft dura can also be used, but use of
autogenous material is less likely to produce
postoperative aseptic meningitis.
• If clips were used on the midline occipital sinus,
they can be removed as the dura is sutured.
• The suture line may be covered with thrombin-
soaked Gelfoam.
• If a craniotomy was performed, the bone flap can
be secured with wires, plates and screws, or
sutures.

55. • Alternatively, the defect can be covered with a
titanium screen held in place by gently
compressing the screen and allowing it to
insert itself between the dura and inner
margins of the bony defect.
• The fascia is closed with interrupted
absorbable sutures to approximate the muscle
and fascia.

57. • If the fascia is dried and difficult to approximate,
the skeletal fixation apparatus can be loosened
and the neck extended to facilitate closure.
• An adequate amount of tissue must be left at the
superior fascial flap to prevent buttonholes at
superior nuchal line.
• The scalp is then closed in layers, ending with a
subcutaneous re-approximation using interrupted
absorbable sutures with inverted knots.

58. • If in the sitting position, all layers should start
from the caudal end of the wound so that the
tails do not hang in the way.
• The wound is then closed with sutures or
staples.
• The wound is covered with a sterile dressing
and the patient extubated in a supine
position.

60. Complications
• Hydrocephalus
• common
• In the past, many patients with tumors and hydrocephalus underwent temporizing
preoperative shunting to treat hydrocephalus and prevent pseudomeningocele,
CSF leak, and meningitis from fistula.
• However, more recently it has been observed that shunting is associated with
many complications, and the increased incidence of subdural hematoma,
infection, and brain-stem compression from upward herniation may outweigh its
benefits.
• Also, the advent of advanced radiographic imaging has allowed diagnosis of fourth
ventricular tumors much earlier than before, when patients were frequently
moribund with dehydration and malnutrition from vomiting and hydrocephalus
needed to be urgently treated.
• Today, only about 10% to 20% of patients with cerebellar and posterior fossa
tumors require permanent shunting and most of these have slow-growing tumors
such as astrocytoma since more acute tumors distend the ventricles for a short
period of time and do not allow outlet adhesions to form.

61. • Risk factors for shunt dependence include younger age, larger
preoperative ventricle size, and more extensive tumors.
• In many cases, preoperative high dose steroids will produce satisfactory
improvement in hydrocephalus.
• Otherwise, an appropriate alternative to shunting is perioperative external
ventricular drainage, especially if a patient presents lethargic or obtunded.
• This allows for precise pressure monitoring and control of drainage rate to
prevent upward herniation and, if continued postoperatively, clearance of
debris, proteinaceous blood, and air from the operation.
• Although external ventricular drainage does reduce the necessity to use
permanent shunts, the infection rate may be as high as 10%, so it should
be used judiciously.
• If a shunt is required for a malignant tumor, there may be an increased risk
of extraneural metastasis through the shunt tubing (especially to the
peritoneum), although some studies have suggested that such metastases
may occur as often in patients without shunts.

62. • Pneumocephalus
• Common - especially when patients are operated in the sitting position,
• common when patients have preoperative hydrocephalus, and frequently
results from overzealous drainage of CSF through an external ventricular
drain intraoperatively.
• *Since nitrous oxide can diffuse into air filled spaces, it is possible that
nitrous oxide contributes to tension pneumocephalus, although this is
controversial.
• If tension pneumocephalus is recognized intraoperatively, the patient
should be placed in Trendelenburg position and the operative bed
irrigated to replace air with the irrigating fluid.
• Symptomatic postoperative tension pneumocephalus can be treated with
a small frontal burr hole to relieve the pressure caused by the trapped air.
• Intraventricular air may cause ventriculoperitoneal shunt malfunction due
to airlock.

63. • Pseudomeningoceles
• Postoperative pseudomeningoceles affect 10% to
15% of all children with posterior fossa tumors.
• Normally, these are small collections of fluid that
respond well to serial lumbar punctures.
• Occasionally they can put the closure under
tension and eventually produce a leak, which
carries a risk of meningitis.
• Pseudomeningocele may be a manifestation of
hydrocephalus and in some cases may require a
CSF diversion shunt to control.

