1.4 Posterior Fossa Tumour Resection

Question 1

A 34-year-old man is scheduled for a posterior fossa tumour excision.
a) List patient positions that might be employed for this operation. (10%)

Answer 1

> > Sitting.
Lateral.
Prone.
Supine.
Park bench.

Question 2

b) What potential intraoperative problems are associated with posterior fossa craniotomy? (25%)

Answer 2

Surgery itself:

> > Venous air embolism (VAE),

paradoxical air embolism.
Air entrainment can happen whenever the venous sinuses are open,
but the risk is increased if the open sinuses are elevated, as occurs in e.g. sitting
position, thus increasing the pressure differential between them and atmosphere further.

> > Cardiovascular instability (hypo- or hypertension, brady- or tachycardia, arrhythmia) due to stimulation of cranial nerve nuclei and other brainstem structures.

> > Bleeding, cerebellar haematoma.*

> > Brainstem damage:
respiratory and cardiovascular centres
and cranial nerve nuclei.

> > Long tract damage.*

> > CSF leak.*

> > Meningitis, wound infection.*

Positioning:
Sitting:

• Airway:
tube displacement,
jugular venous obstruction due to
flexed neck causing laryngeal and tongue oedema.

• Cardiovascular:
VAE, hypotension due to reduced venous return.

• Neurological: cord or brainstem ischaemia due to head flexion and hypotension,
sciatic nerve damage, pneumocephalus.*

• Cutaneomusculoskeletal:
compartment syndrome, lumbosacral pressure sores

Other positions:
• All other positions that involve moving the patient after induction involve risk of tube dislodgement.

• Each position has its unique pressure points, elbow, knee, ankle for lateral, genitalia and knees for prone.

Also, each position has its individual risks for nerve palsies, such as brachial plexus compression
in lateral position, brachial plexus stretch and ulnar nerve damage with prone, brachial plexus stretch and common peroneal compression with park bench.

• Prone position has the additional issues of reflux due to raised intragastric pressure, decreased venous return, corneal damage, central retinal artery occlusion, ischaemic optic neuropathy.

Question 3

c) What monitoring techniques can specifically detect the presence of venous air embolism during
surgery and for each method used, give the features that would indicate the diagnosis? (40%)

Answer 3

Transoesophageal echo:
» Air in right-sided cardiac chambers. In the presence of patent foramen ovale, it can detect air in the left heart also. Not necessarily suitable for long operations where the head is flexed.

Precordial Doppler:
» Sound heard if air present in cardiac chambers.

Question 4

d) How would you manage a significant venous air embolism in this patient? (25%)

Answer 4

This is a medical emergency, and I would alert the theatre team, call for help and adopt an ABC approach, assessing and managing issues simultaneously. The aims of management are as follows:

> > Prevent further air entry:
flood site with saline, fluid load,
**lower patient so that the surgical site
is below the right atrium if possible,
apply sustained positive airway pressure until this is all achieved.

> > Reduce size:
stop nitrous oxide if it is being used*,
administer 100% oxygen,
aspirate air from right atrium via central line.

> > Overcome mechanical obstruction:
left lateral or Trendelenberg positioning may help force bubble above the right ventricular outflow.

*Inotropic support may be required. If the patient suffers cardiac arrest, chest compressions may assist in dispersing the bubble.

Question 5

Anatomy

Answer 5

The posterior fossa is the deepest cranial fossa
and is surrounded by the dorsum sellae and
basilar portion of the occipital bone (clivus) anteriorly, the petrosal and mastoid components of
the temporal bone laterally and the dural layer
(tentorium cerebelli) superiorly and the occipital
bone posteriorly and inferiorly.

Question 6

Anatomy- Borders

Answer 6

The posterior fossa is the deepest cranial fossa

and is surrounded by the dorsum sellae

anteriorly,
and basilar portion of the occipital bone (clivus)

laterally
the petrosal and mastoid components of the temporal bone

superiorly
and the dural layer (tentorium cerebelli)

posteriorly and inferiorly.
the occipital bone

The foramen magnum in the occipital bone is the largest
opening of the posterior fossa

Question 7

Contents

Answer 7

The posterior fossa contains many important structures

including the
brainstem,
cerebellum and
lower cranial nerves.

The sigmoid, transverse and occipital
sinuses all traverse the fossa.

The cerebrospinal fluid (CSF) pathway is
very narrow through the cerebral aqueduct
and any obstruction can cause
hydrocephalus which can result in a significant
increase in intracranial pressure (ICP)

Question 8

Pathology

Answer 8

Tumours are the commonest pathology affecting the posterior fossa.

