Cranial surgery

Question 1

What is the normal intracranial pressure of dogs and cats?

Answer 1

5-12mmHg

Affected by arterial inflow, venous outflow and resistance of cerebral vasculature

Question 2

How do you calculate cerebral perfusion pressure (CPP)?

Answer 2

CPP = MAP - ICP

ICP closely approximately venous outflow pressure

Question 3

What factors can alter cerebral blood volume without altering blood flow?

Answer 3

• Low head position
• Venous outflow obstruction
• Increases intrathoracic pressure

Question 4

What factors alter cerebral blood volume by altering the blood flow?

Answer 4

• CO2
• O2
• Lactic acid
• Drugs

Question 5

What three primary homeostatic mechanisms maintain intracranial pressure?

Answer 5

Volume buffering
- Increase in one parameter (blood volume) causing a decrease in another (CSF volume)

Autoregulation of blood flow
- Reflex vascular changes regulated by the pial arterioles at pressures between 50 - 150mmHg

Chemical autoregulation
- Cerebral vasodilation with increases PaCO2
- Cerebral vasoconstriction with decreased PaCO2
- Decreased PaO2 will eventually cause cerebral vasodilation (less then 60mmHg)

Question 6

What is the Cushing’s reflex?

Answer 6

• Uncontrolled ICP leads to reduced cerebral blood flow, ischaemia and hypercarbia
• Stimulates catecholamine release leading to systemic vasoconstriction and increased cardiac output (thereby increased CPP)
• Baroreceptors sense hypertensive stare and cause a vagally mediated bradycardia

Indicates imminent brain herniation

Question 7

What is the ideal PaCO2 which is aimed for during intracranial surgery?

Answer 7

30 - 35mmHg

Below 30, neuronal ischaemia can occur and exacerbate intracranial hypertension

Question 8

What monitoring systems are available for monitoring ICP?

Answer 8

• Fiber-optic based systems
• miniaturised strain gauge devices
• Transcranial Doppler ultrasound (indirect)

Question 9

What are the anaesthetic goals for managing intracranial hypertension

Answer 9

• Prevent hypercapnia and hypoxia
• Prevent systemic hypotension (maintain at or above 80mmHg to maintain a CPP of 50 - 90mmHg)
• Reduce cerebral oedema (mannitol/hypertonic saline)
• Corticosteroids (neoplasm associated oedema and primary inflammatory disease)
• Control cerebral venous blood volume (head elevation etc)
• Control cerebral oxygen demand (Hyperthermia, seizures, pain and ketamine all increased oxygen demand)

Question 10

What is unique about burring for cranial surgery?

Answer 10

• Smallest burr size allows a secure fit if planning to replace bone
• Inner cortical bone has highly irregular undulations with marked differences in bone thickness
• Dorsal calvaria is considerably thicker
• Cutting at angle of 30 degree allows for almost complete apposition
• Cancellous bone is called diploe

Question 11

What electrosurgical device is preferred when working on dural and parenchymal tissues?

Answer 11

Irrigation-coupled bipolar device

Standard bipolar on minimum effective setting with constant slow, steady saline drip sufficient

Question 12

What can be used to make the initial dura cut?

If wanting to close the durs, what needs to be done intraop?

Answer 12

• # 11 Bard Parker blade, the tip of a needle or microscissors
• It must be kept stretched during the procedure to prevent shrinkage - attach to surrounding tissues using suture, skin staples or mosquito forceps

Question 13

What can be used to close a dural defect?

Answer 13

• Fascia of temporalis muscle (surface of fascia facing towards brain)
• Porcine SISM

Sutured with 4-0 PDS or tissue glue

Question 14

What are some options for cranioplasty?

Answer 14

• Replacement of excised bone
• Acrylic cranioplasty (PMMA)
• Metallic mesh with low profle self-tapping screws

Avoid placing foreign material where is may become contaminated by air from paranasal sinuses

Question 15

What fluorescent dyes can be used intra-op to aid in lesion localisation?

Answer 15

• 5-aminolevulinic acid
• Indocyanine green

Question 16

List the main approaches to the brain and the area of the brain which they are best used for

Answer 16

Transfrontal craniotomy
- olfactory bulbs and rostral portion of frontal lobe
- Modified transfrontal approach increased visibility and surgical access

Rostrotentorial approach
- frontal, parietal, temporal and occipital lobes of the cerebrum
- Lateral ventricles, falx cerebri and corpus callosum also approached through gyrotomy
- Can be extended caudally to expose tentorium cerebelli following occlusion of transverse sinus

Suboccipital craniectomy or Caudotentorial craniectomy
- Caudal cerebellum, dorsal aspect of medulla, 4th ventricle, cranial cervical spinal cord

Transverse Sinus Occlusion
- Dorsal cerebellopontine angle, cerebellum, lateral aspect of tentorium cerebello
- Usually combined with suboccipital or rostrotentorial approach

Approach to pituitary gland
- transsphenoidally (transorally) or via ventral paramedian approach

Question 17

Why is it extra important to close dural defects after a transfrontal approach?

Answer 17

To prevent infection and pneumocephalus due to communication with the paranasal sinuses

Question 18

What may cause significant haemorrhage in a transfrontal approach?

Answer 18

• Dorsal sagittal sinus
• internal ethmoid artery (ventral aspect of olfactory-rostral frontal lobe region)
• If you need to remove part of cribiform plate

Question 19

What external nerves need to be avoided during the rostrotentorial approach?

Answer 19

• Auriculotemporal nerve/temporal nerve supplying the temporalis muscle . Exit and course ventrally at the level of the zygomatic process
• Palpebral branch or auriculopalpebral nerve if combining with osteotomy of zygomatic arch

Question 20

What vessels can cause significant bleeding during the suboccipital craniectomy?

Answer 20

• occipital emissary vein (exits near mastoid foramen)
• Condyloid vein (adjacent to the condyle)
• Internal venous plexus/interarcuate branch or dura
• Limited laterally by the transverse sinus and dorsally by the confluens sinuum

Question 21

Where is the pituitary gland located?

Answer 21

In the sella turcica of the sphenoid bone

Question 22

What are the broad options for brain biopsy?
What kind of needles are recommended?

Answer 22

• Freehand-guided aspiration through small burr holes
• Larger craniotomy approaches guided by advanced imaging

Procedure specific, minimally traumatic, side-cutting guillotine biopsy needles eg, Nashold needle

Question 23

What are the reported rates of diagnostic yield with stereotactic brain biopsy?
Morbidity?
Mortality?

Answer 23

• Diagnostic yield over 90%
• Morbidity 27% (generally less then 5% with experience and avoiding high risk areas such as brain stem)
• Mortality is rare

Question 24

What are the benefits of brain neoplasia resection?

Answer 24

• Increased survival times and immediate improvement
• Improve efficacy of adjuvent radiation and chemotherapy
• Allow introduction of therapies directly into the neoplasm or resection cavity

Question 25

List some congenital or developmental diseases which may benefit from surgical intervention

Answer 25

• Intracranial arachnoid diverticula
• Dermoid and epidermoid cysts
• congenital hydrocephalus
• malformation of the caudal cranial fossa and craniocervical junction (Chiari-like malformation)

Question 26

Where in the ventricles should a ventriculoperitoneal shunt be placed?
What is the function of the control valve?

Answer 26

• Placed in the frontal horn or temporal horn of the lateral ventricle to avoid irritating the vascular choroid plexus
• Control valve helps to maintain physiological ventricular pressures by controlling the siphoning effect. Simulates normal CSF drainage
• Paralumbar skin incision caudal to last rib for placement of abdominal catheter