64. • Aseptic meningitis
• also called posterior fossa fever, is a rare occurrence after posterior
fossa surgery, especially for epidermoids or dermoids that rupture
intraoperatively leaking cholesterol cyst fluid, although it also
occurs after resection of astrocytoma or medulloblastoma.
• It may be a presenting symptom preoperatively but much more
common as a postoperative complication.
• Patients usually present about 1 week after surgery with fever,
headache, irritability, and CSF pleocytosis.
• It can be difficult in some cases to differentiate aseptic meningitis
from true bacterial meningitis, which should always be carefully
excluded before treating for aseptic meningitis.
• The condition resolves with steroid or anti-inflammatory treatment
and serial lumbar punctures to remove CSF.

65. • Cranial nerve palsies
• Transient or permanent cranial nerve palsies sometimes occur.
• The most common deficit is cranial VI and VII palsy caused by
disruption of the fourth ventricular floor along the facial colliculus
where the intrapontine course of the facial nerve loops around the
abducens nucleus.
• If this area is dissected or excavated, the deficit will often be
permanent, but even gentle diathermy with low-current bipolar can
produce a partial paralysis with total or near-total recovery.
• In most cases, patients with temporary facial weakness should be
treated to prevent corneal desiccation with artificial tears,
temporary tarsorrhaphy, or gold-weight implantation in the upper
eyelid.

66. • Permanent weakness has been treated with facial-hypoglossal
anastomosis, which can partially restore upper eyelid function.
• Abducens palsy is best treated with an eye patch to prevent diplopia (or
amblyopia if the patient is under 5 years of age); if the condition persists
beyond a few months, eye muscle surgery may be appropriate.
• Cranial nerve XII palsy can occur from injury to the hypoglossal trigone.
• While less common than facial palsy, this is a very serious complication
since it is usually bilateral since the nuclei are close together by the
median raphe.
• Patients present with dysarthria, swallowing apraxia, and continuous
drooling.
• When combined with cranial nerve VII or IX/X deficits, even aggressive
treatment with tracheostomy and feeding tubes may not prevent serious
complications due to aspiration.

67. • Skewed ocular deviation
• Skewed ocular deviation is a rare condition that is
sometimes seen after fourth ventricular surgery during
which the aqueductal opening is manipulated.
• This usually occurs with damage to the region of the
cerebral aqueduct.
• It is thought to occur because vertical yoking of eye
movements involves pathways that pass through the
periaqueductal gray matter in the mesencephalic
tegmentum.
• This condition usually resolves within weeks after surgery,
and can be avoided by gentleness when working around
the aqueduct.

68. • Posterior fossa syndrome
• The “posterior fossa syndrome,” also called
posterior fossa mutism or pseudobulbar palsy, is
characterized by the delayed onset of mutism,
emotional lability, and supranuclear lesions that
occurs within a few days after midline posterior
fossa operations.
• The syndrome has been seen in as many as 15% of
intraventricular approaches to lesions near the
brain stem, but has also been described with
supracerebellar infratentorial approach to the
pineal region and retromastoid lateral cerebellar
approach to the side or front of the brain stem.

69. • Patients present with global confusion, disorientation,
combativeness, paranoia, or visual hallucinations.
• They are generally alert and will follow simple commands, but will
sometimes refuse to speak or present scanning speech.
• Orofacial apraxia, drooling, dysphagia, pharyngeal dysfunction, and
flat affect are common, but there is no actual weakness, hence the
term pseudobulbar palsy.
• Because of the delay in onset, it has been suggested that edema
from operative manipulation may play a role, for example through
transmission of retractor pressure from the medial cerebellum
through fiber pathways along the middle and superior cerebellar
peduncles into the upper pons and midbrain.
• There are no consistent neuropathologic findings, and most
patients have some improvement over several weeks to months.