They account for more than 60% of all brain tumours in children.

Fifteen per cent of intracranial aneurysms occur
in the posterior fossa vasculature,

and other vascular malformations causing compression
and neuralgia of the trigeminal nerve may
warrant surgical decompression.

Pathologies of the posterior fossa
which may require surgical intervention

1. Tumours
   a. Axial tumours
   b. Cerebellopontine angle tumours
2. Vascular malformations
3. Cysts
4. Cranial nerve lesions
5. Craniocervical abnormalities

Question 9

Table 1 Posterior fossa pathologies

Answer 9

1. Tumours

a. Axial tumours

Medulloblastoma (commonest)
Cerebellar astrocytoma
Brainstem glioma
Ependymoma
Choroid plexus papilloma
Dermoid tumours
Hemangioblastoma
Metastatic tumours

b. Cerebellopontine angle tumours
Schwannoma
Meningioma
Acoustic neuroma
Glomus jugulare tumour

2. Vascular malformations

Posterior cerebellar artery aneurysm
Vertebral/vertebrobasilar aneurysm
Basilar tip aneurysm
AV malformations
Cerebellar hematoma
Cerebellar infarction

3. Cysts
   Epidermoid cyst
   Arachnoid cyst
4. Cranial nerve lesions
   Trigeminal neuralgia (cranial nerve V)
   Hemifacial spasm (cranial nerve VII)
   Glossopharyngeal neuralgia (cranial nerve IX)
5. Craniocervical abnormalities

a. Atlanto-occipital instability
Congenital
Acquired

b. Atlanto-axial instability
Congenital
Acquired

c. Arnold–Chiari malformation

Question 10

Positions - extra

Answer 10

Acoustic neuroma and
cerebellopontine angle tumours

may be carried out in the supine position

with the head turned to the opposite side,
and placement of a sandbag under
the ipsilateral shoulder to minimize stretching of
the brachial plexus

The prone and sitting positions offer good access to structures

in the midline,

but care should be taken to avoid abdominal compression
to minimize surgical bleeding.

The lateral position facilitates gravity assisted drainage of blood and CSF and gives good surgical access for unilateral procedures.

The park bench position is a modification of the
lateral position where the patient is positioned
semi-prone with the head flexed and facing the
floor.

This facilitates greater access to midline
structures and,

in selected patients, avoids the
need for the prone position.

Meticulous care should be taken during positioning
to avoid dislodgement of lines and the tracheal tube,
and protection of pressure areas

Question 11

Sitting position

Why is it beneficial

Answer 11

The sitting position improves surgical access to

the posterior fossa by facilitating gravity assisted
drainage of blood and CSF

and decreasing ICP

It improves surgical orientation,

access to the midline structures and
decreases the amount of surgical retraction needed to gain
access to deeper structures.

Patients in the sitting position must be
returned to the supine position
rapidly for resuscitative measures in case of an
acute cardiovascular collapse.

Question 12

Venous air embolism

What is it

Incidence?

Different in kids?

Answer 12

Venous air embolism (VAE) is a

potentially life-threatening complication

associated with all surgery in the steep head-up position,

including posterior fossa surgery in the sitting position.

The cited incidence of VAE varies from 25 to 75% during surgery
in the sitting position depending on the sensitivity of the monitoring used.

In one study, the incidence of VAE in children was found to be less
than in adults because of the higher pressure in the venous sinuses

Question 13

Why VAE

Answer 13

In the sitting position,

the site of surgery is above the level of the heart

and this results in a negative venous pressure at the level of surgical wound.

Open veins thus entrain atmospheric air into the circulation,
resulting in VAE.

Air entrainment usually occurs through the
diploic veins and open venous sinuses,

but entrainment through head pin sites has also been reported.

Dehydration exacerbates the low venous pressure
and increases the risk of air entrainment,

so normovolaemia must be maintained at all times.

When air passes into the pulmonary circulation,

it causes an increase in pulmonary vascular
resistance and pulmonary hypertension.

This results in elevated right heart pressure and the
risk of paradoxical air embolism.

Question 14

VAE Manifestations

Answer 14

Clinical features of VAE depend on the rate and volume of air
entrained.

The spectrum of manifestations includes

cardiovascular,
respiratory and
neurological changes.

Elevated right atrial pressure results in
decreased venous return, hypotension and shock.

