Ch 35 Cranial surgery Flashcards
What is the normal intracranial pressure of dogs and cats?
5-12mmHg
Affected by arterial inflow, venous outflow and resistance of cerebral vasculature
brain is protected by the confines of the cranial cavity, where it exists in equilibrium with blood and cerebrospinal fluid
Normal function of neurons depends on an adequate supply of oxygen and glucose.
High metabolic demands + limited energy storage capacity necessitate keeping cerebral blood flow within a relatively normal range at all times.
An increase in the volume of one component requires a reciprocal decrease in one or more of the others if intracranial pressure is to remain relatively unchanged (Monro-Kellie doctrine)
How do you calculate cerebral perfusion pressure (CPP)?
CPP = MAP - ICP
ICP closely approximately venous outflow pressure
What factors can alter cerebral blood volume without altering blood flow?
Low head position
Venous outflow obstruction
Increases intrathoracic pressure
What factors alter cerebral blood volume by altering the blood flow?
CO2
O2
Lactic acid
Drugs
What three primary homeostatic mechanisms maintain intracranial pressure?
autoregulation is the ability to maintain normal cerebral perfusion
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)
What is the Cushing’s reflex?
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
pre-op consdierations
- minimum database, concurrent health problems
- thorax/abdo rads/us
- assess for coagulopathy (no nsaid, PT/aPTT
- pre-Sx corticosteroid
anaesthetic
- all monitoring equipment
- mechanical ventilation and blood products are readily available.
- Premedication limited to an analgesic
- induction and entube: avoid increase ICP
- TIVA (prefered over GA)
- Intermittent positive pressure ventilation is important in order to maintain optimal PaCO2
What is the ideal PaCO2 which is aimed for during intracranial surgery?
30 - 35mmHg
Below 30, neuronal ischaemia can occur and exacerbate intracranial hypertension
What monitoring systems are available for monitoring ICP?
Fiber-optic based systems
miniaturised strain gauge devices
Transcranial Doppler ultrasound (indirect)
What are the anaesthetic goals for managing intracranial hypertension (7)
1.Prevent hypercapnia
2.Prevent hypoxia
(Oxygen supplementation, blood transfusions, controlled ventilation PaCO2 30)
3.Prevent systemic hypotension (maintain at or above 80mmHg to maintain a CPP of 50 - 90mmHg, use vasopressors)
4.Reduce cerebral oedema (mannitol/hypertonic saline) : Hydration status, electrolyte concentrations, and acid-base status should be determined
5.Corticosteroids (neoplasm associated oedema and primary inflammatory disease)
6.Control cerebral venous blood volume (head elevation etc)
7.Control cerebral oxygen demand (Hyperthermia, seizures, pain and ketamine all increased oxygen demand)
Mannitol is believed to have two effects on intracranial pressure: an immediate (minutes) plasma-expanding effect that reduces blood viscosity and increases cerebral blood flow and O2 delivery to the brain and a delayed, osmotic effect that occurs 20 to 30 minutes after
0.5 to 1.0 g/kg IV over 10 to 20 minutes.
7.5% NaCl solution 4 mL/kg IV administered over 10 minutes.
frusemide 0.7 to 2 mg/kg IV.
pre-op imaging
(MRI) is generally the imaging modality of choice; however, the additional data provided by computed tomography (CT) imaging may be particularly useful where bony lesions such as fractures, skull-based multilobular tumors of bone, or osteo/chondrosarcomas are present or for surgical localization in hypophysectomy procedures
Positron emission tomography (PET) imaging has potential to define both the extent of intracranial disease and the functional characteristics of neoplasms; however, its use is very limited in veterinary patients to date.
surgical instruments
magnification
Gelatin foam, oxidized regenerated cellulose, and bone wax are ideal products for control of hemorrhage
Ultrasonic aspirators are particularly useful for resection or debulking of extra- and intraparenchymal neoplasm
Intraoperative ultrasonography often facilitates removal of intraparenchymal lesions,
What is unique about burring for cranial surgery?
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
What electrosurgical device is preferred when working on dural and parenchymal tissues?
irrigation-coupled bipolar device
Standard bipolar on minimum effective setting with constant slow, steady saline drip sufficient
What can be used to make the initial dura cut?
If wanting to close the dura, what needs to be done intraop?
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
what consdierations for durotomy?
The dura should be opened with the following considerations in mind:
(1) how to best access the pathology;
(2) the location of major vessels within and under the dura—hemorrhage early on will obscure the view and potentially lead to brain swelling;
(3) how the dural defect will be closed (graft vs. direct closure); and
(4) the effect of potential brain swelling.
What can be used to close a dural defect?
Fascia of temporalis muscle (surface of fascia facing towards brain)
Porcine SISM
Sutured with 4-0 PDS or tissue glue
haemorrhage control
If meningeal or parenchymal > locate and coagulate the vessel rather than “chase” a vessel under a bony ledge
large venous sinuses (dorsal sagittal sinus, transverse sinus). Light bleeding > gelatin foam or other hemostatic agent placed over the tear in the sinus. Heavier bleeding may require that the sinus be packed with gelatin foam or bone wax
excising brain mass
extraparentchymal
- Craniectomies should be large enough to allow dissection
- electrosurgical coagulation of all visible blood vessels that appear to be supplying the mass prior to removal
- dentifying a plane of dissection between the mass and normal brain
- cotton nibs or paddies are helpful to aid in gentle retraction
- Keep the brain surface moist at all times
intraparenchymal
- Intraparenchymal lesions require incision into a gyrus of the cerebrum
- surface vessels in the region of the proposed incision should be gently coagulated
- mass excised using an ultrasonic aspirator
Transfrontal craniotomy
olfactory bulbs and rostral portion of frontal lobe
- Modified transfrontal approach increased visibility and surgical access
- achieved by removing additional bone overlying the frontal sinus than is noted in the standard description.
Because the frontal sinus is in direct communication with the outside environment via the nasal cavity, a watertight closure of the graft over the surface of the brain is of prime importance for preventing various surgical complications (
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
> enlarging the foramen magnum
Significant bleeding from the occipital emissary vein (exiting from the skull near the mastoid foramen) or condyloid vein (adjacent to the condyle) can be encountered.
the internal vertebral venous plexus may be present circumferentially around the spinal cord (interarcuate branch).