70. • Seizures
• Generalized and focal seizures
• The incidence is higher in faster growing
tumors and in the presence of ventricular
drainage or shunting.
• Late-onset seizures may be related to remote
hemorrhage, meningitis, or hydrocephalus.

71. • Ipsilateral limb ataxia, dysmetria, dysdiadokinesis, and hypotonia
usually results from damage to the cerebellar hemisphere,
especially the dentate nucleus, which is located along the
superolateral margin of the roof of the fourth ventricle adjacent to
the upper pole of the tonsil.
• Most injuries to the dentate nucleus occur during dissection of a
hemispheric tumor.
• Retraction during dissection of the superior vermis can injure the
superior cerebellar peduncle (which) producing similar symptoms.
• Unless the dentate is completely ablated, most patients recover
well within a few months with only minor residual intention tremor
that does not interfere with motor development.

72. • Since the superior and inferior cerebellar peduncles
make up the lateral walls of the superior roof of the
fourth ventricle, they are susceptible to damage during
intraventricular procedures.
• The superior cerebellar peduncle contains pathways
connecting the dentate nucleus to the red nucleus and
thalamus, so damage to the superior cerebellar
peduncle produces a similar clinical syndrome to
damage of the dentate nucleus with ipsilateral ataxia
and intention tremor.

73. • Injury to the inferior cerebellar peduncle
produces a syndrome similar to ablation of the
flocculonodular lobe with equilibrium
disturbances, truncal ataxia, staggering gait, and
oscillation of head and trunk on assuming erect
position without ataxia of voluntary movement of
the extremities.
• Injury to the middle cerebellar peduncle (which
causes ataxia and dysmetria) is rare during
intraventricular procedures, but can occur during
an approach to the cerebellopontine cistern.

74. • Postoperative dysarthria can result when
resections extend into paravermian part of
cerebellar hemisphere.
• This occurs more frequently from left
hemisphere injury than from vermal or right
hemisphere injury.

75. • Acute urinary retention is an uncommon complication of dissection
of the fourth ventricular floor near the striae medullaris,
presumably due to injury to the pontine micturition center in the
pontine tegmentum, the structure that integrates the cortex with
sacral and pelvic sensory pathways that apprise bladder filling
status.
• Patients with this condition demonstrate inability to initiate voiding
in spite of a full bladder with high intravesicular pressure.
• Since the pontine micturition center is deep in the pons near the
reticular activating system, this symptom is usually associated with
a disturbance in sensorium, but can occur in conscious patients.
• It is usually reversible but does not respond to detrusor augmenting
agents or alpha-adrenergic blockers.
• Patients are best managed by intermittent catheterization.

76. • Radiation hazards
• Patients treated with radiation sometimes have significant learning
disabilities, and should undergo follow-up neuropsychiatric
evaluation.
• Radiation treatment has also been associated with endocrine
dysfunction, growth dysfunction, hypothyroidism, delayed or
precocious puberty, and secondary malignancy.
• Patients that have extensive laminectomies are predisposed to
development of swan neck deformity, and should be kept in a soft
cervical collar for 6 to 8 weeks until the paraspinal muscles reattach
and monitored with cervical spine x-rays every few months for a
few years to check for spinal deformities.

77. • Injury to major vessels is rare.
• The most likely artery to be injured is PICA.
• Most patients with PICA injury present with postoperative flocculonodular
dysfunction with nausea, vomiting, nystagmus, vertigo, and inability to
stand or walk without appendicular dysmetria.
• Venous injury is extremely rare even if veins are sacrificed due to diffuse
anastomosis in this region.
• Veins near the tonsils, vermis, and inferior roof can be safely sacrificed.
• Medial retraction of the cerebellar hemisphere to expose the lateral
recess and cerebellopontine cistern can stretch bridging veins to the
sigmoid sinus, but it is seldom necessary to sacrifice these.
• Most venous infarctions of the posterior fossa have followed sacrifice of
the petrosal veins or veins of the cerebellomesencephalic fissure
(including the precentral cerebellar vein).

78. Reference
Thank you