Tachyarrhythmia and myocardial ischaemia may ensue.
A large embolus obstructing the outlet of the right ventricle

can result in a sudden onset right heart failure and cardiac arrest.

Pulmonary signs of VAE include
wheeze, crepitations, and sudden decrease in end tidal carbon dioxide (E0
CO2).

Arterial blood gas analysis may reveal
hypoxia and hypercapnia.

Neurological manifestations include cerebral hypoperfusion
as a result of shock and stroke in the event of a
paradoxical embolus.

Question 15

VAE - Decreasing the risk

Answer 15

The risk of VAE can be minimized in many ways.

1. Use of Trendelenburg tilt and leg elevation minimize
   the gradient between the surgical field
   and the right heart and,

2 Hydration status should be carefully optimized.

3. Though Military Anti Shock Trousers can be used to
   elevate right atrial pressure, the potential
   benefits must be weighed against the risks of
   decreasing vital capacity and hypoperfusion to intra-abdominal organs.

Question 16

Suspected VAE

Answer 16

When VAE is suspected,

the surgical field should

immediately be covered with saline soaked swabs
to prevent further air entrainment,

and any suspected air entry point sealed.

If possible, the surgical field should be positioned
below the level of heart.

Though jugular venous compression reduces air entrainment,
it should be used with caution as it increases ICP

and decreases cerebral blood flow
because of the simultaneous compression

of the carotid artery.

Question 17

Dx VAE

Answer 17

Once diagnosed,

the management of VAE is generally supportive.

Oxygen should be administered in high concentration

and nitrous oxide if used must be discontinued immediately.

Cardiovascular stability should be maintained with

i.v. fluid loading and a vasopressor.

A carefully positioned central venous catheter can be used to aspirate
air from the right atrium.

It may be possible to relieve the air lock in the right ventricle by positioning the patient in partial left lateral position (Durant manoeuvre)

or in the Trendelenburg position.

In the event of massive VAE with cardiac standstill,

immediate initiation of chest compression can result in the breaking down of a large air bubble obstructing the right ventricular outflow tract with return of
spontaneous circulation.

Question 18

Pneumocephalus

Answer 18

1. Pneumocephalus is a recognized complication of surgery in the
   sitting position.
2. During surgery,
   various techniques may be used to
   reduce the volume of the brain
   which encourages air to enter the
   intracranial space and,

when the cranial cavity is closed and the brain expands,

the air is compressed causing a mass effect with elevated ICP.

_

3. The condition is worsened with the use of nitrous oxide,
   but avoiding its use does not eliminate the problem.
4. Pneumocephalus can present after operation as delayed recovery,
   neurological deficit, headache, confusion, agitation or convulsion.
5. CT scan enables early diagnosis.

High-flow oxygen reduces pneumocephalus but,

in severe cases, neurosurgical treatment by means
of burr hole and aspiration of air should be considered

6. Untreated tension pneumocephalus can result in brain herniation and cardiac arrest

Question 19

Macroglossia

Answer 19

Macroglossia can occur as a result of obstruction

to the venous and lymphatic drainage
of the tongue because of a flexed neck during
prolonged surgery in the sitting position.

It may cause postoperative
respiratory obstruction, particularly in children.

Question 20

Quadriplegia

Answer 20

Quadriplegia is a rare but potentially disastrous complication

that is caused by prolonged focal pressure on the spinal cord

secondary to the acute flexion of the head in the sitting position.

Regional spinal cord blood flow can be compromised

particularly during episodes of significant
hypotension resulting in ischaemic damage to the spinal
cord.

-

Meticulous attention during positioning and avoiding significant and prolonged hypotension during surgery can help avoid this
complication.

Question 21

Preoperative evaluation

Answer 21

Thorough preoperative evaluation of the

neurological condition and
cardiorespiratory status is very important for the
safe conduct of anaesthesia
for posterior fossa surgery.

Routine assessment of coexisting
medical conditions with a view to optimization
should be performed.

Other important aspects of preoperative assessment include:

(i) Evaluation for cerebellar and cranial nerve dysfunction:

Question 22

Preoperative evaluation

Answer 22

Thorough preoperative evaluation of the

neurological condition and
cardiorespiratory status is very important for the
safe conduct of anaesthesia
for posterior fossa surgery.

Routine assessment of coexisting
medical conditions with a view to optimization
should be performed.

Other important aspects of preoperative assessment include:

(i) Evaluation for cerebellar and cranial nerve dysfunction:

(ii) Evaluation for the presence of elevated ICP

(iii) Evaluation of hydration status and electrolyte disturbance:

(iv) Evaluation for intraoperative positioning:

(v) Evaluation of the airway:

Question 23

(i) Evaluation for cerebellar and cranial nerve dysfunction:

Answer 23

presence of lower cranial nerve compression and dysfunction

can result in loss of the gag reflex and aspiration pneumonitis.

In some patients with bulbar dysfunction,
postoperative ventilation or tracheostomy
may be necessary to protect the airway.

Cerebellar signs may include
ataxia, dysarthria, gait disturbances and intentional tremors.

Question 24

(ii) Evaluation for the presence of elevated ICP

Answer 24

hydrocephalus and elevated ICP are common in patients with posterior fossa
pathology.

Decrease in the level of consciousness and altered
respiratory pattern may indicate the presence of elevated ICP
and, under such circumstances, CT or magnetic resonance
imaging is mandatory.

External ventricular drainage or other
shunt procedures may be indicated to manage hydrocephalus
before surgery or intraoperatively

Question 25

(iii) Evaluation of hydration status and electrolyte disturbance:

Answer 25

dehydration is common and multi-factorial.

Reduced oral intake because of
decreased conscious level,
vomiting,
administration of diuretics,
presence of diabetes insipidus and the use
of i.v. contrast agents to facilitate imaging contribute to dehydration and electrolyte disturbances.

Preoperative administration of i.v. fluids and
optimization of electrolytes should be
considered on an individual basis

Question 26

(iv) Evaluation for intraoperative positioning:

Answer 26

patients should be carefully assessed for the suitability of surgical positioning.

The prone position can be difficult in obese patients.

The presence of a patent foramen ovale (PFO),
which has an incidence of 10–35%,

may preclude the use of the sitting position
though paradoxical embolism can occur even in its absence.

As the complications of paradoxical air embolism
can be devastating,

screening for PFO with bubble contrast
echocardiography and closure if present should be considered
where the sitting position will have a major advantage in the
outcome of surgery

Question 27

(v) Evaluation of the airway:

Answer 27

patients with atlanto-axial subluxation

and lack of neck movement secondary to cranio-cervical
fusion can present challenges during

airway management and positioning.

Question 28

Monitoring
Appropriate intraoperative monitoring is crucial during posterior
fossa surgery.

Answer 28

Routine monitoring should include pulse oximetry, ECG, capnography and temperature. Invasive arterial monitoring will allow the
measurement of beat-to-beat variability and accurate control of
blood pressure, and is mandatory. The arterial transducer should be
placed at the level of external auditory meatus to correlate with cerebral perfusion.

The insertion of a central venous catheter in the right internal
jugular vein is a common practice in patients who undergo surgery
in the sitting position as it allows assessment of volume status and
the aspiration of air during VAE. Measurement of urine output with
an indwelling urinary catheter is also essential

Question 29

Monitoring for venous air embolism

Answer 29

Monitoring for venous air embolism

Various monitors can be used to detect VAE (Table 3).

Precordial Doppler is the most sensitive non-invasive monitoring device used
to detect air embolism, and as little as 0.015 ml kg21 min21 of intracardiac air can be detected.9

It is usually placed in the right parasternal region.

Verification of the position of the probe is vital for early
diagnosis of VAE and can be carried out by injecting a small bolus
of saline through the central venous catheter, although a peripheral
venous catheter can be used as an alternative.

Transoesophageal echocardiography and Doppler is more sensitive than precordial Doppler but is invasive, expensive and can be associated with complications such as oesophageal injury

Question 30

Other signs of VAE

Answer 30

VAE results in an increase in dead space ventilation and a decrease in the level of E0 CO2

Setting tight alarm limits for ECO2 will
issue early warning for possible VAE.

Measurement of end-tidal
nitrogen (E0N2)
is a more sensitive and specific method for the
detection of VAE and,
when the nitrogen present in the air embolus
is expelled by the lungs,

EtN2 levels suddenly increase.

Though a pulmonary artery catheter can be used as a monitor to detect VAE,
it is not a sensitive indicator and not routinely used for this purpose.

An oesophageal stethoscope was previously used to detect VAE
but a significant volume of air has to be entrained before the classic
‘mill-wheel’ murmur is heard, and this is usually already associated
with signs of cardiovascular collapse. With the availability of more
sensitive non-invasive monitors to detect VAE, the oesophageal
stethoscope is of little clinical use

Question 31

Neurological monitoring

Answer 31

Neurological monitoring
The electroencephalogram can be used to detect cerebral hypoperfusion and cortical ischaemia, and somatosensory evoked potentials
(SSEP) to monitor spinal cord ischaemia in the sitting position.

Brainstem auditory-evoked potentials monitor the function of the
VIII cranial nerve and continuous electromyographic monitoring of
the VII cranial nerve minimize damage to the nerve during
cerebellopontine angle surgery. The use of these monitors requires
modification to the anaesthetic technique to minimize interference
with the monitoring. These include maintaining consistent and
modest levels of inhalation or i.v. anaesthetic agents to minimize
interference during SSEP monitoring, avoidance of neuromuscular
blocking agents, and the use of total i.v. anaesthesia during motorevoked potential monitoring

Question 32

Goals of Anaesthesia

Answer 32

Anaesthetic technique

The goals of anaesthetic management are to avoid

1. significant increase in ICP,
   maintain cerebral perfusion pressure,
   avoid haemodynamic instability,
   enable intraoperative neuro-monitoring and
   ensure the early detection and management of complications.
2. Haemodynamic instability during induction and positioning
   should be avoided and interruptions in monitoring during positioning minimized.
3. Remifentanil can be used in patients with elevated
   ICP to reduce the stress response to intubation.
4. Either an inhalation or an i.v. technique
   can be used for the maintenance of anaesthesia,
   but nitrous oxide should be avoided,

particularly in patients with a high risk of VAE or pneumocephalus.

5. Any unexpected haemodynamic change or instability should be notified to the surgeon immediately because it may indicate close surgical proximity to vital centres.

6.Normothermia should be maintained throughout. Careful
observation of blood loss and volume status of the patient should be
ensured.

Question 33

Postoperative management

Answer 33

1. Extubation depends on the preoperative condition
   of the patient and the intraoperative course.
2. In a patient who is neurologically intact
   and who has had uneventful surgery,
   smooth emergence and extubation should be carried out and the patient monitored for signs of neurological changes in a high dependency area.
3. The presence of lower cranial nerve dysfunction
   and potential for aspiration pneumonia
   may warrant postoperative ventilation.
4. Extensive intraoperative dissection,

particularly in the floor of the fourth ventricle and around
the cranial nerve nuclei, may result in postoperative airway
compromise after extubation.

5. Airway oedema after prolonged prone positioning
   and tongue swelling after the sitting position are not uncommon.
6. ICP monitoring should be considered if postoperative ventilation
   is required because hydrocephalus remains a risk.

7 Postoperative hypertension should be
carefully managed to avoid
bleeding complications.

8. Postoperative nausea and vomiting (PONV) and pain are important considerations after posterior fossa surgery.
9. Occipital and infratentorial approaches are associated
   with severe postoperative pain
   due do extensive muscle cutting and
   reflection and subsequent spasm.

10 All patients undergoing surgery on the posterior fossa
should be considered to be at high risk of PONV because of the
proximity of the vomiting centre to the surgical locality.

11. Use of opioids to control the pain after extensive muscle cutting surgery
    compounds the problem

Question 34

Paediatric considerations

Answer 34

Brain tumours are the most common solid tumours in children

Peak incidence is between 5 and 8 years of age and up to two-thirds
are located in the infratentorial region.

Presentation includes obstructive hydrocephalus,
signs of elevated ICP and cranial nerve dysfunction.

It is important to ensure the function
of a shunt if already present.

As in adults, preoperative screening for the presence of a
PFO is very important as its presence may preclude the use of the
sitting position.

Though establishing the sitting position can be difficult in very young children, in one large review, the age range for the sitting position in paediatric practice was 6 weeks to 17 years,
with the median age of 5.4

Anaesthetic management combines the principles of paediatric
and neuro-anaesthesia.

Premedication should be avoided in the presence of symptomatic hydrocephalus.

Blood loss should be meticulously monitored and appropriately replaced. As the surface area of the head is comparatively large in children,
care should be taken to prevent hypothermia.

Extubation can be performed after full recovery and in the presence of intact airway reflexes.

Elective admission to a high dependency area or paediatric intensive care unit will be required.

Analgesia can be established by the regular administration
of paracetamol and codeine, and an appropriate dose of morphine
along with an antiemetic can be used effectively in the high dependency area with regular observation.

The posterior fossa syndrome can complicate the postoperative
course in children.

This syndrome is defined as temporary and complete loss of speech after posterior fossa surgery and risk factors include medulloblastoma and midline location of tumours.

Early diagnosis is important to facilitate speech and language therapy and
to promote parental understanding and coping with the syndrome.